Abstract:
A disk storage system with a RAID architecture, where the disk memory has N units of disks, includes a control unit to write data to the N units of disks or read data therefrom based on instructions from a host equipment, a volatile memory connected to the control unit, and a non-volatile memory connected to the control unit. The volatile memory includes a time stamp memory and a conversion map. The non-volatile memory includes a write buffer, which has a capacity of N×K logical block data (N is a positive integer not less than 2 and K is an integer indicating the number of blocks), and a buffer management table. The control unit accumulates logical block data to be updated in the write buffer until the number of logical blocks reaches N×K−1. It generates logical address tag blocks, including time stamps stored in the time stamp memory and adds them to the N×K−1 logical blocks. The controller then performs a continuous write operation to write N×K logical blocks into empty address areas different from the areas in which data to be updated is stored.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to disk array storage devices such as RAID (Redundant Array of Inexpensive Disk), etc. using a plurality of disk memories and more particularly to storage data reading methods from disk array memories and control devices. The RAID system storage disclosed in U.S. Pat. No. 5,124,987, Japanese Patent No. 214720/1994 and 266510/1994, proposes a high speed writing method to memories. A method to write updated-data collectively in previously arranged separate empty areas in disk memory units (hereinafter simply referred to as disk memory units) instead of rewriting old data areas and a rewriting method of old data areas in idle times when made available thereafter were proposed. 
     The conventional methods described above will be briefly described using FIG.  18 . In FIG. 18, an example involving updating data blocks stored in logic block addresses (hereinafter simply referred to as logic addresses) L 6 , L 4 , L 2 , L 132 , L 7  and L 11  is considered. In these logic block addresses L 6 , L 4 , L 2 , L 12 , L 7  and L 11 , old data is stored in physical block addresses (hereinafter simply referred to as physical addresses) P 6 , P 4 , P 2 , P 12 , P 7  and P 11  in 3 disk units  181 ,  182  and  193 . First, the data blocks  16 Data, L 4 Data, L 2 Data, L 12 Data, L 7 Data and L 11 Data that are newly to be updated are stored temporarily in a buffer memory  184  which is normally composed of a non-volatile memory. These data blocks are collectively written into physical addresses P 51 , P 52 , P 53 , P 54 , P 55  and P 56  which are previously arranged separate empty areas instead of directly replacing data that are the contents of the physical block addresses P 6 , P 5 , P 2  and P 12  in which old data to be updated are stored, with old data left therein. As these data are written into continuous physical addresses P 51 -P 52 , P 53 -P 54  and P 55 -P 56  in 3 disk units  181 ,  182  and  183 , 6 times of writing operation required for the direct rewriting are physically reduced to 3 times of the writing operation and thus, the writing performance is largely improved. 
     On the other hand, in this type of conventional disk array storage systems, there is provided a conversion map  185 , that is a table showing the correspondence between logical addresses and physical addresses in which data blocks are stored. When updating data, as latest data in the logical addresses L 6 , L 5 , L 2 , L 12 , L 7  and L 11  are actually existing in the physical addresses P 51 , P 52 , P 53 , P 54 , P 55  and P 56  in the disk units as described above, the contents of the conversion map are rewritten to indicate proper disk locations. In other words, for instance, data blocks in the logical address L 6  must originally be stored in the physical address P 6  in the disk unit  181  but as actually they are stored in the physical address P 51 , the physical address P 6  corresponding to the logical address L 6  in the conversion map  185  is rewritten to P 51 . Similarly, the physical addresses corresponding to the logical addresses L 5 , L 2 , L 12 , L 7  and L 11  in the conversion map  185  are rewritten to P 52 , P 53 , P 54 , P 55  and P 56 , respectively. 
     Further, when reading data stored in the disk array storage, data is read out by obtaining physical addresses wherein latest data blocks corresponding to designated logical addresses are stored are read out and therefore, there is no possibility to read out old data. 
     Further, in order to make the explanation simple, although only two physical blocks were written in the example shown in FIG. 18 as data blocks that are stored in one disk unit, several ten blocks are actually written in one disk unit. 
     SUMMARY OF THE INVENTION 
     In the conventional technique described above, there was a problem of data maintenance, in that all data stored in a disk memory unit was lost if latest data was erased by failure or erroneous operation of an conversion map as the positional information of latest data was managed by the conversion map. Further, there was also a problem that a conversion map becomes highly expensive because it was needed to provide an conversion map for all logic blocks and to retain the conversion map in a large capacity for power source failure. 
     The present invention solves the problems described above and it is an object of the present invention to provide an inexpensive and high speed data updating method for a control system for a disk storage system. 
     The disk storage data updating method of the present invention is characterized in that, the disk memory is equipped with N units of disk unit, a control unit to write data to the N units of disk unit or read data therefrom according to instructions from a host equipment, a volatile memory connected to this control unit, including a time stamp memory and a conversion map memory and a non-volatile memory connected to the control unit, including a write buffer memory having a memory capacity equivalent to N×K (an integer) logical block data and a buffer management table memory. The data updating method accumulates logical block data to be updated in the write buffer memory until the number of logical blocks reaches N×K−1, generates logical addresses for these logical blocks and logical address tag blocks including time stamps stored in a time stamp memory and adds them to the N×K−1 logical blocks to a total N×K logical blocks and sequentially writes into empty address areas separate from logical address areas that are retaining data to be updated on the N units of disk storage successively. 
