Digital data management system for maintaining consistency of data in a shadow set

A digital data management system for managing a shadow set of storage media includes a plurality of storage media each accessible by at least one data processing device for I/O operations. Successive comparisons are carried out between data stored in corresponding locations in the storage media while maintaining access to the storage media for I/O operations. When inconsistency between data in corresponding locations is detected, a management operation is performed on at least one of the shadow set storage media. The management operation includes interrupting I/O operations to at least the storage medium on which the operation is performed, modifying data on one of the shadow set storage media to correct the inconsistency, and resuming availability of the storage media for I/O operations.

RELATED APPLICATIONS 
This application is related to the following applications, all of which are 
filed concurrently herewith, and each of which is incorporated herein by 
reference: U.S. patent application Ser. No. 07/374,551, in the name of 
Scott H. Davis, William L. Goleman and David W. Thiel, Robert G. Bean and 
James A Zahrobsky, and entitled DATA STORAGE DEVICE FOR A DIGITAL DATA 
PROCESSING SYSTEM; U.S. patent application Ser. No. 07/374,528, in the 
name of Scott H. Davis, William L. Goleman and David W. Thiel, and 
entitled Transferring Data In A Digital Data Processing System, and U.S. 
patent application Ser. No. 07/374,253, in the name of Scott H. Davis, 
William L. Goleman, David W. Thiel, Robert G. Bean and James A Zahrobsky, 
and entitled Transferring Data In A Digital Data Processing System. 
BACKGROUND OF THE INVENTION 
This invention relates to a device for storing digital data. The preferred 
embodiment is described in connection with a system for establishing and 
maintaining one or more duplicate or "shadow" copies of stored data to 
thereby improve the availability of the stored data. 
A typical digital computer system includes one or more mass storage 
subsystems for storing data (which may include program instructions) to be 
processed. In typical mass storage subsystems, the data is actually stored 
on disks. Disks are divided into a plurality of tracks, at selected radial 
distances from the center, and sectors, defining particular angular 
regions across each track, with each track and set of one or more sectors 
comprising a block, in which data is stored. 
Since stored data may be unintentionally corrupted or destroyed, systems 
have been developed that create multiple copies of stored data, usually on 
separate storage devices, so that if the data on one of the devices or 
disks is damaged, it can be recovered from one or more of the remaining 
copies. Such multiple copies are known as the shadow set. In a shadow set, 
typically data that is stored in particular blocks on one member of the 
shadow set is the same as data stored in corresponding blocks on the other 
members of the shadow set. It is usually desirable to permit multiple host 
processors to simultaneously access (i.e., in parallel) the shadow set for 
read and write type requests ("I/O" requests). 
It is sometimes necessary to "merge" two (or more) storage devices to 
reassemble a complete shadow set, where the devices were previously 
members of the same shadow set, but currently contain data that is valid, 
although possibly inconsistent. Data in a particular block is valid if it 
is not erroneous, that is, if it is correct, as determined by an error 
correction technique, or, if it is incorrect but correctable with use of 
the error correction technique. Shadow set members have data that is 
inconsistent if they have corresponding blocks whose data contents are 
different. For example, if one of the hosts malfunctions (e.g., fails), it 
may have had outstanding writes that completed to some shadow set members 
but not to others, resulting in data that is inconsistent. A merge 
operation ensures that the data stored on corresponding blocks of the 
shadow set members are consistent but does not determine the integrity 
(i.e., accuracy) of the data stored in the blocks which were inconsistent. 
The integrity of the data is verified by higher level techniques (e.g., by 
an applications program). 
