Data distribution utilizing a master disk unit for fetching and for writing to remaining disk units

The control unit arbitrarily selects a disk unit among the disk units that are inactive when the control unit receives an input/output request involving either read or staging. For a write request, the control unit selects a master disk unit in the disk unit group for the immediate writing of data, and a disk unit other than the above-mentioned master disk is preferably selected to execute the read and staging unit. After writing is used to write the data into the other disk units after the write request execution is completed by writing to the master disk unit.

BACKGROUND OF THE INVENTION 
The present invention relates to the control of the execution of a load for 
a storage, particularly the control of parallel processing with respect to 
input and output for disk drives. 
In Japanese Patent Laid-Open No. 114947/1985, a double write control has a 
cache (hereinafter referred to simply as a cache). Two disks, called 
double write disks, are each written with the same data. A control unit 
processes an input output request from a CPU for one of the two disk 
units. In the case of receiving a read request (input request) from the 
CPU, the control unit executes the request as it is. In the case of 
receiving a write request (output request) from the CPU, data is written 
in a specific one of the double write disks and at the same time the same 
data is written in the cache. At a later time, making use of available 
processing time when the control unit and disks have nothing else to do, 
the control unit writes the same data from the cache into the other disk 
unit, which is called a write after process. In this manner, the same data 
is written to each disk unit of the double write disk units. 
In Japanese Patent Publication No. 28128/1986, there is disclosed a double 
filing control for load distribution with respect to double write disk 
units. There is no write after process. The control is designed to achieve 
a higher processing speed by selecting an inactive disk unit, among the 
disk units, when an input/output request is received. An inactive disk 
unit will be defined herein as a disk unit that is currently not 
undergoing any disk accessing, that is not undergoing any read or write 
operation. 
In a thesis found in the Information Process Institute Bulletin "Nishigaki 
et al: Analysis on Disk Cache Effects in a Sequential Access Input 
Process", Vol. 25, No. 2, pages 313-320 (1984), there is disclosed with 
respect to a single disk unit a read ahead control having a cache, which 
involves the staging, in the cache, data not requested by the CPU but 
which will be requested in an instruction shortly following the current 
instruction. This staging process is executed by the control unit 
independently of any execution of an input/output request from the CPU. 
SUMMARY 
It is an object of the present invention to solve problems, analyzed below, 
that the inventors have found with respect to the above-noted controls. 
Japanese Patent Laid-Open No. 114947/1985 does not give any attention to a 
potential advantage of the double write disk system, namely that a 
plurality of disk units can be controlled by the control unit, but instead 
the document discloses that the CPU input/output request is limited to one 
specific disk unit as requested by the CPU. Therefore, even though there 
is another disk unit that may be inactive, the request cannot be fulfilled 
by the control unit if the CPU requests the one specific disk unit that 
happens to be active at the time. The disk unit is considered active when 
it is undergoing some type of input/output process. 
On the other hand, Japanese Patent Publication No. 28128/1986 has excellent 
performance by selecting an inactive disk unit by the control unit for an 
input/output request from the CPU. However, this is applied to the double 
write function by utilizing a cache without a write after control, and 
therefore it's reliability is lowered. This is due to the high possibility 
that write data is received from the CPU that is applicable to all the 
disk units, but is stored in the cache without being immediately written 
to a disk unit. Therefore, if power failure occurs in the cache in 
combination with the breakdown of any one of the disk units, the write 
data received from the CPU is lost. 
Furthermore, in the case of a control unit having a cache, the control unit 
can execute an input/output between the cache and the disk unit 
independently of an input/output request from the CPU, as disclosed in the 
thesis in the Information Process Institute Bulletin, mentioned above. In 
view of this, the inventors think that attention should be given to the 
possibility that a plurality of disk units can be selected for an 
input/output process by the control unit independently of an input/output 
request from the CPU. 
Japanese Patent Laid-Open No. 135563/1984 does not have any relation to the 
double write system. This patent relates to the cache disk control unit 
with a write after control. The disk control unit stores the write data 
received from the CPU to both the cache memory and the non-volatile 
memory. The write data in the cache memory is written to the disk unit by 
utilizing a write after process. Therefore, the write request issued by 
the CPU can be processed at high speed without accessing the disk unit, 
moreover, this can realize the highly reliable write after process. If the 
write data in the cache memory is lost because of the breakdown of the 
cache memory, the write data remains in the non-volatile memory. However, 
this patent does not relate to the double write function. Japanese Patent 
Laid-Open No. 37418/1990 relates to the double write system including the 
cashe disk control unit with a write after control. In this patent, the 
cashe disk control unit has the bit map information to recognize the area 
in the disk unit where the write data in the cashe has not been written 
yet. The bit map information is utilized to reduce the recovery time when 
the cashe unit breaks down. However, this patent does not relate to the 
load distribution among the disk units in the disk unit group. 
Specifically, the present invention relates to the control for providing a 
write after process using a cache so that the same data may be written to 
a disk unit group, comprising one or more disk units. If the disk unit 
group comprises one disk unit, the disk unit has a plurality of disks on 
each of which is written the same data. If the disk unit group comprises a 
plurality of disk units, each disk unit may have one or more disks, with 
the same data being written to each disk unit of the group. 
The object of the present invention is to provide control for improving 
parallel execution of input/output processes by distributing the processes 
among disk units in the disk unit group, for distributing the load of the 
input/output processes under the control of the control unit. 
To better understand the present invention, input/output processes, which 
the control unit executes between the control unit and the disk units can 
be classified into four kinds, as follows: 
(1) A write request received from the CPU, which requires access to a disk 
unit. 
(2) A read request received from the CPU, which requires access to a disk 
unit. 
(3) A staging process performed independently of an input/output request 
from the CPU (that is independently of a read request or a write request 
from the CPU), which transfers the data from a disk unit to a cache. 
(4) A write after process executed between the control unit and a disk 
unit. 
Of the above mentioned four kinds, the write after process is not an object 
for load distribution, as will be explained later. The write after process 
is executed, with respect to a disk unit group, for all of the disk units 
other than those to which the same data has already been written, when a 
write request received from the CPU is executed. Therefore, there is no 
freedom for selecting a disk unit which should be used to execute the 
write after process. Therefore, in the above four processes, the first 
three processes are objects for load distribution. 
In the present specification, two kinds of load distribution according to 
the present invention will be discussed. 
In the first kind of load distribution, the control unit selects a disk 
unit among the disk units that are inactive when the control unit executes 
an input/output process involving either the second (read) or third 
(staging) kind of process. When a disk unit should be selected for a write 
request from the CPU, which requires access to the disk unit according to 
the first type of the four mentioned input/output processes, the control 
unit selects a specific disk unit in the disk unit group for the immediate 
writing of data. 
In the second kind of load distribution, when the control unit selects a 
disk unit for an input/output process of the first type, that is for the 
write request received from the CPU which requires access to a disk unit, 
a specific disk unit in the disk unit group is selected. When a disk unit 
is selected to execute an input/output process of the second and third 
types (read and staging), a disk unit other than the above-mentioned 
specific disk unit is selected, preferably arbitrarily. 
The functions of the first kind of load distribution will be discussed. 
When a control unit receives from the CPU a read request which requires 
access to a disk unit, the control unit executes the following process. 
For the read request, the control unit selects arbitrarily (that is 
independently of the CPU, which includes according to an algorithm 
implemented in the control unit), a disk unit among the inactive disk 
units in the disk unit group (each of the disk units in the disk unit 
group has on it the same data to be read). If no inactive disk unit is 
found among the disk units of the disk unit group, the control unit will 
place the read request in a wait state. In the case of receiving a write 
request from the CPU requiring access to a disk unit, the control unit 
selects one specific disk unit, hereinafter called the master disk unit, 
among all the disk units of the disk unit group. If the specific disk 
unit, particularly the master disk unit, is active with respect to some 
other input/output process, the control unit will place the write request 
in a wait state. In the case of executing a staging performed by the 
control unit independently of an input/output request from the CPU, an 
inactive disk unit among the disk units of the disk unit group is selected 
for the staging, that is for transfer of information between the disk unit 
and the cache. If all of the disk units subject to such a selection are 
active with some other input/output process, the control unit places the 
staging in a wait state. 