     In the disk storage data updating method of the present invention, it is further characterized in that data is written in a stripe area extending over a plurality of disk units. 
     In the disk storage data updating method of the present invention, it is further characterized in that the time stamp memory is incremented whenever N×K logical blocks accumulated in the write buffer memory are written into N units of disk unit. 
     In the disk storage data updating method of the present invention, it is further characterized in that physical storage locations on the disk units corresponding to logical addresses are detected by reading and inspecting logical address tag blocks in the stripe areas recorded in the disk units and data is written in or read out of the detected storage locations. 
     In the disk storage data updating method of the present invention, it is further characterized in that when inspecting the logical address tag blocks, if there are a plurality of stripe areas containing the same logical address, when the time stamp in the logical address tag block is latest, the logical address blocks in that stripe area are judged to be valid blocks and blocks having the same logical address as those in other stripe areas are judged to be invalid blocks. 
     In the disk storage data updating method of the present invention, it is further characterized in that when inspecting the logical address tag blocks, by detecting the maximum time stamp value, a time stamp to be added in the next writing is reproduced. 
     In the disk storage data updating method of the present invention, it is further characterized in that when inspecting the logical address tag blocks, the time stamp value that becomes the writing sequence standard is obtained by detecting the minimum time stamp value. 
     In the disk storage data updating method of the present invention, it is further characterized in that by reading out the logical block data in a plurality of stripe areas stored in the disk storage and inspecting the logical address tag blocks, only valid logical blocks in the stripe areas are moved in the write buffer memory, new logical address tag blocks corresponding to these valid logical blocks are generated, and by sequentially writing logical blocks for one stripe comprising the valid data moved in the write buffer and newly generated logical address tags in empty areas separate from a plurality of read out stripe areas, an empty area is produced on the disk storage, into which logical blocks can be written successively. 
     In the disk storage data updating method of the present invention, it is further characterized in that when generating a new logical address tag block, if the number of valid blocks is less than N×K−1, a NULL address is set for a logical address corresponding to a block in which data in a new logical address tag block is not stored. 
     In the disk storage data updating method of the present invention, it is further characterized in that by performing the inspection of the logical address tag blocks in a plurality of stripe areas when starting up the disk unit after writing data in the empty area of the disk unit, the stripe numbers for the logical addresses which are judged to be valid, the block numbers in the stripes and the time stamps of the valid data are recorded in the conversion map. 
     In the disk storage data updating method of the present invention, it is further characterized in that logical address tag blocks of the stripes in the time zone with less access to the disk unit are read and compared with the conversion map and a correction is made. 
     In the disk storage data updating method of the present invention, it is further characterized in that when the disk units in which the logical address tag blocks are dispersed and arranged by stripe and the logical address tag blocks are inspected, the logical address tag blocks of different disk units are read out in parallel. 
     In the disk storage data updating method of the present invention, it is further characterized in that the logical address tags are sequentially written in the stripe areas together with the logical block data and are also written in the dedicated tag areas in parallel, and when inspecting the logical address tag blocks, the logical address tag blocks are checked by sequentially reading this dedicated tag areas. 
     In the disk storage data updating method of the present invention, it is further characterized in that the storage areas of the disk unit are divided into a plurality of segments in a plurality of stripes and controlled so that stripe data are written in only one segment in a fixed period. In addition, when the write object segment is changed over, the contents of the conversion map at the point of time and the change-over segment number are recorded in the disk unit. In the subsequent conversion map preparation, the contents of the conversion map when the segment was changed over and only those logical address tag blocks where a bit map is set among the logical address tags written in the stripe area in the segment number recorded in the disk unit are inspected. 
     In the disk storage data updating method of the present invention, it is further characterized in that a bit map corresponding to the stripe in the segment on the non-volatile memory is prepared and when a write objective segment is changed over, this bit map is cleared, when writing data in the stripe area, a bit corresponding to the stripe area into which data were written is set and when preparing a conversion map, the conversion map at the time when the disk unit segment was changed over and only a logical address tag in which the bit map is set among the logical address tags having the segment numbers recorded in the disk unit are inspected. 
     In the disk storage data updating method of the present invention, it is further characterized in that in order to advance the minimum value of time stamp, by reading stripes with less invalid blocks periodically, valid blocks only are moved to the write buffer, a logical address tag block is generated from the logical addresses in the corresponding logical tag block and a new time stamp and stripes comprising the valid data in the write buffer and the generated logical address tag blocks are sequentially written in an empty area separate from the areas retaining the read stripes. 
     In the disk storage data updating method of the present invention, it is further characterized in that in order to advance the minimum value of time stamp, by reading out the logical address tag blocks only from stripes with less invalid blocks periodically, the logical address tag blocks added with a new time stamp and the logical address of invalid blocks made NULL address are generated and the logical address tag blocks generated here are overwritten on the read out logical address tag blocks. 
     In the disk storage data updating method of the present invention, it is further characterized in that after preparing the conversion maps, invalid blocks are determined by comparing the time stamp of the logical address tag blocks on the disk unit with the time stamp of the corresponding conversion map. 