SUMMARY OF THE INVENTION 
The invention generally features a method and apparatus for performing a 
management operation in a data storage device, the preferred embodiment 
describing an improved method and apparatus for merging the data of two 
storage media (e.g., hard disks) in a shadow set of storage media. The 
preferred method of managing a shadow set of storage media accessible by 
one or more data sources (e.g. host processors) for I/O operations, 
includes the steps of: A. carrying out successive comparisons of data 
stored in corresponding locations in a plurality of shadow set storage 
media, respectively, while maintaining access to the storage media by the 
data sources for I/O operations; and B. performing a management operation 
on at least one of the shadow set storage media. The management operation 
preferably includes the steps of: (a) interrupting I/O operations to at 
least the shadow set storage medium on which the management operation is 
performed; (b) modifying data on the shadow set storage medium whose I/O 
is interrupted, based on the results of the comparisons performed in step 
A; and (c) resuming the availability of the modified storage medium for 
I/O operations by the data sources. 
In the preferred embodiment, the step of modifying comprises making data on 
the modified storage medium consistent with data on another of the shadow 
set storage media, by reading data from one of the storage media and 
writing the read data to the modified storage medium. The shadow set 
storage media are accessible by a plurality of data sources. 
The invention allows inconsistencies in duplicate copies of stored data to 
be corrected while maximizing the availability of the data in the various 
copies comprising the shadow set during the correction process. Maximum 
availability is achieved since I/O operations, initiated by the hosts, are 
interrupted only when an inconsistency is found, and only for a period 
long enough to correct the inconsistency. 
Other advantages and features of the invention will be apparent from the 
following detailed description of the invention and the appended claims.

STRUCTURE AND OPERATION 
Referring to FIG. 1, a computer system including the invention includes a 
plurality of hosts 9a-c, each of which includes a processor 10, memory 12 
(including buffer storage) and a communications interface 14. The hosts 
9a-c are each directly connected through a communications medium 16 (e.g., 
by a virtual circuit) to two or more storage subsystems illustrated 
generally identified by reference numerals 17a-b (two are shown). 
Each storage subsystem 17a-b includes a disk controller 18 that controls 
one or more disks 20, which form the members of the shadow set. Disk 
controller 18 includes a buffer 22, a processor 24 and memory 26 (e.g., 
volatile memory). Processor 24 receives I/O requests from hosts 9a-c and 
controls reads from and writes to disk 20. Buffer 22 temporarily stores 
data received in connection with a write command before the data is 
written to a disk 20. Buffer 22 also stores data read from a disk 20 
before the data is transmitted to the host in response to a read command. 
Processor 24 stores various types of information in memory 26, described 
more fully below. 
Each host 9a-c will store, in its memory 12, a table that includes 
information about the system that the hosts 9a-c need to perform many 
operations. For example, hosts 9a-c will perform I/O operations to storage 
subsystems 17a-b and must know which storage subsystems are available for 
use, what disks are stored in the subsystems, etc. As will be described in 
greater detail below, the hosts 9a-c will slightly alter the procedure for 
I/O operations if a merge operation is being carried out in the system by 
a particular host 9a-c. Therefore, the table will store status information 
regarding any ongoing merge operation (as well as other operations). The 
table also contains other standard information. 
While each storage subsystem may include multiple disks 20, the members of 
the shadow set are chosen to include disks in different storage subsystems 
17a-b. Therefore, a host may directly access each member of the shadow 
set, through its interface 14 and over communication medium 16, without 
requiring it to access two shadow set members through the same disk 
controller 18. This will avoid a "single point of failure" in the event of 
a failure of one of the disk controllers 18a-b. In other words, if members 
of a shadow set have a common disk controller 18, and if that controller 
18 malfunctions, the hosts will not be able to successfully perform any 
I/O operations. In the preferred system, the shadow set members are 
"distributed", so that the failure of one device (e.g., one disk 
controller 18) will not inhibit I/O operations because they can be 
performed using another shadow set member accessed through another disk 
controller. 
In some cases a host 9 initiates a merge operation in which it makes data 
on two disks 20 comprising members of a shadow set consistent. In a merge 
operation, the host 9 initiates by means of write and read commands, read 
and write operations. Before proceeding further, it will be helpful to 
describe these commands in greater detail. 