In general, an input/output process placed in a wait state will be 
periodically reviewed to see if it can be executed, and if it can be 
executed, it will be executed. 
The first type of a load distribution according to the present invention 
has improved reliability and improved features, with respect to the 
control disclosed in the above-mentioned documents. As compared with the 
control disclosed in Japanese Patent Laid-Open No. 114947/1985, the first 
type of distribution according to the present invention is slightly 
inferior in the distribution effect for the write request, but as compared 
with the control disclosed to Japanese Patent Publication No. 28128/1986, 
the present invention provides superior and excellent performance. The 
first type of load distribution according to the present invention has a 
restriction with respect to the free selection of the disk unit for a 
write request. Accordingly, the distribution effect is lower as compared 
with the control of Japanese Patent Laid-Open No. 114947/1985 that can 
select any disk unit within the disk unit group. However, for a read 
request, any inactive disk unit is selected by the present invention. 
Usually, there is a far greater number of read requests than the number of 
write requests, for disk units in general, and the ratio is approximately 
3:1 to 4:1. Therefore, the first load distribution type shows not so large 
a degradation in the performance as compared with the control disclosed in 
Japanese Patent Laid-Open NO. 114947/1985. On the other hand, as compared 
with the control disclosed in Japanese Patent Publication No. 28128/1986, 
which uses one disk unit intensively for all input/output requests, the 
first type of load distribution according to the present invention shows a 
far better performance. 
The reliability of the first type of load distribution according to the 
present invention is higher than the reliability provided by the 
disclosure of Japanese Patent Laid-Open No. 114947/1985, and is almost 
equal to that of the method disclosed in Japanese Patent Publication 
28128/1986. For the first kind of load distribution according to the 
present invention or the Japanese Patent Publication No. 28128/1986, there 
is no data for the write after process for the disk unit for which write 
requests are intensively assigned. The write after process does not write 
data to any specific disk unit for which write requests are intensively 
assigned. Therefore, even if there is a power failure in the cache, no 
write data received from the CPU is lost unless the specific, master disk 
unit intensively storing all of the write requests is also damaged. If, 
according to Japanese Patent Publication No. 28128/1986 the write request 
was immediately executed for a random one of the disk units and the write 
data was saved in the cache for a later write after. Therefore, if the 
cache lost its data before the write after could be completed and any one 
of the disks in the disk unit group is damaged, the data could be 
completely lost. Whereas in the present invention, the write request is 
always immediately executed with respect to one specific disk, a master 
disk, so that even if the data is lost in the cache before the write after 
process can be completed, the data can be read from the master disk 
reliably. Accordingly, the load distribution of the first type according 
to the present invention has high performance and high reliability with 
respect to a disk unit group, in a well balanced manner. 
The function of the second type of load distribution, according to the 
present invention, will be discussed. 
When the control unit receives from the CPU a write request requiring 
access to a disk unit in the disk unit group, the control unit selects one 
specific disk unit, hereinafter called the master disk unit, among all the 
disks units of the disk unit group for immediate execution of the write 
request, and also writes the same data to the cache for later execution of 
the write after process. However, if this specific disk unit, the master 
disk unit, is in an active state, the control unit places the write 
request in a wait state. When receiving a read request from the CPU 
requiring access to a disk unit in a certain disk unit group, the control 
unit executes the following process. First, one arbitrary (arbitrary with 
respect to the CPU and selectable according to random distribution or some 
algorithm by the control unit) disk unit in an inactive state is selected 
from among the disk units of the disk unit group other than the 
above-mentioned specific disk unit, that is other than the master disk 
unit. That is, the read request is performed with respect to any of the 
disk units of the disk unit group except for the master disk unit. If no 
inactive disk unit is found among the disk units other than this master 
disk unit, the master disk unit is then examined to determine whether or 
not it is inactive. If the master disk unit is inactive, as determined by 
such examination, the control unit selects the master disk unit to 
complete the read request, and if the examination reveals that the master 
disk unit is currently active, the control unit will place the read 
request in a wait state. 
When attempting to execute a stage process independently of an input/output 
request from the CPU, the control unit performs the following process, for 
the second load distribution kind in the present invention. First one 
arbitrary disk unit is selected among the inactive disk units of the disk 
unit group other than the master disk unit. If no inactive disk unit is 
found for such selection, the master disk unit is examined to determine 
whether or not it is inactive. If this determination finds the master disk 
unit inactive, the control unit selects the master disk unit for execution 
of the staging, and if the examination finds that the master disk unit is 
active, the control unit places the staging in a wait state. 
The reason why the second load distribution kind according to the present 
invention is more desirable than the first load distribution kind is as 
follows. As an example, let it be assumed that a read request is assigned 
to a specific disk unit for which write requests from the CPU are 
intensively assigned, more specifically, the master disk unit, by the 
first load distribution kind. If a write request is received before the 
process for the read request is completed, the control unit cannot start 
executing the write request. Therefore, the disk units other than the 
master disk unit should preferably be assigned for any processes other 
than the write request from the CPU. Thus, the load distribution effect 
can be enhanced by the second type of load distribution of the present 
invention as compared with the first type of load distribution and as 
compared to the load distribution of the above-mentioned documents.

DETAILED DESCRIPTION 
All of the following figures and description apply equally to the first 
type of load distribution according to the present invention and the 
second type of load distribution according to the present invention, 
except where the differences are specifically disclosed with respect to 
the second type of load distribution that is a modification of the first 
type of load distribution. 
FIG. 2 is a block diagram showing the configuration of a computing system 
of the present invention. The computing system comprises: a plurality of 
processors 210, each having a CPU 200, a main memory (MM) 201 and channels 
202; a control unit 203; and a plurality of disk units 204 grouped into a 
lesser plurality of disk unit groups 211. In this respect, it will become 
clear from the following description that the present invention is 
applicable to the control unit 203 connected to a single or a plurality of 
processors 210, as indicated. There are a plurality n of disk units 204 
grouped into each of a plurality m of disk unit groups 211, that is, each 
of the m disk unit groups 211 has more than n disk units 204. The number 
of n disk units 204 belonging to each disk unit group 211 may vary among 
the disk unit groups. Each disk unit 204 belongs to a specific disk unit 
group 211. The method for designating the disk unit group 211 to which the 
respective disk unit 204 belongs is not directly related to the present 
invention and therefore will be omitted from the description. 
The control unit 203 comprises more than one director 205, a cache (for 
example a volatile memory such as a DRAM) 206, control information memory 
207 and directory 208. Each of the directors 205 selectively transfers 
data between a channel 202 and a disk unit 204, between a channel 202 and 
the cache 206, and between the cache 206 and a disk unit 204. The cache 
206 stages data that requires more frequent access, which data is also 
stored along with other less frequently accessed data in the disk units 
204. The directory 208 stores information needed to manage the cache 206. 
The staging is executed by the directors 205. A specific example of 
staging data can be the object data accessible from the CPU 20, and the 
stored data in the vicinity of this data in the disk unit 204. 
The control unit 203, to which the present invention is specifically 
directed, has the function to write the same data to all of the disk units 
204 belonging to a certain disk unit group 211, that is, the so-called 
multiple writing function. Therefore, it can be considered that the 
processor 210 issues an input/output request selectively to each of the 
disk groups 211. 
From the viewpoint of the control unit 203, acceptable input/output 
processes from the processor 210 can be classified as follows: 
(1) An input/output request process that does not gain access to a disk 
unit 214, which is a request for data transfer between the cache 206 and 
the processor 210. For example, the process executed when the data for a 
read request received from the processor 210 has already been staged in 
the cache 206 as shown by the cache directory 208. 
(2) A process involving input/output request from a CPU requiring access to 
a disk unit 204 in a specified disk unit group 211. 
