United States Patent [19J
`Miyamoto et al.
`
`I 1111111111111111 11111 111111111111111 1111111111 1111111111111111 Ill lllll llll
`US005845061A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,845,061
`Dec. 1, 1998
`
`[54] REDUNDANT CLIENT SERVER SYSTEM
`
`[75]
`
`Inventors: Takahisa Miyamoto, Yokohama;
`Toshikazu Yasue, Chigasaki; Shuji
`Ohno, Kawasaki; Norihiro Goto,
`Ebina, all of Japan
`
`[73] Assignee: Hitachi, Ltd., Tokyo, Japan
`
`[21]
`
`Appl. No.: 548,754
`
`[22]
`
`Filed:
`
`Oct. 26, 1995
`
`[30]
`
`Foreign Application Priority Data
`
`Oct. 31, 1994
`Nov. 30, 1994
`
`[JP]
`[JP]
`
`Japan .................................... 6-267552
`Japan .................................... 6-297028
`
`[51]
`[52]
`[58]
`
`Int. Cl.6
`...................................................... G06F 11/00
`U.S. Cl. ................................. 395/182.02; 395/200.33
`Field of Search ......................... 395/182.02, 182.01,
`395/180, 181, 431, 200.02, 200.11, 600,
`182.09, 182.11, 200.33; 364/269.2, 943.91,
`265, 268, 268.9, 284.4; 371/7, 8.1; 345/335
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`5,131,089
`5,163,131
`5,343,477
`5,394,526
`5,396,613
`5,420,998
`5,452,447
`
`7/1992 Cole ........................................ 364/200
`11/1992 Row et al. .............................. 364/200
`8/1994 Yamada .............................. 395/182.02
`2/1995 Grouse et al. . ... ... ... ... ... .... ... ... 364/200
`3/1995 Hollaar ............................... 395/182.02
`5/1995 Horning .................................. 364/200
`9/1995 Nelson et al.
`.......................... 395/600
`
`5,452,448
`5,535,399
`5,537,585
`5,553,242
`5,555,404
`5,566,297
`5,566,299
`5,615,353
`
`9/1995 Sakuraba et al. ....................... 395/600
`7/1996 Blitz et al. ......................... 395/182.04
`7 /1996 Blickenstaff et al.
`... ... ... ... ... ... 395 /600
`9 /1996 Russel et al. .. ... ... .... ... ... ... ... ... 364/200
`9/1996 Torbj0rnsen et al.
`.................. 395/600
`10/1996 Devatakenda et al. ............ 395/182.13
`10/1996 Billings et al. .................... 395/182.02
`3/1997 Lautzenheiser ......................... 395/463
`
`OTHER PUBLICATIONS
`
`Draft proposed American National Standard for information
`systems SCSI.
`"Design and Mounting of UNIX4, 3BSD", pp. 191-193.
`
`Primary Examiner-Albert DeCady
`Attorney, Agent, or Firm-Antonelli, Terry, Stout & Kraus,
`LLP
`
`[57]
`
`ABSTRACT
`
`A client server system has clients, a server of execution
`system, and a server of fault auxiliary system are connected.
`A process administrator is provided in each one of the server
`of execution system and the server of fault auxiliary system.
`When a fault has occurred in a disk processor in the server
`of execution system, the process administrator in the server
`of execution system transmits only "requests" relating to the
`disk processor among "requests" received from clients to the
`server of fault auxiliary system. A process administrator in
`the server of fault auxiliary system conducts corresponding
`server processing on the "requests" transmitted from the
`server of execution system.
`
`12 Claims, 29 Drawing Sheets
`
`SEAVER OF
`EXECUTION SYSTEM
`
`.......
`
`__,.,
`SHARED
`DISK 1
`....__
`
`v-/106
`
`1~1
`J
`
`109
`(
`J
`
`AULT
`SEAVER OF F
`TEM
`AUXILIARY SYS
`
`5107
`101 -"'--" I COMMUNICATIONI
`UNIT
`
`DISK
`PROCESSOR
`
`105
`)
`
`PROCESS
`ADMINISTRATOR
`
`(
`
`108
`
`DISK
`PROCESSOR
`
`PROCESS
`ADMINISTRATOR
`<.
`I
`110
`
`V"'--- 102
`
`I
`
`CLIENT
`
`r--103
`
`~
`104
`
`NETWORK
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 1
`
`

`

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`SERVER OF
`
`DISK 1
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`
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`
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`
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`,,.
`
`FIG. 1
`
`---
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 2
`
`

`

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`
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`
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`
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`
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`
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`aurnl I
`
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`
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`
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`
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`
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`PROCESSOR
`
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`
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`
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`
`)
`(
`
`EXECUTION SYSTEM
`SERVER OF
`
`__;~•vu
`
`"-
`r
`
`FIG. 2
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 3
`
`