When a host 9 wishes to write data to a disk 20, which may comprise a 
member of a shadow set, the host issues a command whose format is 
illustrated in FIG. 2A. The command includes a "command reference number" 
field that uniquely identifies the command, and a "unit number" field that 
identifies the unit (e.g. the disk 20) to which data is to be written. To 
accomplish write operations for each disk 20 comprising a member of the 
shadow set, the host issues a separate write command to each disk 20 with 
the proper unit number that identifies the disk 20. The "opcode" field 
identifies that the operation is a write. The "byte count" field contains 
a value that identifies the total number of bytes comprising the data to 
be written and the "logical block number" identifies the starting storage 
location on the disk at which the data is to be written. The "buffer 
descriptor" identifies the location in host memory 12 that contains the 
data to be written. 
The "host reference number," "entry locator" and "entry id" are used in 
connection with a "write log" feature, described in detail below. 
The format of a read command is illustrated in FIG. 2B, and includes fields 
that are similar to the write command fields. For a read command, the 
buffer descriptor contains the location in host memory 12 where the data 
read from the disk is to be stored. The read command does not include the 
fields in the write command that are associated with the write log 
feature, namely, the host reference number field, entry locator field, and 
entry ID field (FIG. 2A). 
Once a host transmits a read or write command, it is received by the disk 
controller 18 that serves the disk 20 identified in the "unit number" 
field. For a write command, the disk controller 18 will perform the write 
operation in connection with the identified disk 20 and return an "end 
message" to the originating host 9. The format of the write command end 
message is illustrated in FIG. 3A. The end message includes a number of 
fields, including a command reference number field whose contents 
correspond to the contents of the command reference number field of the 
write command which initiated storage operation, and status field that 
informs the host whether or not the command was completed successfully. If 
the disk 20 was unable to complete the write operation, the status field 
can include an error code identifying the nature of the failure. 
In response to a read command, the disk controller 18 will read the 
requested data from its disk 20 and transmit the data to memory 12 of the 
originating host. After the data has been transmitted, an end message is 
generated by the disk controller and sent to the originating host, the 
format of the read command end message being illustrated in FIG. 3B. The 
read command end message is similar to the end message for the write 
command, with the exception that the read command end message does not 
include an entry locator or entry ID field associated with the write 
history log feature, described below. 
The disk controller 18 also maintains a write history log that includes a 
number of "write history entries" (FIG. 4), each of which stores 
information regarding a recent write operation. As described above, each 
of the storage subsystems that form each shadow set member includes a 
processor 24 and an associated memory 26. When a write operation is 
performed to a shadow set member, its disk controller 18 stores, in a 
write history log in its memory 26, information in a write history entry 
indicating the data blocks of the shadow set member to which data has been 
written. The write history entry also stores information identifying the 
source of the write command message (e.g., the originating host) which 
initiated the write operation. 
Thereafter, if a merge operation becomes necessary, a host 9 managing the 
merge operation accesses the write history log for each shadow set member 
engaged in the merge operation and determines from the log entries which 
data blocks may be inconsistent. For example, if one of hosts 9 in FIG. 1 
should fail while initiating write operation to members of the shadow set, 
one shadow set member may have completed the write operation but not 
another shadow set member, leaving the data on the shadow set inconsistent 
to the extent of the one write operation. Therefore, merge operation may 
be performed by a properly functioning host, but it need only be performed 
for the data blocks that the failed host has recently enabled to be 
written because other data blocks will be consistent. Therefore, the host 
performing the merge operation will access the write history log 
associated with each member of the shadow set, determine which blocks have 
been written by the host that failed, and perform a merge on corresponding 
blocks in the shadow set. Since only the blocks that were written in the 
shadow set are merged, the operation is completed much more quickly than 
if a host managing a merge operation enabled the contents of an entire 
shadow set member to be copied to another shadow set member. 
We will first describe how the write history log is created and maintained 
and then will describe how this information is utilized in a merge 
operation. When a write command is received by one of the disk controllers 
18, the disk controller prepares and stores a write history entry in its 
memory 26. The format of a write history entry is shown in FIG. 4. The 
"entry flags" indicate the state of a write history entry. An "Allocated" 
flag is set if the write history entry is currently allocated (i.e., being 
used to store information about a write operation). The contents of a 
"unit number" field identify the specific disk to which the write 
operation associated with the write history entry was addressed. A 
"command identifier/status" field is used to identify and give the status 
of a current command (examples of the information stored in this field are 
described below). 