     In the disk storage data updating method of the present invention, it is further characterized in that it is equipped with a write buffer having a capacity equivalent to (N−1)×K logical blocks, the logical blocks of data to be updated are accumulated in this write buffer memory, the updating of the logical block is retarded until the number of logical blocks accumulated reaches a selected number, a logical address tag block composed of logical addresses of the logical blocks accumulated in the write buffer is generated, K parity blocks from (N−1)×K data logical blocks with the logical address tag block added to the selected number of logical blocks are generated and N×K logical blocks with parity block added to this data logical block are sequentially written in an empty area separate from the areas retaining data to be updated on N units of disk unit. 
     In the disk storage data updating method of the present invention, it is further characterized in that the number of selected logical data blocks is (N−1)×K−1 and the logical address tag blocks are recorded in one disk unit. 
     In the disk storage data updating method of the present invention, it is further characterized in that the number of selected logical data blocks is (N−1)×K−2 and two logical address tag blocks are allocated so that the logical tag blocks are recorded in two disk units in one parity stripe. 
     In the disk storage data updating method of the present invention, it is further characterized in that to inspect the logical address tag blocks recorded in the disk units, in addition to the sequential write in unit of parity stripe, the logical address tags are written in the dedicated tag areas in which logical address tags are collected and the written data in this dedicated tag areas are not protected by parity but the dedicated tag areas are so allocated that the disk unit in which logical address tags in the parity stripes are recorded is different from the disk unit in which the logical address tags in the dedicated tag area are recorded. 
     The disk storage controller of the present invention is characterized in that it is equipped with N units of disk unit, a control unit to write or read data in or from N units of disk unit according to instructions from a host equipment and a disk storage comprising a volatile memory connected to the control unit, including a time stamp memory and a conversion map memory, a non-volatile memory connected to the control unit, including a write buffer memory having a storage capacity equivalent to N×K (an integer) logical block data and a buffer management table memory, and logical block data to be updated in the write buffer are accumulated until the number of logical blocks reaches N×K−1, a logical address tag block containing logical addresses for these logical blocks and time stamps stored in the time stamp memory are generated, this logical address tag block is added to the N×K−1 logical blocks to a total N×K logical blocks, and this total N×K logical blocks are sequentially written in an empty address area separate from the logical address areas retaining data to be updated on the N units of the disk unit successively. 
     In the disk storage controller of the present invention, it is characterized in that it is equipped with a volatile memory storing time stamps to maintain the time sequence of write, a write buffer memory to retain data to be written into the disk units in a log structure and a non-volatile memory storing buffer management information retaining logical address information on empty areas in the write buffer memory and logical addresses of written data retained therein. 
     The disk storage controller of the present invention is characterized in that it is equipped with a disk storage comprising N units of disk units, a write buffer memory having a capacity equivalent to (N−1)×K logical blocks and a sequential write control unit which accumulates logical blocks of data to be updated in this write buffer, retards the updating of the logical blocks until the number of accumulated logical blocks reaches a selected number, generates a logical address tag block comprising logical addresses of logical blocks accumulated in the write buffer, generates K parity blocks from (N−1)×K data logical blocks with the logical address tag block added to the selected number of logical blocks and writes N×K logical blocks with parity blocks added to this data logical blocks in an empty area separate from areas retaining data to be updated on N units of disk unit successively. 
     The disk storage controller of the present invention is characterized in that it is equipped with a redundant disk unit to form a redundant disk structure using parity, a volatile memory to store time stamps to maintain a time sequence of writing, a write buffer to retain data to be written in the disk units in a log structure and a nonvolatile memory to store buffer management information on empty areas in the write buffer and logical address information of retained data written therein. 