(3) Execution process by the control unit 203 between a disk unit 204 and 
the cache 206, whereby the control unit 203 executes a subsequent 
input/output process independently of the input/output request received 
from the processor 210. For example, a data transfer without any 
relationship to the processor 210, which is an input output process 
between the cache 206 and a disk unit 204 in one of the disk unit groups 
211. 
The present invention relates to a load distribution control between the 
disk units 204 in the same disk unit group 211. Therefore, the 
input/output request process that does not gain access to a disk unit 
group 211 referred to as (1) above is not directly related to the present 
invention. Among the processes executed by the control unit 203, the 
processes mentioned above as (2) and (3) are the objects of the present 
invention. In this respect, the disk unit 204 to which no input/output 
process (2) or (3) mentioned above is assigned (that is, a disk unit not 
executing any process), is referred to as a disk unit in an inactive 
state. 
The load distribution control of the first type according to the present 
invention will now be described along with all features that are common to 
the load distribution control of the second type according to the present 
invention. 
FIG. 1 is a block diagram illustrating the operation of the control unit 
203 in accordance with the first type of distribution of the present 
invention. In FIG. 1, there are a plurality of master disk units, namely 
AO, BO and CO that are respectively a part of the disk unit groups 211A, 
211B and 211C. The difference between the master disk unit and the other 
disk units is that the master disk unit is broadly a specific disk unit 
defined in advance in each of the disk unit groups 211 to more intensely 
receive the write requests, more specifically, to more intensely 
immediately receive the write data directly in accordance with a write 
request without passing the write data through the cache in a write after 
process, and even more specifically the master disk immediately receives 
the write data for all write requests to its disk unit group, whereas the 
other disk units of the same disk unit group receive the write data in a 
write after process from the cache. 
In FIG. 1, the disk unit group 211A, the disk unit group 211B and the disk 
unit group 211C are connected to the control unit 203. The disk unit group 
211A comprises a master disk unit AO and a plurality of disk units A1 
through AI, which can vary in number from one to many disk units and which 
are disk units other than the specific unit, that is other than the master 
disk unit AO. Likewise, the disk unit group B comprises a master disk unit 
BO, and a plurality of other disk units B1 through Bj, and the disk unit 
group C comprises a master disk unit CO and other disk units C1 through 
Ck. 
The input/output requests received by the control unit 203 from the 
processor 210 that require access to a disk unit 211 will be described 
separately for the write and read requests. In FIG. 1, data flow 110 is 
for a write request, and data flow 113 is for a read request. The control 
unit 203 receives a write request 110 from the processor 210, which 
requires access to the disk unit group 211A. The control unit 203 selects 
the master disk AO within the disk unit group 211A with selection (a). In 
other words, the master disk AO is regarded as the disk unit in which the 
write requests which require an access to the disk unit group 211A are 
concentrated. According to the more limited aspect of the present 
invention, all of the write requests immediately transfer write data to 
the master disk AO and the cache 206. The same write data is later 
transferred by the cache to all of the other disk units of the disk unit 
group 211A by the write after process. The control unit 203 writes to the 
master disk unit AO the data of the write request received from the 
processor 210 and at the same time writes this write data to the cache 206 
as write data 111. The control unit 203 later executes writing this same 
write data 111 to each of the disk units belonging to the same disk unit 
group 211A other than the master disk AO, that is to the disk units A1 
through AI with selection (b) and write after process flow (c). The write 
data 111 is stored in the cache 206 until all of the write after processes 
to each of the disks A1 through Ai are completed. 
In the case of receiving the write request which requires access to the 
disk unit group 211A, the reason why the master disk AO is preferably 
always selected for immediately receiving the write data is as follows: If 
it is so arranged that any write request must necessarily be assigned to 
the master disk AO, all write data 111 received from the processor 210 is 
written to the master disk AO. As a result, the complete data is always 
held in the master disk AO, even, if for example, there is a breakdown in 
any one of the disk units A1 through Ai other than the master disk and a 
power outage affecting the cache 206. However, this arrangement results in 
a restriction in selecting the disk unit freely for the write request 
received from the processor 210. Hence, the system performance is lowered 
as compared to a system wherein the write request can be allocated to an 
arbitrary disk unit in the requested disk unit group without specifying a 
master disk. That is, the present invention has an advantage over such a 
system with respect to reliability, but has a slightly reduced 
performance, e.g., speed. Specifically, in the present invention when the 
control unit 203 receives the write request 110 requiring access to the 
disk unit group 211A, the control unit cannot start its processing unless 
the master disk AO is inactive. 
FIG. 1 also illustrates the case where the control unit 203 receives a read 
request from the processor 210 requiring access to the disk unit group 
211C. At this time, the control unit 203 selects any one of the disk units 
204 arbitrarily (including an algorithm within the control unit), among 
the disk units 204 that are in an inactive state within the disk unit 
group 211C, which in the example of FIG. 1 involves the selection (e) of 
disk unit 211C. The control unit 203 transfers the requested read data 
from the disk unit 211C to the processor 210 along the path 113. At this 
time, it may be arranged that the read data requested by the processor 210 
is stored not only in the disk C1 but also staged in the cache 206 as 
stage data 114, and such storing is indicated by the broken line. By 
staging the data, a read request for the same data 114 at a later time can 
be executed from the cache at a higher speed than it can be executed from 
the disk unit group 211A. 
As shown in FIG. 1, there are input/output processes between the disk unit 
groups and the cache 206 that are executed by the control unit 203 
independently of the input/output request received from the processor 210. 
Specifically, there is the write after process involving data flow (c), 
which writes write data to the disk units in an independent stage process 
independently executed from the input/output request of the processor 210. 
Another example of the independent stage process performed separately from 
the processor 210 is the execution of an advanced read from the control 
unit 203 involving the inactive state disk unit selection process (e) and 
the advance read process (d). The write after process (c) is a process to 
write the write data 111 stored in the cache 206 to the disk unit selected 
as Ai, where no write data 111 has been written yet. There is no need of 
performing a write after process with respect to the master disk AO in the 
disk unit group A because the write data 111 received from the processor 
210 has already been directly written to the master disk unit AO. 
Accordingly, with the exception of the master disk unit AO, the write 
after process (b, c), is performed sequentially with respect to each of 
the other disk units, namely disk units A1 through Ai, with a sequence of 
execution not necessarily in that order. In the case where the control 
unit 203 executes a read ahead staging process (d, e) for a disk unit 
group 211B independently of the process at 210, the control unit 203 
arbitrarily selects any one of the disk units in an inactive state among 
all of the disk units of the disk unit group 211B. In FIG. 1, the control 
unit 203 stages the stage data 114 in the cache 206 read from the disk Bj, 
by way of an example of the read ahead staging process. 
For the write request received from the processor 210 that requires access 
to a disk unit group, the reliability is obscured by selecting the master 
disk for immediate writing of the write data. On the other hand, in the 
case of reading data from a disk unit group, any inactive disk unit, 
including the master disk unit, is selected for reading. This way, high 
reliability and high performance are realized in a well balanced manner 
according to the first type of load distribution of the present invention. 
The present invention is particularly applicable to parallel processing, as 
shown with respect to different parallel processing examples set forth in 
FIGS. 3 through 5. Also, all of the processing shown in FIG. 1 is 
preferably parallel. 
FIG. 3 illustrates the control unit 203 executing parallel processing for 
the input/output processes that are: a first process that requires access 
to the disk unit group A pursuant to a request from the processor 210; and 
a second process requiring execution by the control unit 203 and the cache 
206 independently of the processor 210. As shown in FIG. 3, by way of 
example, the control unit 203 is parallel executing a stage process (a) 
with the disk unit A1 independently of an input/output process required by 
the processor 210, a write after process (c) between the control unit and 
the disk A2, and a read request (b) from the processor 210, all of which 
require access to the disk unit group 211A. In this case, the control unit 
203 selects an inactive disk unit Ai in the disk unit group 211A so that 
it can start executing the read request that has been received from the 
processor. In FIG. 3, the write after process (c) and the read ahead stage 
process (a) are performed each independently of the processor 210 and are 
each executed in parallel processing with the other. However, if there are 
many inactive disk units, the control unit 203 can parallel execute the 
corresponding greater number of multiple write after processes and read 
ahead staging processes independently of the processor 210. However, it is 
impossible to perform a write request that requires the master disk unit 
AO to be in an inactive state if some other process, for example a read 
process, is already being performed with respect to the master disk unit 
so that the master disk unit is not in an inactive state: this is a 
disadvantage of the first type of load distribution according to the 
present invention, which disadvantage is solved by the second type of load 
distribution of the present invention, as described hereinafter. 