The "starting logical block number" gives the address (position) on the 
disk volume at which the associated write operation begins and the 
"transfer length" specifies the number of bytes of data written to the 
member's disk. I.e., these fields specify what part of the shadow set 
member has been potentially modified. The "host reference number" field 
identifies the host from which the write operation originated. 
The "entry id" field contains a value assigned by a host to uniquely 
identify a write history entry. 
The "entry locator" field contains a value assigned by the shadow set 
member that uniquely identifies the internal location of the write history 
entry, i.e., the location within memory 26. 
When a shadow set disk controller 18 receives a write command from a host, 
the controller performs the following operations. First, the controller 
validates the command message fields and checks the state of the disk 20 
to perform the write operation in a standard manner for the protocall 
being used. If the field validation or state checks fail, the shadow set 
member rejects the command issuing a write end message (FIG. 3A) whose 
status field contains the appropriate status. 
The controller then checks a flag in the write command that indicates the 
contents of whether a new write history entry for the command is to be 
allocated or whether a previously allocated write history entry will be 
reused. If a new write history entry is to be allocated, the "Host 
Reference Number/entry locator" field will contain the Host reference 
number. The controller will search the set of write history entries in 
memory 26 for an entry that is not currently allocated--i.e., a write 
history entry with a clear "allocated entry" flag. 
If an unallocated write history entry cannot be found, the controller 18 
completes the command and sends an end message to the host whose "status" 
field indicates that the write history log is invalid with respect to that 
host. The controller will also invalidate all write history entries for 
the host that issued the write command in order to prevent another host 
from relying on these entries in a merge operation. The entries can be 
invalidated by using a "history log" modifier contained in each entry that 
indicates whether that entry is valid or invalid. 
If an unallocated write history entry is found, the controller 18 performs 
the following operations in connection with the write history entry: 
a. Sets the "allocated entry flag". 
b. Copies the contents of the command message's "unit number" field to the 
entry s "unit number" field. 
c. Copies the contents of the command message s "opcode" and "modifiers" 
fields to the entry's "command id/status" field. 
d. Copies the contents of the command message s "byte count" (or "logical 
block count") field to the entry's "transfer length" field. 
e. Copies the contents of the command message s "logical block number" (or 
"destination lbn") field to the entry's "starting logical block number" 
field. 
f. Copies the contents of the command message's "host reference number" 
field to the entry's "host reference number" field. 
g. Copies the contents of the command message's "entry id" field to the 
entry's "entry id" field. 
h. Continues normal processing of the command. 
If a previously used write history entry is to be reused, then the "Host 
Reference number/ entry locator" field will contain the "entry locator" 
that defines the location of the write history entry to be used. This may 
occur if, for example, a host 9 is re transmitting a previous write 
command message, which will have the same value in the "host reference 
number" field. To accomplish this, the controller 18 will first determine 
if the value contained in the "entry locator" field identifies one of the 
set of write history entries in its write log. If the contents of the 
"entry locator" field does not identify an entry in the set, the 
controller 18 rejects the command as an invalid command. If the contents 
of the "entry locator" identifies one of the entries in the set of write 
history entries, the disk controller 18 uses the value in the "entry 
locator" field as an index into the set of write history entries to find 
the write history entry to be reused. The controller then checks the 
setting of the allocate entry flag of the found write history entry. If 
that flag is clear, indicating that the entry was not actually already 
allocated, the controller 18 rejects the command with a status of Write 
History Entry Access Error. 
The controller then checks the "command identifier/status" field of the 
identified write history entry to see if the entry is currently associated 
with an in progress command such as a write command that is being carried 
out--i.e., the controller determines if an "encode" flag within the 
"opcode" field is clear. The encode flag is set when an operation begins 
and is cleared when the operation completes. If the entry is associated 
with an in progress command, the controller rejects the command and sends 
an end message to the host whose status field identifies a Write History 
Entry Access Error. 