     By forming the above described structure, it is possible to construct a cheap and high speed disk storage principally requiring no indirect map and a disk storage control system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating an embodiment of the present invention; 
     FIG. 2 is a diagram illustrating the relation between a write buffer memory and buffer management information in the embodiment of the present invention; 
     FIG. 3 is a diagram illustrating the contents that are stored in an empty area of a disk unit in the embodiment of the present invention; 
     FIG. 4 is a diagram quoted for illustrating one block size data write sequence from a host equipment; 
     FIG. 5 is a diagram illustrating the contents of logical address tags TG 1 /TG 2  of stripes ST 1 , ST 2  in the embodiment in FIG. 4; 
     FIG. 6 is a diagram illustrating an example to integrate stripes ST 3 /ST 4  into one stripe ST 5 ; 
     FIG. 7 is a diagram illustrating an example of a case wherein a logical address tag TG 5  is produced from the logical address tags TG 3 /TG 4  in the stripe integration; 
     FIG. 8 is a block diagram illustrating a configuration example of a conversion map that is used in the embodiment of the present invention; 
     FIG. 9 is a flowchart for explaining a preparation method of a conversion map  32  when starting the system; 
     FIG. 10 is a diagram illustrating an example wherein logical address tags are dispersed and stored in 4 disk units by stripe; 
     FIG. 11 is a diagram illustrating the allocation of the storage areas of the disk unit in the segment division; 
     FIG. 12 is a diagram illustrating the entry structure of the segment management information; 
     FIG. 13 is a diagram illustrating the contents of the dedicated tag area wherein the logical address tags are successively stored; 
     FIG. 14 is a block diagram illustrating an embodiment of the disk unit by RAID 5  that is constructed with the present invention applied; 
     FIG. 15 is a diagram illustrating the operating concept of the embodiment shown in FIG. 13; 
     FIG. 16 is a diagram illustrating an example to control so as to write the same logical address tag in two disk units; 
     FIG. 17 is a diagram illustrating an example wherein the dedicated tag areas are allocated and used for achieving the high speed conversion map preparation; and 
     FIG. 18 is a diagram illustrating a system configuration to achieve a data updating method in a conventional example. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 1 is a block diagram illustrating the concept of a disk storage consistent with the present invention. The disk storage control system of the present invention is composed of a control unit  1 , a disk unit  2 , a volatile memory  3  and a non-volatile memory  4 . The disk unit  2  is composed of a plurality of disk storage units. In order to make the explanation simple, this embodiment will be explained as being composed of 4 disk storage units  21 ,  22 ,  23 ,  24 . The volatile memory  3  is provided with a time stamp area  31  and an indirect mapping storage area  32  for storing time sequence of write operation. The non-volatile memory  4  is provided with a write buffer area  41  to retain data to be written in the disk unit  2  by forming them in a log structure and a buffer management table  42  to retain information on empty areas in the write buffer area  41  and logical addresses of write data retained therein. The control unit  1  manages the time stamp area  31 , the write buffer area  41  and the buffer management table  42  according to an instruction from a host equipment  5  and controls the write operations to the disk unit  2 . 
     FIG. 2 illustrates the relation between the write buffer area  41  and the buffer management table  42  allocated in the non-volatile memory  4 . The control unit  1  does not write data requested for writing from an externally connected host equipment directly into the disk unit  2  but divides the data in unit of block and stores in order (in a log format) in the write buffer area  41 . Here, the buffer table  42  is in a table structure comprising a plurality of entries and the logical address of each data block viewed from a host equipment to be written in these entries while corresponding to the block locations B 0 , B 1 , . . . , B 15  in the buffer area  41 . Further, the flag “F” is provided to an entry to which data is allocated and the flag “U” to an entry to which no data is allocated. 
     In the example illustrated in FIG. 2, the write data is stored in the block locations up to B 9  in the write buffer area  41  and it is indicated that the logical addresses of B 0 , B 1 , . . . B 9  are LA 134 , LA 199 , . . . , LA 678 . 
     Further, for the disk storage units  21 - 24  of the disk unit  2 , data in the length equivalent to K blocks, which are integer (K) times of block size in a unit of storage area called as stripe unit (a size close to 1 track length of the disk unit is better) is written. Also, to the stripe units at the physically same locations of the disk storage units  21 - 24 , data is written at the same time regarding them as one stripe area (ST) as a whole. 
     Further, the disk unit  2  is shown to the host equipment  5  as a disk unit in a capacity less than actual combined storage capacity of a plurality of disk storage units  21 - 24 . In other words, if the host equipment is first inquired for its storage capacity, a less capacity is returned as a response. Therefore, in addition to the storage areas that can be logically read/written from the host equipment  5 , an excess storage area, that is, an empty area is secured. 
     Further, the time stamp  31  is an information that is added to write data from the host equipment  5  when the data is actually written into the disk unit  2  and is used to determine the data write sequence into the disk unit  2 . So, the time stamp  31  is incremented every time when data of the write buffer  41  is written in the disk unit  2 . 
     Next, the operation of the embodiment of the present invention will be described in detail referring to FIG.  2  through FIG.  8 . 
     First, the write operation will be described. After receiving data to be written and its logical addresses from the host equipment  5 , the control unit  1  divides the data in unit of block and sequentially stores them in the empty area of the write buffer  41  on the non-volatile memory successively as illustrated in FIG.  2 . Further, in FIG. 2, data is sequentially written in an empty area equivalent to the length of  15  blocks comprising B 0 , B 1 , . . . , B 15  of the write buffer  41  successively. 
     Further, the received logical addresses are converted into addresses for every blocks and stored in the corresponding entries B 0 , B 1 , . . . B 15  of the buffer management table  42 . Further, in the case of update data for those data already stored in the write buffer  41 , they are not stored sequentially in the empty areas of the write buffer  41  but old data stored in the write buffer  41  is directly updated. 
     At the stage where write data from the host equipment  5  is accumulated in the write buffer  41  in the numbers less by 1 block than 1 stripe (ST) of the disk unit  2 , that is, (4K−1), the control unit  1  writes these data in the disk unit  2 . In FIG. 2, at the stage where data is accumulated in the write buffer  41  by K=4, that is, for 15 blocks, they are written in the disk unit  2 . At this time, as the last write block, the logical address tag block LA-TAG as illustrated in FIG. 3 is prepared from the logical addresses of the blocks stored in the write management table in the buffer management information  42  and the time stamp  31  on the volatile memory  3 . The  1  for  1  relation is preset between the address data and data blocks in this logical address tag blocks so that the logical addresses of all data blocks can be seen. 