`

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`
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`HANDIND OVER
`
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`
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`
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`
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`
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`
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`
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`
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`
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`
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`
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`
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`
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`
`DISK 1
`SHARED
`
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`
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`
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`SERVER OF
`
`FIG. 3
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 4
`
`

`

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`
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`
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`
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`
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`
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`
`108
`
`.___ __ -,--________ __.REQUEST'---+---~-----'
`
`NOTICE
`
`I I
`
`ADMINISTRATOR
`
`PROCESS
`
`COMMUNICATION
`
`UNIT
`
`107
`
`101
`
`PROCESSOR
`
`DISK
`
`105
`
`AUXILIARY SYSTEM
`SERVER OF FAULT
`
`111
`
`109
`
`EXECUTION SYSTEM
`SERVER OF
`
`106
`
`FIG. 4
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 5
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 5 of 29
`
`5,845,061
`
`FIG. 5
`
`803
`
`804
`
`PROCESS
`ADMINISTRATOR
`
`FAULT
`MONITOR
`
`802
`
`REQUEST
`QUEUE
`
`REQUEST CONTROLLER
`
`FAULT
`RECOVERY
`UNIT
`
`STATE
`ADMINISTRATION
`TABLE
`
`INTERSERVER
`COMMUNICATION
`UNIT
`
`801
`
`805
`
`108
`
`806
`
`FIG. 6
`
`901
`
`902
`
`DISK
`PROCESSING
`ID PORTION
`
`NORMAL
`STATE
`
`806
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 6
`
`

`

`~
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`0
`....
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`
`FAULT
`
`SYSTEM
`AUXILIARY
`
`102
`7
`
`105
`
`106
`
`DISK 1
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`
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`
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`
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`
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`7
`UNIT
`
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`'
`QUEUE
`REQUEST
`
`ADMINISTRATION
`
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`
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`
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`•-----------------
`: NORMAL ST ATE
`: STATE SETTING :
`
`COMMUNICATIO
`
`UNIT
`
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`
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`
`RECOVERY
`
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`
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`
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`
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`
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`
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`
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`
`, MONITORING
`
`PROCESSOR
`
`DISK
`
`109
`
`101
`
`EXECUTION SYSTEM
`SERVER OF
`
`FIG. 7
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 7
`
`

`

`~
`
`.... = 0--,
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`
`SYSTEM
`AUXILIARY
`
`FAULT
`
`105
`
`106
`
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`SHARED
`
`STATE
`
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`
`ADMINISTRATOR
`
`PROCESS
`
`1003 f MONITORING
`
`PROCESSOR! 1 004
`
`DISK
`
`109
`
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`
`EXECUTION SYSTEM
`SERVER OF
`
`FIG. 8
`
`FAULT 11 RECOVERY
`
`UNIT
`
`MONITOR
`
`L--------------
`:NORMAL STATE
`:STATE SETTING : ___ __._____...
`I 1006
`
`1001
`
`1005
`
`108
`
`1001
`
`COMMUNICATION
`INTERSERVER
`
`UNIT
`
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`J--....__._ _..__.___ QUEUE
`..) I I ( (REQUEST'
`
`II
`
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`
`COMMUMICATIONI
`
`UNIT
`
`I
`
`'STATE ACQUISITION
`,
`
`TABLE
`
`ADMINISTRATION
`
`1002
`
`I
`
`107
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 8
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 8 of 29
`
`5,845,061
`
`FIG. 9
`
`SERVER OF
`EXECUTION SYSTEM
`
`101
`
`OCCURRENCE OF FAULT
`109
`1 004
`PROCESS
`ADMINISTRATOR
`
`FAULT
`RECOVERY
`UNIT
`
`1006
`
`SHARED
`DISK 1
`
`106
`
`1003
`
`107
`
`FAULT
`MONITOR STARTING
`: STATE SETTING :
`: OCCURRENCE
`: OF FAULT
`•-----------------
`1002
`
`105
`
`STATE
`ADMINISTRATION
`,
`TABLE
`SERVER
`OF FAULT
`'STATE ACQUISITION
`AUXILIARY
`INTERSERVER
`SYSTEM
`COMMUNICATION -- - --
`UNIT
`HANDING '----.----'
`'-------'OVER OF
`~ - - -+ - - - - - - . - - - -+ -_ PROCES(cid:173)
`102
`SING
`1 005
`108
`1001
`INFOR-
`MATION
`
`FIG. 10
`
`1501
`
`1502
`
`DISK
`PROCESSING
`ID PORTION
`
`OCCURRENCE
`OF FAULT
`
`1006
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 9
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 9 of 29
`
`5,845,061
`
`FIG. 11
`
`111
`
`SERVER OF FAULT
`AUXILIARY SYSTEM
`
`SHARED 1 - - - - - -1
`DISK 1
`
`106
`
`1306
`
`105
`
`DISK
`PROCESSOR
`I HANDING
`:OVER OF
`: PROCESSING
`: INFORMATION
`
`FAULT
`MONITOR
`
`1302
`
`REQUEST
`QUEUE >--'-'~
`
`REQUEST CONTROLLER
`
`PROCESS
`ADMINISTRATOR
`
`1304 1 303
`
`FAULT
`RECOVERY
`UNIT
`
`STATE
`ADMINISTRATION
`TABLE
`
`INTERSERBER
`COMMUNICATION
`UNIT
`
`110
`
`102
`
`1301
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 10
`
`