Finally, if the entry is not associated with an in progress command, the 
controller performs the following operations: 
a. Copies the contents of the command's command message "unit number" field 
to the entry's "unit number" field. 
b. Copies the contents of the command's command message "opcode" and 
"modifiers" fields to the entry's "command id/status field. 
c. Copies the contents of the command's command message "byte count" (or 
"logical block count") field to the entry's "transfer length" field. 
d. Copies the contents of the command's command message "logical block 
number" (or "destination lbn") field to the entry's "starting logical 
block number" field. 
e. Continues normal processing of the command. 
After a write command has aborted, terminated, or completed, the disk 
controller copies the "encode," "flags," and "status" end message fields 
into the appropriate fields of the write history entry associated with the 
command and then continues standard processing. 
In addition, prior to returning the end message of a write command, the 
disk controller sets the "host reference number," "entry id," and "entry 
locator" end message fields equal to the values contained in the 
corresponding fields of the write history entry associated with the 
command. Note that with one exception the requirement just described can 
also be met by copying those fields directly from the command message to 
the end message. The only exception is that when a new write history entry 
has been allocated, the controller must set the "entry locator" end 
message field equal to the value contained in the "entry locator" field of 
the associated write history entry. 
As will be explained below, the system utilizes a "Compare Host" operation 
in performing a merge of two shadow set members. The command message 
format for the Compare Host operation is shown in FIG. 6A. The Compare 
Host operation instructs the disk controller supporting the disk 
identified in the "unit number" field to compare the data stored in a 
section of host memory identified in the "buffer descriptor" field, to the 
data stored on the disk in the location identified by the "logical block 
number" and "byte count" fields. 
The disk controller receiving the Compare Host command will execute the 
requested operation by reading the identified data from host memory, 
reading the data from the identified section of the disk, and comparing 
the data read from the host to the data read from the disk. The disk 
controller then issues an end message, the format of which is shown in 
FIG. 6B, to the host that issued the Compare Host command. The status 
field of the end message will indicate whether the compared data was found 
to be identical. 
One of the system's hosts will control operations for merging two storage 
devices. The system can select any host to execute the operation. For 
example, the host with the best transmission path (e.g., the shortest 
path) to the shadow set may be chosen. 
During a merge operation, the host controlling the merge will utilize a 
"Write History Management" command to perform a number of operations. The 
command message format (i.e., the message sent by the host to the shadow 
set) of a Write History Management command is shown in FIG. 5A. FIG. 5B 
illustrates the end message format (i.e., the message returned to the host 
by a shadow set member s disk controller ). The host selects a particular 
operation as the operations are needed during the merge, and specifies the 
operation in the "operation" field of the command message. The other 
fields contain other information, explained more fully below as each 
operation is explained. 
The "DEALLOCATE ALL" operation is used to deallocate all of the write 
history entries for the disk identified in the "unit number" field. The 
DEALLOCATE ALL operation makes all of the write log spaces available for 
new entries. For example, after a merge operation is performed, the 
entries stored in the write history log are no longer needed because the 
members are assumed to be consistent immediately following a merge. The 
DEALLOCATE ALL operation will deallocate (i.e. free up) all of the write 
history entries to make them available for new information as new writes 
are made to the shadow set. 
A host can deallocate only those write log entries associated with a 
particular host using a "DEALLOCATE BY HOST REFERENCE NUMBER" operation. 
This may be desirable if a merge was performed as a result of a particular 
host having failed. Such a merge, as described more fully below, would 
involve merging only those blocks that were written with information from 
the failed host. Once that merge is completed, the write history entries 
associated with that host will no longer be needed and may be deallocated. 
All write history entries are not deallocated because if a different host 
fails, its entries would be needed to perform a similar merge. To execute 
this operation, each disk controller deallocates all of the write history 
entries that are associated with the host identified in the "host 
reference number" field for the disk identified in the "unit number" 
field. 