     Thereafter, as illustrated in FIG. 3, data for 1 stripe including this logical address tag block are simultaneously written into the empty areas of the disk storage units  21 - 24  collectively. In FIG. 3 the empty areas for 1 stripe (ST) of the disk storage units  21 - 24  are shown in  4  unit stripes D 1 -D 4  and the logical addresses of  4  data blocks that are written in these unit stripes D 1 -D 4  areas are shown. Further, the value of the time stamp  31  illustrated in FIG. 1 is incremented when the write is completed. Thus, the disk writing performance is sharply improved as many fine disk writings can be made at one time. 
     Next, the data block refilling process will be described. In the disk write method that is an object of the present invention, that is, a method to accumulate updating data and write in a previously provided other empty area in the disk unit  2  collectively instead of directly rewriting the old data area, it is imperative to have available an empty area for collectively writing data collected in the disk unit  2 . Therefore, it is possible to make an empty area by collecting invalid data as data were already written in other areas in an idle time when the disk access from the host equipment  5  is not executed. This process is called the refilling process. This refilling process is composed of two steps; the invalid block judging and the stripe consolidation steps. 
     As an example of the invalid block judgment, a case is considered, where there is  1  block size data writing from the host equipment  5  in the order illustrated in FIG.  4 . LXX in the figure indicates logical addresses given by the host equipment and SXX indicates the writing sequence. In the embodiment of the present invention, as the write buffer  41  is able to retain data of  15  blocks, write data of the first S 1  through S 15  are collected in one stripe (ST 1 ) and written into an empty area with the time stamp TS 1  added. Similarly, write data of S 16 -S 30  are written in another empty area as another stripe (ST 2 ) with the time stamp TS 2  added. Further, as the time stamp  31  is incremented whenever the writing is made, there is the relation of TS 1 &lt;TS 2 . 
     Here, as seen in FIG. 4, the data of the logical addresses L 9  and L 18  exist as S 5  and S 2  in the stripe of the time stamp ST 1  and also as S 19  and S 21  in the stripe of the time stamp TS 2 . That is, there are two data that are to be written into the same logical addresses L 9  and L 18 . However, when considering the sequencer of written data blocks, the data blocks of S 19  and S 21  that were written later are effective and therefore, S 5  and S 2  data must be judged to be invalid. 
     By the way, the writing order SXX that was used here for the convenience was not recorded on an actual disk. So, this judgment is made using the logical addresses added to the stripes. The contents of the logical address tags TG 1 , TG 2  of two stripes ST 1 , ST 2  in the example in FIG. 4 are as illustrated in FIG.  5 . That is, for the logical address tags TG 1 , TG 2 , the logical addresses of the blocks are stored in the storage areas corresponding to 15 blocks; B 0 , B 1 -B 15  that are written into the buffer  41  and the time stamps TS 1 , TS 2  when the stripes ST 1 , ST 2  are written will be written in the 16th storage area. 
     As can be seen in FIG. 5, the data of the same logical addresses L 9 , L 18  are contained in two logical address tags TG 1 , TG 2  and either data of the blocks B 5 , B 2  of the stripe ST 1  or the blocks B 4 , B 6  of the stripe ST 2  are invalid. Further, when the time stamp TS 1  of the logical address tag TG 1  is compared with the time stamp TS 2  of the logical address tag TG 2 , it is possible to judge that the blocks B 5 , B 2  of the stripe ST 1  are invalid from the relation of TS 1 &lt;TS 2 . As described above, it is possible to find an invalid data block by checking the logical address tags in the disk unit  2 . 
     FIG. 6 is a diagram illustrating an example of consolidating stripes and a case to consolidate two stripes ST 3  and ST 4  into one stripe ST 5  is shown. In FIG. 6, it is assumed that five blocks B 2 , B 7 , B 8 , B 12  and B 13  are valid in the stripe ST 3  and other 10 blocks are invalid (hatching). Similarly, 9 blocks B 18 , B 19 , B 20 , B 21 , B 22 , B 24 , B 25 , B 27  and B 29  are assumed to be valid in the stripe ST 4  and other 6 blocks are invalid (hatching). Therefore, since there are only  14  valid blocks in two stripes ST 3 , ST 4 , when the valid blocks of these two stripes ST 3 , ST 4  are taken out and consolidated into one stripe ST 5 , an empty area equivalent to one stripe can be produced as a result. 
     A definite method of the stripe consolidation is as follows: read two stripes ST 3 , ST 4  shown in FIG. 6 in the volatile memory  3 , take out only valid blocks of these two stripes ST 3 , ST 4  and move them into the write buffer  41  successively. In consonance with this, move logical addresses of the valid blocks only from TG 3 , TG 4  to corresponding locations, prepare a new logical address tag TG 5  and update a time stamp at the point of that time to ST 5  as shown in FIG.  7 . 
     As there are only 14 valid blocks in this example, a stripe having one write block from the host CPU  5  is completed and the data is written in an empty area of the disk unit  2  in bulk. In this case, the disk areas are efficiently used. If, however, a disk access is made from the host CPU  5  in the first mode, since write processing is waited, excessive disk accesses are likely to be made. For this reason, the last data block may be kept empty to allow data to be written therein while no disk access is made. No problems are posed as it is possible to indicate that no data is in the last data block of the logical address tag TG 5  by inserting NULL address such as −1, etc. in the logical address of the last data block. 