FIG. 4 and FIG. 5 illustrate parallel processing for plural read requests. 
FIG. 4 shows a plurality of processors 210, each connected to a single 
control unit 203, and specifically shown are the processors 210 and 210a. 
By way of example, the control unit 203 receives from each of the 
processors 210 and 210a a read request that requires access to the disk 
unit group 211A. Then the control unit 203 arbitrarily selects an inactive 
disk unit for each, for example, disk unit A1 and Ai among the disk units 
of disk unit group 211A to start parallel processing the read requests 
that have been received, which processing may involve staging (b) of read 
data. As a matter of course, if there is any inactive disk in the disk 
unit group 211A when the control unit 203 receives the read request from 
the processor 210a, the read request is immediately executed. If the 
master disk is inactive, the write request is immediately executed. 
However, because of competition with respect to the master disk unit AO, 
it is impossible to parallel perform a plurality of write requests, each 
of which requests the disk unit group 211A. Also, if three or more 
processors are connected to the control unit 203, it is possible to 
perform parallel three or more read requests respectively from the three 
or more processors, where each request requires access to disk unit group 
211A so long as there are at least three or more inactive disk units among 
the disk units 204 of disk unit group 211A. 
FIG. 5 illustrates parallel processing with respect to one processor 210 
connected to the control unit 203. The processor 210, by way of example, 
can issue a new input/output request to the disk unit group 211A before 
the current processing of the input/output request to the disk unit group 
211A is completed. In FIG. 5, by way of example, the control unit 203 can 
be considered as in the middle of executing with respect to disk unit A1 a 
read request (a) received from the processor 210 that requires access to 
the disk unit group 211A. Before finishing this read request (a), the 
control unit 203 receives another read request (b) from the processor 210 
that involves access to the same disk unit group 211A. The control unit 
203 arbitrarily selects any one of the inactive disk units 204 of the disk 
unit group 211A, for example disk unit Ai, to start this second received 
read request (b) before the first read request (a) has been completely 
executed. Although not shown, the control unit 210 can also immediately 
start to process a write request that is received before the read requests 
(a) and (b) are completely executed, which write request requires access 
to the disk unit group 211A only if the master disk unit AO is inactive. 
However, because of the competition for the master disk AO, it is 
impossible to parallel execute a plurality of write requests that each 
require access to the same disk unit group, for example disk unit group 
211A. 
Furthermore, even in the case where the single processor 210 issues three 
or more input/output requests including one write request all specifying 
the disk unit group 211A, the control unit 203 can parallel execute the 
requests if in each case there is an inactive disk unit, with the write 
request requiring the master disk unit to be inactive. 
Even though much of the following description of the first type of load 
distribution also relates to the second type of load distribution 
according to the present invention, the difference between the second type 
of load distribution and the first type of load distribution, each 
according to the present invention, will now be described. In the second 
type of distribution load, the read request from the processor requiring 
access to a specific disk unit group is executed with respect to any one 
of the inactive disk units other than the master disk unit. In a similar 
manner, a staging process that is performed independently of the processor 
210 is executed with respect to any one of the inactive disk units other 
than the master disk unit. 
The reason why the second type of load distribution according to the 
present invention differs only in this manner from the first type of load 
distribution according to the present invention is that selecting a disk 
unit other than the master disk unit 204 for processes other than the 
write request allows the processing to be executed at a higher speed than 
with the first type of load distribution. This is possible, because then 
there will be less conflict between a write request and a process other 
than the write request, because the write request involves only the master 
disk unit (the write after can be performed at a later time) and the 
processing other than the write request is executed preferably with 
respect to disk units other than the master disk unit, all of which 
increases the probability that parallel processing can be performed with 
less wait states. That is, according to the second type of load 
distribution according to the present invention, there is a higher 
possibility that the master disk is in an inactive state when the write 
request requiring access to the disk unit group is received, which 
increases the possibility of parallel processing and increases the speed 
of the process. 
In FIG. 6, the following parallel processing is being accomplished as an 
example of the second type of load distribution according to the present 
invention. Write request 110 from the processor 210 requiring disk unit 
group 211A places write data 111 in the cache 206 of the control unit 203 
and also performs master disk selection (a) to thereby place the same 
write data in the master disk unit AO. At the same time (including 
overlapping but not coincided execution cycles), according to parallel 
processing, a read request 113 from the processor 210 requesting read data 
from the disk unit group 211C involves a selection (e) of disk unit C1 by 
the control unit 203 as an arbitrarily selected inactive disk unit among 
the disk units 204 other than the master disk unit CO to transfer read 
data to the processor 210 and as stage data 114 to the cache 206. Also 
according to parallel processing, staging conducting the read ahead 
process selects disk unit Bj by process (e) under control of control unit 
203 to pass data along flow (d) to be stored in cache 206 as stage data 
114. In parallel with the above or at a later time, the control unit 203 
can perform select (b) of disk unit Ai for the write after of write data 
111. 
Unless otherwise indicated, the following description is applicable to both 
the first type and the second type of load distribution according to the 
present invention. 
FIG. 8 illustrates the structure of a disk unit 204. A plurality of 
rotatably driven coaxial disks 801 are provided in the illustrated disk 
unit 204. A read/write head 802 is provided for reading and writing data 
for each of the disks 801 control unit interface 803 controls the 
operation, including movement, of the heads 802 with respect to the disks 
801. A unit of recording medium for each of the disks 801 to which the 
read/write head 802 can gain access while the disk 801 completes one 
revolution is called a track 800. A plurality of tracks 800 are present on 
each disk 801. 
FIG. 9 illustrates the structure of a single track 800. The track 800 has 
its track head 902 and track tail 903 defined at certain fixed positions, 
as references. Also, one or more records 900 can reside on each track 800. 
A record 900 is a minimum unit of input/output processing between the 
processor 210 and the control unit 203. The position of the record 900 on 
the track 800 is determined by a unit of fixed byte length called a cell 
901. The storage of a record 900 must be started at a head of a cell 901 
and it cannot be started from anywhere within the cell 901. Therefore the 
length of a record 900 is an integer multiple of the length of a cell 901. 
The numbering of the cells 901 is in ascending order, one by one, 
beginning with the track head 902 of the track 900 as number 0. 
FIG. 10 illustrates the structure of the cache 206. The cache 206 may be 
DRAM or a portion thereof mapped to comprise segments 1000. In this 
embodiment, one segment 1000 is assigned to one track 800, and the entire 
data in each track 800 is stored in a corresponding segment 1000. However, 
according to the present invention, the assigned unit of the segment 1000 
is not necessarily limited to the entire track 800. A smaller unit, such 
as a record, which is a read/write unit between the processor 210 and the 
control unit 203, can also be adopted freely as the assigned unit. 
FIG. 11 illustrates the structure of the directory 208. The directory 208 
comprises a plurality of segment management informations 1100, a track 
table 1101, and an empty segment head pointer 1102. Each segment 
management information 1100 resides in the segment unit 1000. Each one of 
the track tables 1101 and empty segment pointer 1102 resides in the 
control unit 203. 