A specific write history entry can be deallocated using a "DEALLOCATE BY 
ENTRY LOCATOR" operation. The disk controller deallocates the specific 
write history entry that is located within the write log at the location 
specified in the "entry locator" field. If the contents of the entry 
locator field does not specify an entry within the limits of the write 
history log (i.e., if there is no write history entry at the identified 
location), the command is rejected and an end message is returned as an 
invalid command. If the value contained in the "host reference number" 
field of the write history entry identified by the entry locator does not 
equal the value contained in the command message "host reference number" 
field (i.e., if the host identified in the command is also not the same as 
the host identified in the write history entry), the command is rejected 
as an invalid command. Similarly, if the value contained in the "entry id" 
field of the write history entry located via the "entry locator" does not 
equal the value contained in the command message "entry id" field, the 
controller rejects the command as an invalid command. 
The "READ ALL" operation is used to read information from the write log of 
a shadow set member supporting the disk identified in the unit number 
field. If a host wishes to determine the total number of write history 
entries stored in the write log, the "count" field is set to zero, and the 
disk controller sets the command's end message "count" field (see FIG. 6B) 
equal to the number of write history entries that are associated with the 
identified unit. 
The READ ALL operation is also used to read all of the write log entries 
from an identified shadow set member. In this case, the "count" field is 
nonzero, and the disk controller transfers the number of the write history 
entries specified in the "count" field to the host memory 12 (specified in 
the "write history buffer descriptor" field) beginning with the first 
write history entry. Note that only those write history entries that are 
associated with the unit identified in the "unit number" field are 
included in the transfer. 
The "READ BY HOST REFERENCE NUMBER" operation is used to read information 
from the write logs associated with a specific host. If the "count" field 
message is zero, the disk controller sets the command's end message 
"count" field equal to the number of write history entries that are 
associated with both the host identified in the "host reference number" 
field and the unit identified in the "unit" field. Therefore, the 
controller counts only those entries that resulted from writes to the 
identified unit from the identified host. 
If the "count" field in a "READ BY HOST REFERENCE NUMBER" operation is 
nonzero, the controller transfers the number of write history entries 
specified in the "count" field to the location in host memory 12 specified 
in the "write history buffer descriptor" field, beginning with the first 
write history entry that is associated with both the host identified in 
the "host reference number" field and the unit identified in the "unit 
number" field. 
As noted above, the end message format for the Write History Management 
command is shown in FIG. 5B. The "unit alloc" field will contain the total 
number of write history entries currently allocated and associated with 
the unit identified in the command message "unit number" field. The 
"server alloc" field will contain the total number of write history 
entries currently allocated across every disk served by the particular 
disk controller. The "server unalloc" field contains the total number of 
write history entries currently available. 
Now that the Write History Management command, and its associated 
operations, have been described in detail, a merge operation utilizing the 
write log feature will be described with reference to the flowchart of 
FIGS. 7A-C. 
In step 1, the host issues one of the Write History Management commands 
described above to each shadow set member in order to obtain information 
from the write logs stored in memory 26 of each disk controller 18. The 
specific commands used will depend on the circumstances that created the 
need for the merge operation. For example, if a host fails, and a merge is 
performed by one of the properly functioning hosts, the host performing 
the merge will obtain, from the write log associated with each shadow set 
member, a complete list of all data blocks on that member disk to which 
the failed host had performed a write operation. As discussed above, the 
only data blocks that can possibly be inconsistent are those to which data 
was written by the failed host. 
To obtain this information, the host performing the merge will first issue 
a Write History Management command to perform a "Read Host Reference 
Number" operation to each disk controller that supports a shadow set 
member, with the "count" field set to zero. The command will identify the 
failed host in the "host reference number" field. As described above, each 
disk controller will receive its command and will send an end message to 
the host with the count field set to the total number of write history 
entries in its write log that were created due to writes from the failed 
host (step 2). 