     Next, the read operation of data blocks written as described above will be explained. By performing the judgment of invalid blocks of the refilling process for the logical address tags of all stripes in the disk unit  2 , the physical locations of valid blocks for all logical addresses can be detected. Therefore, it is theoretically possible to detect physical blocks to be read by checking all stripes whenever logical addresses of read blocks are received from the host equipment  5 . However, this method requires an enormous time for block reading and is not practical to use. 
     So, the logical address tags of all stripes are checked only when starting up the system and a conversion map  32  of logical addresses to physical addresses is prepared on the volatile memory  3  illustrated in FIG.  1 . Then, the access to valid blocks is performed using this conversion map  32  for the read request from the host equipment  5 . As a result, it becomes unnecessary to check address tags whenever the block read is requested by the host equipment  5  and the performance will never drop at the time of reading. Further, as this conversion map  32  can be reproduced any time by checking all stripes and it is also unnecessary to store this conversion map in the non-volatile memory  4  providing for the power source failure as in a conventional indirect map. 
     Here, the conversion map  32  will be explained using FIG.  8 . As illustrated in FIG. 8, the conversion map  32  retains the stripe number ST# storing blocks for the logical addresses L 0 -Ln, block numbers BLK# in the stripes and further, the time stamp TS# in a format of table. So, if the logical addresses L 0 -Ln are given, actual physical addresses can be obtained simply from ST# and BLK# by referring to this table. 
     Further, to prepare the conversion map  32  at the time when the system is started, the logical address tags TG 1 , TG 2 , TG 3 , . . . of the stripes ST 1 , ST 2 , ST 3 , . . . stored in the disk unit  2  are read in order (STEP  1 ) as illustrated in the flowchart in FIG.  9 . Then, extract the time stamps TS 1 , TS 2 , TS 3 , . . . and read logical address tags TG 1 , TG 2 , TG 3 , . . . (STEP  2 ). Further, extract the logical addresses LAXX in the read logical address tags TG 1 , TG 2 , TG 3 , . . . in order (STEP  3 ). When the logical address LAn is the same as the extracted logical address LAn, it is registered in the conversion map  32 , the time stamp TSn of the logical address LAn registered in the conversion map  32  is compared with the time stamp TSn of the extracted logical address LAn (STEP  4 ) . As a result of this comparison, if the time stamp TSi of the extracted logical address LAn is newer than the time stamp TSj of the logical address LAn registered in the conversion map  32 , that is, if TSi is larger than TSj, the stripe number ST# of the disk unit  2  from which the logical address LAn is extracted is registered in the conversion map  32  and further, the data block location BLK# is stored (STEP  5 ). Further, the time stamp TS# for the extracted address LAn is stored as the time stamp TS# for the logical address LAn in the conversion map  32  (STEP  6 ). As a result of the comparison in STEP  4 , if the time stamp TSi of the extracted logical address LAn is older than the time stamp TSj of the logical address LAn registered in the conversion map  32 , that is, if TSi is smaller than TSj, then the contents of the stripe ST# for the logical address LAn, the data block location BLK# and the time stamp TS# registered in the conversion map  32  are left as they are, and a determination is made whether all valid logical addresses that were read have been checked (STEP  7 ) . If all logical addresses that were read were not checked, then return to STEP  3 , repeat the processes up to STEP  6 . If the same processes were completed on all of the logical addresses in the read out logical address tags, check whether the above described processes were executed on all of the logical address tags TG 1 , TG 2 , TG 3 , . . . stored in the disk unit  2  (STEP  8 ). If the same processes were not completed, return to STEP  3 , repeat the processes up to STEP  7 . And if the same processes were completed, make the contents of the stripe ST#, data block location BLK# and time stamp TS# for the logical addresses that were left at that point of time as the contents to be registered in the conversion map  32  (STEP  9 ). 
     In other words, only when the time stamps of the logical address tags are larger than the time stamps of the table in the conversion map  32  for all logical addresses in the taken out logical address tags, register the stripe number and corresponding block numbers on the table. When this check is executed for all stripes, a conversion table indicating only valid blocks is obtained. Further, only valid blocks are always registered on this conversion map  32  when the same process is executed for logical address tags whenever stripes are written in the disk unit  2 . Further, even when the data registered on this conversion table becomes improper due to failure in the memory, they can be detected and corrected by comparing and inspecting the logical address tags of the stripes with the conversion map. 
     As described above, the principal process of the preparation of the conversion map is to inspect the logical address tags. Therefore, when the number of logical address tags is many as in a large capacity disk unit, an extended time will be required to prepare a conversion map. In particular, if the logical address tag blocks are concentrated on one disk unit  24  as illustrated in FIG. 2, the access is concentrated to this disk at the time when the system is started and the logical address tags cannot be checked in parallel with them. So, the time required for preparing this conversion map can be reduced to ¼ by dispersing the disk unit into which the logical address tags are stored by the stripe into 4 units and checking the logical address tags in parallel, as illustrated in FIG.  10 . 