FIG. 12 shows the required information for the present invention, which is 
provided in each segment management information 1100. An empty segment 
pointer 1200 indicates the segment unit 1000 which is not used in the 
track 800. A cache track number 1201 is the number of the track 800 of the 
disk unit group 211 stored in the segment unit 1000 for the corresponding 
segment management information 1100. The record bit map 1202 shows the 
starting position of a record 900 on the track 800 stored in the segment 
1000 for the corresponding segment management information 1100. Here the 
bit position is in the corresponding number of the starting cell 901. If, 
for example, the nth bit in the record bit map 1202 is on, the storing of 
the corresponding record 900 is started at the nth cell 901, for the 
corresponding segment management information 1100. If the nth bit is off, 
a record 900 stored starting at the nth cell 901 does not exist. 
FIG. 13 illustrates the storing format of the data on the track 800 in the 
disk unit 204 for data also in the cache 206. The structure shown in FIG. 
13a is the same as that of FIG. 9 that has already been described and 
which is also contained within the segment unit 1000. In the segment unit 
1000, shown in FIG. 13b, the records 900 are sequentially recorded 
starting from the record at track head 902 on the track 800. Therefore, if 
the number of the cell 901 that stores the start of the record 900 on the 
track 800 is known, the storage starting position of the record 900 in the 
segment unit 1000 of the cache 206 is also known. 
With respect to FIG. 12, partially described above, an updated record bit 
map 1203 for a record 900 is stored in the segment unit 1000 for the 
corresponding segment management information unit 1000, which requires a 
write after process. The record 900 that requires the write after process 
is hereinafter called a write after record. The respective bits reside in 
the corresponding number of the cell 901 as in the case of the record bit 
map 1202. Specifically, if the nth bit in the updated record bit map 1203 
is on, the record 900, storing of which is started at the nth cell 901 for 
the corresponding segment management information 1100, is a write after 
record. A separate update record bit map 1203 is provided for each one of 
the disk units 204. The specific relationship between an updated record 
bit map 1203 and a disk unit 204 will be referred to when the structure of 
the control information memory 207 is described. The areas for the updated 
record bit map 1203 are prepared for the number of the disk units 204 that 
can be defined in one disk unit group 211. However, the number of the 
updated record bit maps 1203 that can be used is the number of the disk 
units 204 comprising the corresponding disk unit group. 
In FIG. 12, the store completion flag 1204 shows whether or not the record 
900 is stored on the assigned track 800 of the disk unit with respect to 
the record in the segment unit 1000 for the corresponding segment 
management information 1100. The active flag 1205 shows that the input 
output process is being executed for the track 800 assigned to the 
corresponding segment management information 1100. The segment pointer 
1206 indicates a segment unit 1000 for the corresponding segment 
management information 1100. 
FIG. 14a illustrates the structure of the track table 1101, and FIG. 14b 
indicates the structure of the inactive segment head pointer 1102. 
The track table 1101 shows whether or not each of the segment units 1000 is 
assigned to a track 800, for a set of tracks 800 in the same disk unit 
group 211. A track table 1101 is provided for each of the disk unit groups 
211. If assigned, a pointer 1100A is set to provide the address in memory 
where the segment management information is to be found. As shown for each 
of the segment units 1000 assigned to the track 800 if there is no segment 
management information 1100, 1200, the pointer is reset. The track table 
1101 has the information regarding tracks 800 in the same disk unit group 
211, all stored in the numerical order of the tracks 800, that is in 
ascending numerical order in the direction of the arrow shown in FIG. 
14(a). The segment management information 1100 for the corresponding 
segment unit 1000 to which no track 800 has been assigned is all combined 
sequentially in storage at an address identified by the empty segment head 
pointer 1102. A set of the unassigned combined segment management 
information 1100 is called an empty segment que 1400 shown in FIG. 14(b). 
FIG. 15 illustrates the structure of the control information memory 207. In 
the control information memory 207, disk unit group information 1500 is 
included, which has information for each disk unit group 211. The number 
of disk unit group informations 1500 corresponds to the number of the disk 
unit groups 211 that can be controlled by one control unit 203. 
FIG. 16 illustrates the structure of one disk unit group information 1500, 
which is the same for all. Disk unit number 1600 is the number of the disk 
units 204 currently in the corresponding disk unit group 211. A plurality 
of the disk unit informations 1601 are provided, respectively for each of 
the disk units 204 comprising the corresponding disk unit group 
information 1500. The prepared number of disk unit informations 1601 is 
equal to the definable number of the disk units 204 defined in one disk 
unit group 211. Effective information is stored from the first disk unit 
information head 1601 up to the number of the disk unit information 1601 
defined by the disk unit number 1600. Here the disk unit information head 
1601 is information for the master disk. Also, the nth updated record bit 
map 1203 of FIG. 12 in the segment management information 1200 is a disk 
unit 204 corresponding to the nth disk unit information 1601. A processor 
input/output wait bit 1602 shows that an input/output request received by 
the corresponding disk unit group 211 from the processor is in the wait 
state. This bit number can be expressed as follows. The number of the 
processor input/output wait bit 1602 equals the number of the processor 
210 that can be connected to the control unit 203 (here the number is 
given as I) times the number of the input/output process requests, (here 
the number is given as J) that can be executed in parallel by one 
processor 210 for one disk unit group 211. 
Therefore, when each processor 210 issues the input/output request to the 
control unit 203, the processor 210 sends the following two points of 
information to the control unit 203. A first point of information is the 
identity of the processor 210 that issues the input/output request, 
indicated by one of the numbers from 1 to I that are respectively assigned 
to the processor 210. The second point of information identifies for the 
specified disk unit group 211 the input/output requests by one of the 
numbers 1 to J. 
FIG. 17 illustrates the structure of one of the disk unit informations 1601 
and the others have the same structure. A disk unit number 1700 is given 
for identifying the disk unit 204 for the corresponding disk unit 
information 1601. A processor input/output execution single bit 1701 shows 
whether or not a disk unit 204 for the corresponding disk unit number 1700 
is active in executing an input/output request received from the processor 
210. A write after execution single bit 1702 shows whether or not a disk 
unit 204 for the corresponding disk unit number 1700 is active in 
executing a write after process. An independent staging execution single 
bit 1703 shows whether or not a disk unit 204 for the corresponding disk 
unit number 1700 is active in executing a staging process performed 
independently of the processor 210. For the processor input/output 
execution bit 1701, write after execution bit 1702 and independent stage 
execution bit 1703, only one may be set on at a single time. Also, a disk 
unit 204 for which the processor input/output execution bit 1701, write 
after execution bit 1702 and independent staging execution bit 1703 are 
off is a disk unit 204 in an inactive state. When one of these bits is on, 
the disk unit 204 is in an active state. Segment management information 
pointer 1704 indicates the address of the stored segment management 
information 1100 assigned to the track 800 accessed by an input/output 
process in execution by a disk unit 204 for the corresponding disk unit 
1700. The segment management information pointer, when set, shows the 
address in storage for the segment management information for the disk 
unit 204 identified by disk unit number 1700. 
It is desirable that the control information memory 207 is non-volatile, 
otherwise there is a problem that information stored in the control 
information memory 207 can be lost due to power outage and other reasons. 
Input/output processes to be executed by the control unit 203 are actually 
carried out in parallel by the respective directors 204 in the control 
unit 203. 
FIG. 18 shows each procedure used by each of the respective directors 204 
for carrying out the required parallel executions according to the present 
invention. Each function of the procedures will be described. An 
input/output request receipt part 1800 processes the input/output request 
received from the processor 210. A write after process schedule part 1801 
provides a schedule for the write after process. An independent stage 
process schedule part 1802 provides a schedule for the staging performed 
independently of the processor 210. A disk unit transfer part 1803 
executes the read/write transfer to and from the disk units 204. 
FIG. 19 is a flowchart for the input/output receipt part 1800 of FIG. 18. 
When the input/output receive part 1800 receives a new input/output 
request from the processor 210, this part starts its execution. The 
execution is as follows. 