The host will then determine whether it needs to issue more Write History 
Management commands (step 3). In this example, the host has received end 
messages specifying the number of write history entries contained in each 
write history log for the failed host. The host controlling the merge will 
therefore need to issue another command to each shadow set member's disk 
controller that indicated it had write history entries for the failed 
host. A disk controller that returned an end message with the "count" 
field set to zero has no write history entries for the failed host and the 
host does not send a second command to these disk controllers. (Note that 
in the rare case where every shadow set controller returns a valid end 
message with the "count" field set to zero, indicating that no shadow set 
member has been written with data from the failed host, no merge operation 
is necessary and the process terminates.) 
Therefore, a second Write History Management command is sent to each disk 
controller having needed write history entries (step 1), the command again 
specifying the "Read by Host Reference Number" operation and identifying 
the failed host, but this time setting the "count" field to the number of 
write history entries that the particular write history log has for the 
failed host. This time each disk controller reads the write history 
entries from memory 26, sends them to the controlling host's memory 12, 
and issues an end message (step 2). The host receives the end messages and 
will determine, in this case, that it does not need to issue another Read 
By Host Reference Number command since it will now have all of the needed 
write history information. 
The host (step 3) proceeds to establish a Logical Cluster Number table in 
its memory 12. The table contains numbers that identify the "clusters" 
(i.e., groups of data blocks) in the shadow set that are to be merged. 
The controlling host sets up the table by listing logical cluster numbers 
that identify all of the clusters that are identified by the write history 
entries transmitted by the shadow set members. 
As each write history entry is received, the host prepares a new entry, 
each of which is identified by a logical cluster counter (which starts at 
1 and increases sequentially). The entry contains a number identifying the 
cluster to which the data had been written, and a disk controller ID that 
identifies the disk controller that sent the write history entry. By 
sequentially going through the logical cluster number table, the host will 
be able to identify every cluster to which data has been written by the 
failed host. 
The host may discount logical cluster number table entries if they are the 
result of write history entries where corresponding entries were received 
from every disk controller supporting a shadow set member, because if a 
write has been performed in every shadow set member, the shadow set will 
not be inconsistent due to that write. In other words, since shadow set 
inconsistencies occur due to a write succeeding on some members but 
failing on other members, if a write history entry identifying a specific 
write command can be found in the write log associated with every shadow 
set member, then we know that the write operation succeeded on every 
member, and logical cluster numbers formed from these write history 
entries need not be used. 
Next, in step 5, the host sets a logical cluster counter equal to the first 
logical cluster count number in the logical cluster number table. The 
logical cluster counter is used to access a logical cluster number. When 
initialized to one in step 5, the first entry in the logical cluster 
number table is identified. 
The host then selects one of the members as a "source" and the other member 
as the "target" (step 6). The host issues a read command to the disk 
controller serving the source to read the data stored in the section of 
the disk identified by the current logical cluster counter (step 7). The 
read command issued by the host is described above and shown in FIG. 2B. 
The "unit number" is set to describe the source disk 20 with the "Logical 
Block Number" and "Byte Count" being set according to the cluster 
currently identified by the logical cluster counter. 
The source receives the read command and, after reading the identified data 
from the disk 20 to buffer 22, will transmit the data to the host memory 
12 in the section identified by the "Buffer Descriptor" field in the read 
command. The source will then transmit an end message of the type 
illustrated in FIG. 4B, which informs the host that the read operation has 
been performed (step 8). 
After the host receives the end message (step 9), the host will issue a 
Compare Host command to the target controller to compare the data read 
from the source to the data in the corresponding cluster in the target to 
determine if the data is identical (step 10). 
If a "yes" result is obtained, the host will check to see if the logical 
cluster counter identifies the last logical cluster number in the logical 
cluster number table (step 13). If the result of step 13 is "yes", the 
merge operation is finished. Otherwise, the logical cluster counter is 
incremented (step 14) and the method returns to step 7 to process the next 
cluster. 
If a no result was returned, indicating that the data read from 
corresponding clusters on the two merge members are not identical, the 
host will implement the following steps to make the data consistent. 