     In addition, by dividing and managing the storage area of the disk unit  2  into a plurality of segments, the number of inspections of the logical address tags required for preparing the conversion map can be reduced. In FIG. 11, the structure of the storage area of the disk unit in the segment division system is shown. As illustrated in FIG. 11, the storage area of the disk unit is divided into a segment management information (hatching) and 4 segments in unit of stripe. Here, the segment is a unit area wherein the collective writing of buffer data or the disk writing in the refilling process is concentrated in some period. For instance, as long as the segment  2  is an object of the disk writing, the selection of an empty area is so controlled that no writing is made in the segments  1 ,  3 ,  4 . 
     Further, when the empty area of a segment becomes less and the disk writing is changed over to another segment, the segment management information is stored in the disk unit. The segment management information is composed of a segment number and a change-over conversion map as illustrated in FIG.  12 . The segment number is the segment number to which the data writing is changed over and the change-over conversion map is the state of the conversion map on the volatile memory  3  when a segment is changed over. Furthermore, all the data of a change-over conversion map is not overwritten whenever a segment is changed over, but only the entries of the logical addresses written in the preceding segment may be written back. If, therefore, the time stamp is stored when the preceding segment is switched and compared with the time stamp of the conversion map, the logical addresses written in the preceding segment can be determined. 
     In this segment division system, the segment management information is retained when a segment is changed over. So, by simply reading the conversion map at the time of segment change-over from the segment management information and then, by inspecting the logical address tag of the segment indicated by the segment number of the segment management information, a conversion map which is the same as that when all logical address tags were inspected can be reproduced. Therefore, the number of inspections required for logical address tags is one segment and a time required to prepare a conversion map in this example can be reduced to ¼ according to this system. 
     Further, a bit map corresponding to all stripes in the segments on the non-volatile memory  4  is prepared. When a segment is to be switched, this bit map is cleared. When a bulk/collective write is performed or when a write in compaction/refilling is performed, the bit corresponding to each written stripe is set to “1”. Thus, the bit map corresponding to a stripe that was changed after the segment change-over will become “1”. Accordingly, when preparing a conversion map, it is possible to further reduce the number of inspections and a time required for preparing a conversion map by referring to this bit map and inspecting a logical address tag for a stripe that was changed. 
     The size of a logical address tag is normally 512-1024 bytes and there is a performance difference of about 50 times between the sequential access and the random access of a disk. In the system illustrated in FIG. 2, the logical address tag information are scattered for every stripes and therefore, the time consuming random access was used when preparing a conversion map. So, a dedicated tag area (for each segment when dividing segments) for contiguously storing only logical address tags is prepared so that the logical address tags can be read at the sequential access as fast as 50 times of the random access as illustrated in FIG.  13 . For the collective writing data from a host equipment or refilling data writing, logical address tags are written not only in an empty area but also in a corresponding dedicated tag area. In the system illustrated in FIG. 2, the number of disk writings was 4 times per stripe but according to this method, it is increased one time for the writing of logical address tags into the dedicated area. However, as the speed of the conversion map preparation becomes as high as 50 times, this method is a very efficient means when a startup time of a disk unit becomes a problem. In order to minimize the time required to write data in the dedicated tag area, the dedicated area is set around a target area as a center, as illustrated in FIG. 13, so as to shorten the seek time of each disk drive. Further, the write to the disk unit  2  is made in unit of sector (512 bytes, etc.) and logical address tags are allocated in the dedicated tag area in unit of sector so that the reading is not required when writing logical address tags. 
     Lastly, the time stamp will be explained. As time stamps are stored on the volatile memory  3  as illustrated in FIG. 1, the time stamps stored on the volatile memory  3  may be erased because of power source fault, etc. So, similarly to the conversion map, the logical address tags for all stripes are checked only when starting the system and a value next to the largest time stamp  5  is set in the time stamp  5  in the volatile memory  3 . Further, the time reduction technique described in the explanation of the conversion map preparation is applicable directly to the reproduction of time stamps. 
     Further, the value of time stamp  5  is incremented whenever the time stamp  5  is written into the disk unit and is used for judging the write sequence. As an example, a case where the time stamp  5  is constructed by a 24 bit counter will be explained. In case of the 24 bit counter, the counter turns round by 16 M times and a count value returns to zero. In general, the effective minimum value of the time stamp  5  is set as a reference, and 16 M is added to a value smaller than the reference to be used for comparison and determination. This minimum value is also obtained by investigating logical address tags of all stripes only when the system is started. 
     However, use of this technique is based on the assumption that the maximum value of time stamp does not exceed the minimum value, that is, a difference between the maximum and minimum values of time stamps is within a range that can be expressed by 24 bits. It is therefore necessary to update the time stamp value to a new value by updating all stripes before the time stamp  5  turns one turn. To achieve this, stripes in which invalid blocks have not been updated in at least an interval corresponding to a predetermined number of writes are selected to be refilled, or only the logical address tags of stripes in which NULL addresses are set as the logical addresses of invalid blocks are rewritten. The method using NULL addresses is a method of rewriting logical address tag blocks, and hence is a light-load process as compared with a refilling process. 