At step 1900, it is determined if an input/output request that has been 
received requires access to the disk unit 204. To specifically define the 
type of an input/output request which requires access to the disk unit 204 
is not directly related to the present invention and therefore is omitted 
from the detailed description. If the input/output request received does 
not require access to the disk unit 204, the processing proceeds to the 
step 1918. If the answer to the determination in step 1900 is yes, step 
1901 starts executing the input/output request by first determining if the 
requested input/output track resides in the cache. Specifically, a track 
800 which the input/output request wishes to gain access to, is checked to 
determine whether or not a segment 1000 is assigned thereto. If an 
assignment is made, processing proceeds to step 1903. If no assignment is 
made as determined by step 1901, step 1902 assigns segment management 
information 1100 to the track 800 to which the input/output request wishes 
to gain access and links it to the corresponding area of the track table 
1101. Also, the store completion flag 1204 for the assigned segment 
management information 1100 is turned off and the active flag 1205 is 
turned on. At this time, the segment management information 1100 of the 
assignment is one selected from the segment management information's 1100 
that are in an inactive state starting at the empty segment head que 1102. 
If there is no segment management information 1100 in the inactive state, 
a segment management information currently assigned is selected by a known 
method. Any specific method of the selection is not related to the present 
invention and thus will not be described in detail. Following step 1902, 
step 1905 is executed. 
If the determination in step 1901 is yes, step 1903 makes the determination 
if the requested input/output track is in use, that is the active flag 
1205 for the segment management information 1100 assigned to the track 800 
to which the input/output request gains access is checked to determine 
whether or not this flag 1205 is on. If it is on, the requested track 800 
is in use for some other input/output process, therefore, the input/output 
request newly received cannot be executed immediately and hence processing 
proceeds to step 1916. If according to the determination in step 1903, it 
is found that the active flag 1205 is off, the active flag 1205 is turned 
on according to step 1904 and processing proceeds to step 1905. 
In step 1905, it is determined if the input/output request is a write 
request. According to the most preferred form of the present invention, a 
write request which requires access to the disk unit group 211 gains 
access to only the master disk unit. Thus, if the input/output request is 
for reading, e.g., the processing branches conditionally to step 1908 in 
accordance with a negative determination from step 1905. 
In the case of a write request, the master disk unit is checked at step 
1906 to determine if the master disk unit in the requested disk unit group 
is inactive. This check examines the following information in the disk 
unit information 1601 for the corresponding master disk of the requested 
disk unit group 211, that is the head disk unit information 1601 in the 
disk unit group information 1500. In other words, the processor 
input/output execution bit 1701, write after execution bit 1702, and 
independent staging execution bit 1703 are all checked to determine if 
they are all inactive. Then, if the master disk is found to be in the 
inactive state, corresponding to none of the above-mentioned bits being 
set active, the requested master disk is selected according to step 1907. 
The selection is made in step 1907 by turning on the processor 
input/output execution bit 1701 in the disk unit information 1601 for the 
corresponding master disk. When the above process is completed, processing 
proceeds to step 1910 to start the same processing that is already 
described with respect to the first type of load distribution. 
If the determination in step 1906 is that the requested master disk is 
active, processing proceeds to step 1913 where the corresponding 
input/output request is then kept in a wait state until it can be executed 
by turning off the active flag 1205. 
If there is a negative determination from step 1905, that is if there is a 
read request, processing proceeds to step 1908. In step 1908, a 
determination is made if there is any inactive disk unit found in the 
requested disk unit group 211, to start the assignment of the read request 
which requires access to a disk unit 204 of the requested disk unit group 
211. According to the first load distribution type of the present 
invention, an arbitrary disk unit which is in the inactive state is 
assigned to the request which requires access to a disk unit 204. 
Accordingly, the requested disk unit group 211 is checked to determine if 
there is an empty disk unit 204 therein in accordance with step 1908. The 
specific contents of the processing are as follows. In other words, with 
respect to the disk unit informations 1601 for the requested disk unit 
groups 211, the processor input/output execution bit 1701, write after 
execution bit 1702 and independent stage execution bit 1703 are searched 
to see if they are off. If any one of these bits is on for each of the 
searched disk units, it means that there is no inactive disk unit 204 in 
the requested disk unit groups 211, making it impossible to start 
executing the read process and therefore the answer to the determination 
in step 1908 is no and processing proceeds to step 1913 for the wait state 
until processing may be resumed. 
If an inactive disk unit 204 is found in the requested disk unit group 211, 
corresponding to a yes determination from step 1908, an inactive disk unit 
204 of the corresponding disk unit number 1700 in the corresponding disk 
unit information 1601 is selected by step 1909. Specifically, the 
processor input/output execution bit 1701 in the selected disk unit 
information 1601 thus found is turned on. 
In step 1910, the selected segment management information pointer 1704 is 
set to indicate the segment management information 1100 assigned to the 
requested track 800. In step 1911, a positioning process request is issued 
for the disk unit 204 selected according to step 1909 to access the 
requested track 800 with its head. In step 1912, connection between the 
director 205 in use and the processor 210 making the request is cut off 
until the position process for the selected disk unit 204 is completed. 
Thereafter, the processing in the input receipt part 1800 is terminated. 
As mentioned, step 1913 is conducted when no inactive disk unit 204 is 
found in the disk unit group 211 requested. In step 1913, the active flag 
1205 from the corresponding segment management information 1100 is turned 
off. In accordance with step 1914, a determination is made if the store 
completion flag 1204 is on. If it is on, processing proceeds to step 1916. 
If the determination in step 1914 is no, the segment management 
information 1100 is registered in step 1915 in an empty segment que 1400 
because the off value of the store completion flag 1205 indicates that no 
data is recorded in the segment 1000 corresponding to this segment 
management information 1100. 
In step 1916, the corresponding input/output wait bit 1602 in the disk unit 
group information 1500 is set to provide an indication to the processor 
210 that the process of the corresponding input/output request cannot be 
started due to some other input/output process being carried out. 
Specifically, the bit position to be set in the process request 
input/output wait bit 1602 is determined in view of the two points given 
below, and the setting of the bit is performed accordingly. A first point 
is to know the number of the processor 210 which issued the corresponding 
input/output request from those processors 210 numbered from 1 to I. A 
second point is to know subsequently the number of the input/output 
request from those numbered 1 to J issued to the disk unit group 210 
specified by the input/output request. 
In the case where the segment management information 1100 of the requested 
track 800, is being used by some other input/output request, it is not 
particularly necessary to manipulate the segment management information 
1100. Finally, at step 1917, the requesting processor 210 is notified that 
the processing of the corresponding input/output request is in a wait 
state because the execution thereof cannot be started due to some other 
input/output process being executed. After this, the processing in the 
input/output receipt part 1800 is terminated. 
If the answer to step 1900 is no, processing proceeds to step 1918. In step 
1918, there is execution of a process required for the input/output 
request which does not require any access to the disk unit 204. The 
specific contents of this process is not directly related to the present 
invention therefore it is not described further in detail. Thereafter, the 
processing is ended. 
FIG. 20 is a flowchart showing the write after process schedule part 1801. 
The right after process schedule part 1801 executes during a time the 
director 205 is inactive. 
As shown in FIG. 20, step 2000 defines the disk unit group 210 for the 
write after. Because the method of this step is not directly related to 
the present invention, the detailed description thereof is omitted. In 
step 2001, a determination is made if there is an inactive disk unit 204 
other than the master disk unit found in the requested disk unit group 
211. The specific contents of the process of this step 2001 is given. With 
the exception of the master disk, a disk unit information 1601 in which 
the processor input/output execution bit 1701, write after execution bit 
1702 and independent stage execution bit 1703 are all off is searched for. 
If it cannot be found, the negative result of seep 2001 indicates that the 
write after process cannot be executed and processing proceeds to the end 
and the right after process schedule part 1801 is terminated. If found, 
that is if there is an affirmative answer to step 2001, the write after 
execution bit 1702 in the disk unit information 1601 found with step 2001 
is turned on with step 2002. 
In step 2003, a determination is made if the disk unit 204 found in the 
step 2001 has any track 800 which can execute the write after process. The 
specific check information is the segment management information 1100 with 
the on-bit in the updated record map 1203 for disk unit 204 selected from 
the track table 1101. Also, it is necessary that such segment management 
information 1100 is not in use for some other process request. Thus, the 
active flag 1205 in the segment management information 1100 must be off. 