The host will first establish cross system synchronization by transmitting 
a message over communications medium 16 to all other hosts in the system 
(step 15). Each host will receive the transmitted message, will complete 
all outstanding I/O requests to the shadow set and will stall new I/O 
requests. Once this has been accomplished, each host sends an 
"acknowledge" message to the host controlling the merge indicating that it 
has complied with the synchronization request (step 16). 
After receiving all of the acknowledge messages, the host will issue 
another read command to the source for the same cluster read in step 7, 
using an identical command message (step 17). The data is read again 
because another host may have modified the data since it was read in step 
7. The source receives the read command message and executes it in the 
same manner as described in connection with step 8 above (step 18). The 
host will once again receive the end message from the source as in step 9 
above (step 19). 
The host will now issue a write command to the target using the write 
command message format shown in FIG. 2A, and described above (step 20). 
The write command will instruct the target to write the data read from the 
source in step 18, to the cluster in the target identified by the logical 
cluster counter. The target receives the write command, executes it and 
sends an end message to the host (step 21). These steps will result in the 
data in the two corresponding clusters in the source and target being 
consistent. Note that, because no I/O requests are being processed during 
these steps (i.e., the system is synchronized) there is no danger of the 
data in the source being modified after it was read in step 18, so the 
data written to the target's cluster in step 21 will be identical to the 
data now stored at the corresponding cluster in the source. 
After the host receives the end message indicating the write has been 
completed (step 22), the host transmits a message to all other hosts to 
end the cross-system synchronization, the message informing all other 
hosts that normal I/O operations may resume (step 23). The host will then 
return to step 13 and continue as described above. 
Therefore, during a merge operation, the host selects a shadow set member 
as the source and sequentially compares the data stored in each data block 
(a cluster at a time) to data in the corresponding data blocks in the 
target shadow set member. If the host finds an inconsistency, the system 
is temporarily synchronized while the host resolves the inconsistency by 
reading the inconsistent data from the source and writing the data to the 
corresponding cluster in the target. Because the shadow set members being 
merged normally have only a very small amount of data that is 
inconsistent, cross system synchronization will be necessary for only a 
short time, resulting in only minimal disruption to normal I/O operations. 
Because the data on both shadow set members is equally valid, if a cluster 
on the shadow set member selected as the source is corrupted (e.g., cannot 
be read), the corresponding cluster on the target is utilized. I.e., the 
target acts as the source for that cluster. Therefore, data transfer in a 
merge operation can occur in both directions. 
If a host in the system needs to perform a read operation while another 
host is performing a merge as described above, the host must first ensure 
that the specific location to which the read is directed is consistent. To 
accomplish this, the host will effectively perform a merge operation on 
that specific section by first issuing the read to the source and then 
issuing a Compare Host command to the target to determine if the data is 
consistent. If it is consistent, then the host continues with its 
processing of the read data. If it is not consistent, then the host will 
synchronize the system, reread the data from the source and write the data 
to the target as described above. 
Since there is limited space in each disk controller memory 26 for the 
write history log, the hosts will try to keep as many write history 
entries as possible available for use. Therefore, if a host issues a write 
command to each member of the shadow set, and receives end messages from 
each disk controller indicating that the write request was successfully 
performed, the host will reuse the write history entries that were 
allocated for the write requests sent to each disk controller. These 
entries may be reused because, if a write has succeeded to all members of 
the shadow set, then the shadow set will not be inconsistent due to that 
write. As discussed above, a host that performs a merge operation will 
deallocate those entries that were used in the merge operation once the 
operation is completed. 
The illustrative embodiment describes a merge operation performed on a 
single target, but several targets may be used to thereby merge three or 
more storage media. Similarly, the system need not use the same member 
disk as the source throughout the merge operation, but may use other disks 
as the source. 
While the illustrative embodiment describes the use of the write history 
log when performing a merge operation, it should be clearly understood 
that the write history log need not be used. The advantages of the 
invention can be achieved without using the write log feature. The merge 
operation would be carried out for each cluster in the shadow set, or for 
clusters selected by some means other than the disclosed write history 
log. 
Accordingly, the invention shall not be limited by the specific 
illustrative embodiment described above, but shall be limited only by the 
scope of the appended claims.