     Further, in the embodiment described above, only a method to judge invalid blocks by comparing logical address tags of two stripes ST 1 , ST 2  each other was explained. To check all invalid blocks it is necessary to check all combinations between two stripes. With a conversion map, however, in checking the respective logical addresses in the logical address tags, a time stamp of the conversion map indicating invalid data is compared with the time stamp of a corresponding stripe, and a block with a smaller value can be determined as an invalid block. 
     FIG. 1 illustrates the structure of RAID 0  to disperse data to a plurality of disks and the system of the present invention is also applicable to the redundant disk structure (RAID 4 ,  5 ) using the parity. The conceptual diagram of a disk storage in the RAID 5  structure consistent with the present invention is illustrated in FIG.  14 . This is the structure illustrated in FIG. 1 added with a redundant disk unit  25 , and the control unit  1 , disk unit  2  ( 21 ,  22 ,  23 ,  24 ), volatile memory  3 , non-volatile memory  4 , time stamp  5 , write buffer  6  and buffer management information  7  have the same functions as those of the embodiment illustrated in FIG.  1 . 
     The operation of the embodiment illustrated in FIG. 14 will be explained with attention paid to a difference with the embodiment illustrated in FIG.  1 . In the write process, at the stage where write data from a host equipment are accumulated in number less by 1 block than 1 stripe (K*4−1) in the write buffer  6 , the control unit  1  goes to write these data in the disk units  21 - 25 . At this time, the steps to prepare the logical address tag block as the last write block from the logical addresses of the blocks stored in the write management table  7  and the time stamp  5  on the volatile memory  3  are the same as those in the embodiment illustrated in FIG.  1 . 
     Thereafter, from data for  1  stripe added with this logical address tag block, the exclusive OR (XOR) operation is executed for every stripe unit and a parity stripe unit is prepared. Then, the stripe data with this parity are simultaneously written in the empty areas of the disk units  21 - 25  collectively. Further, the value of the time stamp  5  is incremented at the stage where the write is completed. Thus, in addition to the collective one time data writing, it is not required to read old data and old parity block for the parity calculation and therefore, the number of disk accesses can be further reduced. Similarly, the stripe refilling process is also written into the disk unit  2  after preparing a stripe with a parity. This state is illustrated in FIG.  15 . 
     In the parity RAID structure, even when one disk unit becomes faulty, the data of the faulty disk can be reproduced by computing XOR of data of another disk that is composed of a stripe and parity and the service as a disk storage can be provided successively. However, if one disk unit was out of order when starting the system, as a logical address tag is reproduced by reading data out of a disk unit storing no logical address tag and then inspected, time required for preparing a conversion map and thus, time until the system startup is completed will increase sharply. 
     So, as illustrated in FIG. 16, it is controlled so as to write the same logical address tag in two disk units by reducing data blocks composed of a stripe. As a result, even when one disk unit becomes out of order, the remaining logical address tags can be read when preparing a conversion map and a sharp increase of a time needed for the system startup can be avoided. 
     When a dedicated tag area is to be used to create a conversion map at a high speed, allocation of logical address tags to the dedicated area is controlled such that a disk drive in which logical address tags are stored in a dedicated tag area differs from each disk drive in which logical address tags are stored in stripes, as shown in FIG.  16 . With this control, it suffices if one logical address tag is stored in a stripe. 
     Further, when writing the logical address tags in the dedicated tag area, if parity data is used to take countermeasures against disk failures, two times of write and two times of read become necessary instead of one time so far needed and the disk write overhead at the time of a collective writing and the stripe refilling sharply increases. Therefore, the above countermeasures using parity data are not taken for data in the dedicated tag area. 
     The data in the dedicated tag area is used to create a conversion map at a high speed. If a disk drive having a dedicated tag area fails, logical address tags in stripes can be checked (by a random access operation) as those stored in the dedicated tag area. No problem is thus posed. In addition, since the number of logical address tags to be checked by a random access decreases to ⅕, a conversion map can be created at a high speed. The present invention is described in the context of a disk storage system having a RAID architecture. However, the present invention is not limited to this area only. That is, the present invention is principally applicable to not only such disk units as magnetic disks but also magnet-optical disks having largely different performances for sequential write and random write, and storages in RAID structure given with redundancy by parity requiring two readings and two writings in the small block updating. 
     According to the present invention described above, it is not needed to retain conversion maps in a non-volatile memory preparing for the power source failure as the conversion maps can be reproduced any time and accordingly, it is possible to construct a very cheap disk storage. Further, even if the contents of a non-volatile memory are lost due to hardware failure, according to the present invention, only latest written data retained in the write buffer are lost and almost all data on the disks are left, while in a conventional method, all data on the disks are lost as conversion maps cannot be reproduced. Accordingly, durability against failure is largely improved. Further, the recovery process from the power source failure and the normal system startup process are the same and as no special process is needed for the system termination or recovery, the development cost can be reduced. Further, as the process at the system startup can be increased to high speed by the dispersed arrangement of logical address tags to a plurality of disk units, a dedicated tag area capable of sequentially accessing logical address tags, storage area segment division management, etc., the waiting time at the system startup can be suppressed to a range where there is no problem for practical use. In particular, in the parity RAID structure, as logical address tags are recorded in two disk units, the system startup time does not increase even when one of the disk units becomes out of order.