This is another condition required to execute the write after process. If 
the answer to the determination in step 2003 is no, step 2004 turns off 
the write after execution bit 1702 and the processing of the write after 
process schedule part 1801 is terminated. If the answer to the 
determination of step 2003 is yes, step 2005 will select the track 800 for 
the write after process. If there are a plurality of tracks 800 which can 
be used for executing the write after process, one must be selected. 
However, the selection of the track 800 among a plurality of such tracks 
is not related to the present invention and its description is omitted. 
In step 2006, the active flag 1205 in the segment management information 
1100 for the track 800 selected at the step 2005 is turned on. At step 
2007, pointer 1704 to the segment management information 1100 assigned to 
the selected track 800 for the corresponding input/output request is set. 
At step 2008, a positioning process request is issued to the disk unit 204 
selected at the step 2001. After this, the processing of the write after 
process schedule 1801 is terminated. 
FIG. 21 is a flowchart showing the independent stage process schedule part 
1802, which executes during the time that the director 205 is in an 
inactive state. In step 2100, the disk unit group 211 which executes a 
stage process independently of the processor 210 is defined. This step is 
not directly related to the present invention and accordingly a specific 
description is omitted. In step 2101, a determination is made if the disk 
unit group 211 found in step 2100 has a track 800 for executing the 
staging independently of the processor 210. This step is not directly 
related to the present invention and therefore its specific description is 
omitted. If no track 800 is found in step 2101, the processing in the 
independent staging schedule part 1802 is terminated. If the answer to the 
determination of step 2101 is yes, step 2102 is performed. In step 2102, a 
track 800 is selected for the execution of the staging performed 
independently of the processor 210. If there are a plurality of tracks 800 
that can be used to execute the stage process independently of the 
processor 210, a track 800 from among the plurality must be selected. 
However, the selection of a specific track 800 itself is not related to 
the present invention so that the specific description thereof is omitted. 
In step 2103, a segment management information 1100 is assigned to the 
track 800 selected in the step 2102. The track 800 which should be used 
for executing the staging independently of the processor 210 is not a 
track 800 staged in the cache 206. This assignment method is the same as 
shown and described previously with respect to step 1902. Also, the store 
completion flag 1204 in the assigned segment management information 1100 
is turned off; the active flag 1205 is turned on. 
In step 2104, it is determined if there is a disk unit that is in the 
inactive state in the disk unit group defined in the step 2100 for the 
input/output processing. The specific processing for step 2104 is the same 
as that of step 1908 and hence a specific description thereof will be 
omitted. If the determination of step 2104 is negative, that is if no 
inactive state disk unit is found, it is impossible to execute the staging 
independently of the processor 210 and as a result the segment management 
information 1100 assigned at step 2103 is returned to the empty segment 
que 1400 and thereafter the processing in the independent stage process 
schedule 1802 is terminated. If the answer to the determination in step 
2104 is yes, that is if an inactive disk unit is found, step 2106 is 
performed to turn on the independent stage execution bit 1702 in the disk 
information 1601 found at the step 2103. 
In step 2107, a pointer 1704 to the segment management information 1100 
assigned to the selected track 800 by the corresponding input/output 
request is set. In step 2108 a positioning process request is issued to 
the disk unit 204 selected in step 2100. Thereafter, the processing in the 
independent staging schedule 1802 is terminated. 
FIG. 22 is a flowchart for the disk unit read/write transfer part 1803 of 
FIG. 18. Execution by the disk read/write unit transfer part 1803 is 
started when the director 205 is informed that the positioning of the disk 
unit 204 is completed. 
In step 2200, a segment management information 1100 pointed to by the 
segment management information pointer 1704 in the disk unit information 
1601 for the corresponding disk unit 204 is selected. Hereinafter, a 
simple description of the segment management information 1100 means the 
segment management information 1100 selected at the step 2200 unless 
otherwise specified. Also, a simple description of the information in the 
segment management information 1100 means the information in the segment 
management information 1100 selected at the step 2200 unless otherwise 
specified. 
In step 2201, a determination is made if the processor input/output 
execution bit 1701 in the disk information 1601 for the corresponding disk 
unit 204 is on. If the bit is not on, it indicates that the input/output 
process currently in execution is not a process for the input/output 
request received from the processor 210 and processing proceeds to step 
2212. If the determination in step 2201 is yes, that is if the execution 
bit 1701 is on, it indicates that the input/output process currently in 
execution is the process for the input/output request received from the 
processor 210. Accordingly, in step 2202, the completion of positioning is 
communicated to the processor 210 and processing continues with connection 
of the control unit 203 to the processor. 
In step 2203, a determination is made if the input/output request received 
from the processor is a write request. If the determination is no, that 
is, in the case of a read request, processing proceeds to step 2209. 
In the case of a write request, the data is received from the processor 210 
in step 2204 and written to the disk unit 204 and in the segment 1000 
assigned to the corresponding segment management information 1100. 
However, the number of the cell 901 where the data in the request 1000 is 
to be written should be identified before the write execution described 
above starts, because the data to be written in the segment 1000 must be 
written at a position corresponding to the cell 901 thus identified. 
Further, the updated record bit map 1203 for all the disk units 204 other 
than the master disk unit in a segment management information 1100 is 
selected. In other words, the corresponding bit to the cell 901 identified 
as above in the updated record bit map 1203 is turned on. The write data 
received from the processor is also thereby written to the cache; 
thereafter, the completion of the input/output process is communicated to 
the processor 210. 
In step 2205, a determination is made if the storing completion flag 1204 
is on, by checking the corresponding segment management information 1100. 
If the determination is yes, processing proceeds to step 2215 because the 
records 900 on the processed track 800 are staged in the segment 1000. If 
the answer is no indicating that the storing completion flag 1204 is off, 
the record 900 from the process track 800 is not staged in the segment 
1000. Consequently, the process proceeds to step 2206. 
In step 2206, the bit position of the record bit map 1202 corresponding to 
the number of the cell 901 identified in step 2204 is turned on. 
In step 2207, the remaining record 900 and the track 800 currently in 
execution to the segment 1000 is staged. In this case, it is also 
necessary to execute the following process while identifying the number of 
the cell 901 in the stage object record 900. First, the record 900 to be 
staged is also staged at a position corresponding to the identified cell 
901 in the segment 1000. Also, the bit position of the record bit map 1202 
corresponding to the number of the identified cell 901 is turned on. Then, 
with the storing completion flag 1'204 turned on in step 2208, the 
processing proceeds to step 2215. 
Step 2209 is reached from a no determination in step 2203. In step 2209, a 
determination is made as a part of a read request received from the 
processor 210. In step 2209, it is determined if the store completion flag 
1204 in the subject segment management information 1100 is on. If the 
determination in step 2209 is yes, processing proceeds to step 2210 to 
transfer the requested record 900 to the processor 210. The record 900 has 
already been stored in the segment 1000. Consequently, in step 2210 the 
requested record 900 in step 2210 is transferred from the disk unit 204 to 
the processor 210. Then the completion of the input/output process is 
communicated to the processor 210 and processing proceeds to step 2215. 
If the determination in step 2209 is negative, that is if the store 
completion flag 1204 is off, the record 900 on the subject track 800 
currently in execution is not staged in the segment 1000. Therefore, 
process step 2211 is executed. At step 2211, while being transferred to 
the processor 210 from the disk unit 204, the requested record 900 is 
staged in the segment 1000 of cache 206. Also, in step 2211, it is 
necessary to execute the process given below while identifying the number 
of a cell 901 of the stage record 900. First, the record 900 to be staged 
in a segment 1000 is also written at a position corresponding to the 
identified cell 901. Further, the bit for the corresponding number of the 
identified cell 901 in the record bit map 1202 in the selected segment 
management information 1100 is turned on. After this the completion of the 
input/output process is communicated in the processor 210. Subsequently, 
processing proceeds to step 2207, which has been described above, to stage 
the remaining record 900 in the subject track 800. 
With the negative determination from step 2201, step 2212 is reached. In 
step 2212, it is determined if the write after execution bit 1702 in the 
disk unit information 1601 for the corresponding disk unit 204 is on. If 
the answer is no, that is if the bit is off, the processing proceeds to 
step 1214 to execute the stage process independently of the processor 210. 
In step 2214, all the records on the track are staged to the cache and the 
corresponding record bit map is set and the storage completion flag is 
turned on. 
If the determination in step 2212 is yes, step 2213 will write the write 
after record to the disk unit. All the identified write after records are 
identified by the subject bit map 1203 in the defined segment management 
information 1100. After this, the entire updated record bit map 1203 for 
the corresponding disk unit 204 is cleared, that is set to 0. 
Subsequently, the processing proceeds to step 2215. 
In step 2214 reached with a no answer from steps 2212, the stage process is 
executed independently of the processor 210. Specifically, all the records 
900 on the subject track 800 are staged in the segment 1000 of the cache. 
It is also necessary to execute the following process while identifying 
the number of the cell 901 in the stage record 900. First the record 900 
to be staged in the segment 1000 is also written at a position 
corresponding to the identified cell 901. Further, the following process 
is performed for the record bit map 1202 in the subject segment management 
information 1100. In other words, the bit position of the record bit map 
1202 corresponding to the identified cell 901 is turned on. In addition, 
the store completion flat 1204 is turned on. 
The termination process proceeds from step 2215. In step 2215, reached from 
various other steps 2214, 2213, a yes answer from step 2205, and step 
2208. In step 2215, the active flag 1205 in the subject segment management 
information 1100 is turned off. 
In step 2216 all the bits in the process I/O execution bit 1701, write 
after execution bits 1702 and independent stage execution bit 1703 in the 
disk unit information 1601 for the corresponding disk unit 204 are turned 
off, which will show the inactive state for the disk unit 204. 
Finally, in step 2217, the following process is executed to release the 
wait state of the input/output request for which the processor wait bit 
1602 for the corresponding disk unit group 211 is on. In other words, the 
waiting states of all of the input/output requests defined by the 
processor 210 numbered 1 to I and the input/output request numbers of 1 to 
J according to the bits being turned on are released. Specifically, each 
of the processors is notified to issue its input/output request. In 
summary, a search is made against the input/output wait bit of the 
input/output request which is in a wait state for the corresponding disk 
unit group and the wait state is released. After this, the processing in 
the disk unit transfer part 1802 is terminated. 
As mentioned previously, the second type of load distribution according to 
the present invention is the same as the above-described first type, 
except for specifically mentioned differences. Some of these differences 
are given below. In the second type of load distribution, according to the 
present invention, a disk unit 204 other than the master disk is 
preferably selected for the read request requiring access to the disk unit 
group 211 and the staging is performed independently of the processor 210. 
The respective data structure shown in FIGS. 8 through 17 are adapted to 
the second type of load distribution without any change even though they 
were described with respect to the first type of load distribution. The 
modular structure shown in FIG. 18 that is necessary for executing the 
first type load distribution in director 205 can be adapted for the second 
type of load distribution as it is. Although the respective process flow 
of the modules in the input/output request receive part 1800 and the 
independent stage process schedule part 1802 are slightly different from 
those in the first type of load distribution, the process flow in the 
first type of load distribution for the other modular process is 
applicable without modification to the second type of load distribution. 
FIG. 23 is a flowchart showing the input/output request receive part 1800 
in the second type of load distribution of the present invention. The 
execution of the input/output request receive part 1800 is started as in 
the case of the previously described first type of load distribution. 
Therefore, only the difference between the flowchart of FIG. 19 of the 
first load distribution type and the flowchart of FIG. 23 of the second 
load distribution type will be described, and the similarities will not be 
repeated. In this respect, the step numbers are identical where the 
contents of the process steps in FIG. 23 are exactly the same as those in 
the process steps in FIG. 19. The difference in the process flow of the 
flowchart of FIG. 23 and that of FIG. 19 is that a step 2300 is adopted in 
FIG. 23 instead of the step 1908 in FIG. 19. 
In step 2300, a determination is made if any inactive disk unit other than 
the master disk is found. In step 2300, the selection is preferably made 
to determine if the disk unit 204 other than the master disk is in an 
inactive state. This is because the second type of load distribution, for 
a read request requiring access to the disk unit group 211, selects a disk 
unit 204 other than the master disk. Specifically, the following 
information in the disk unit information 1601 for each disk unit 204 other 
than the master disk is checked. In other words, the processor 
input/output execution bit 1701, write after execution bit 1702 and 
independent stage execution bit 1703 are checked to determine whether all 
the bits are off, for those disk units other than the master disk unit. If 
there is an inactive disk unit 204, a yes determination from step 2300, 
processing proceeds to step 1909 to select such disk unit 204, and then 
the same process as described with respect to FIG. 19 proceeds. If there 
is no inactive disk unit 204, that is if the answer to the determination 
of step 2300 is no, the processing proceeds to step 1906 to check whether 
the master disk is inactive, and the processing follows the process flow 
already shown and described with respect to FIG. 19. 
With the exception of the points given above, the process of FIG. 23 is 
exactly the same as that shown in FIG. 19 and the description thereof is 
omitted. 
FIG. 7 is the flowchart of the independent stage process schedule part 1802 
with respect to the second type of load distribution of the present 
invention. The execution of the independent stage process schedule part 
1802 for the second type of load distribution is started the same as in 
the case of the first type of load distribution, already described. 
Subsequently, the difference between the process flow shown in FIG. 21 
with respect to the first distribution load and the process flow shown in 
FIG. 7 will be described. In this respect, the step numbers are identical 
where the contents of processes and the process flow of FIG. 7 are exactly 
the same as those of the process flow in FIG. 21. 
The process flow in FIG. 7 differs from that in FIG. 21 in the following 
points. First, when an inactive disk unit 204 is to be found subsequently 
to the step 2102, an inactive disk unit 204 other than the master disk is 
found in the step 2400 in the case of the second type of load distribution 
according to the present invention. This is because of the preferred 
selection of the disk unit 204 other than the master disk unit in the 
second type of load distribution for the stage process performed 
independently of the processor 210. The specific process is the same as 
that for step 2300 and the description thereof is omitted. If there is an 
inactive disk 204, the processing proceeds to step 2104 to select such 
inactive disk unit 204, and the same process as in the first type of load 
distribution starts. If the disk unit 204 other than the master disk unit 
is not in an inactive state, the master disk unit is checked in step 2401 
to determine if it is inactive. The above process is the same as the 
process in step 1906 and therefore the description thereof is omitted. If 
the master disk unit is inactive, the master disk unit is selected in step 
2402. Specifically, the processor input/output execution bit 1701 in the 
disk unit information 1601 for the corresponding master disk is turned on. 
After this, the processing proceeds to step 2107 to start the same 
processing as already explained with respect to the first type of load 
distribution. If the master disk unit is not inactive, that is if it is 
active, the staging process performed independently of the processor 210 
cannot be executed. Hence, the processing proceeds to step 2105 to start 
the same process as already explained with respect to the first type of 
load distribution. With the exception of the above points, the process 
flow in FIG. 7 and the process flow in FIG. 21 are exactly the same, and 
the description of these same parts will not be duplicated. 
According to the present invention, it is possible to obtain a well 
balanced high performance and high reliability of a control unit with a 
cache having a function of writing the same data into all of the plurality 
of separate storages of a disk unit group comprising one or more disk 
units. This is because of the achievement of the distribution of an 
input/output process between the disk units within a limit not losing any 
reliability and the improvement of the performance of an input/output 
process executable by the control unit brought about by the present 
invention. 
While a preferred embodiment has been set forth along with modifications 
and variations to show specific advantageous details of the present 
invention, further embodiments, modifications and variations are 
contemplated within the broader aspects of the present invention, all as 
set forth by the spirit and scope of the following claims.