`

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`
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`
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`
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`
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`
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`
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`
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`1-----1 SHARED
`
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`
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`
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`
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`COMMU-
`RE-
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`
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`
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`
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`
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`
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`
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`
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`
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`
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`
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`
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`
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`
`1£_ __ . -P'ROCESSING: ADMINISTRATOR
`
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`I
`
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`
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`-------,
`
`og
`1
`
`OCCURRENCE
`
`OF FAULT
`
`101
`
`EXECUTION SYSTEM
`SERVER OF
`
`FIG. 12
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 11
`
`

`

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`
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`
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`
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`
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`
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`
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`ADM1N1sTRAT1ON
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`
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`
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`
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`
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`
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`
`EXECUTION I REQUEST
`SERVER OF~--I
`
`SYSTEM
`
`105
`
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`
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`
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`
`1 1 1
`
`102
`
`FIG. 13
`
`:MONITORING
`
`1303 ............ --
`PROCESSOR
`
`DISK
`
`ADMINISTRATOR
`PROCESS
`
`DISK 1
`SHARED---(cid:173)
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 12
`
`

`

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`
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`
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`
`NETWORK
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`
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`2305
`
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`
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`
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`
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`SERVER OF SECOND
`
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`SERVER OF FIRST
`
`.... /'--""£"VO
`
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`
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`,
`
`--
`FIG. 14
`
`DISK 1
`SHARED
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 13
`
`

`

`~
`
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`THIRD DISK SECOND DISK
`
`5
`1605
`
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`1615 AUXILIARY
`
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`
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`
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`
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`
`1616
`
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`
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`
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`
`r--....
`,---
`~ARED DISK
`
`FIRST
`
`SECOND
`
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`SHARED DISJS
`
`--==~ 1608
`
`SHARED DISK
`
`1613 ....__
`
`THIRD
`
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`
`1610
`
`)
`(
`
`ADMINISTRATOR
`
`PROCESS
`
`1601
`>
`
`/\-,-'" 1609
`
`NICATION
`COMMU-
`
`UNIT
`
`PROCESSOR
`PROCESSOR
`SECOND DISK THIRD DISK
`J
`(
`
`1612
`
`J
`(
`
`1611
`
`I
`
`PROCESSOR
`FIRST DISK
`
`/
`
`SYSTEM
`EXE CUT IOI\
`
`SERVER
`
`OF
`
`=::/'-"' IOUO
`
`r--....
`,---
`
`FIG. 15
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 14
`
`

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`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 15
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 15 of 29
`
`5,845,061
`
`FIG. 17
`1701
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`1702
`
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`STATE
`
`OCCURRENDE
`OF FAULT
`
`NORMAL
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`1705
`
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`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 16
`
`

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`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 17
`
`

`

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`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 18
`
`

`

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`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 19
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 19 of 29
`
`5,845,061
`
`FIG. 22
`PRIOR ART
`
`206
`
`DISK
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`
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`
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`204
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`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 20
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 20 of 29
`
`5,845,061
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`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 21
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 21 of 29
`
`5,845,061
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`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 22
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 22 of 29
`
`5,845,061
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`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 23
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 23 of 29
`
`5,845,061
`
`FIG. 27
`
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`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 24
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 24 of 29
`
`5,845,061
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`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 25
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 25 of 29
`
`5,845,061
`
`FIG. 29A
`700 702 703
`
`701
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`DESTINATION w 7og
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`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 26
`
`

`

`~
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`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 27
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 27 of 29
`
`5,845,061
`
`FIG. 31
`PRIOR ART
`
`11
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`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 28
`
`

`

`U.S. Patent
`
`Dec. 1, 1998
`
`Sheet 28 of 29
`
`5,845,061
`
`FIG. 32
`PRIOR ART
`10a
`
`FILE SYSTEM
`
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`
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`
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`
`53
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 29
`
`

`

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`
`¼._____~
`
`WRITING
`
`69
`
`I '
`
`I
`I
`
`65
`
`TION INFORMATION
`FILE ADMINISTRA-I _r 68
`
`I <67
`
`DATA BLOCK
`
`r5 66
`
`BLOCK LIST
`
`61
`
`CACHE ~o~~
`
`521
`
`BLOCK BUFFER
`
`FILE SYSTEM
`
`PROCESS
`
`20
`
`PRIOR ART
`
`FIG. 34
`
`CACHE FLASH ~ I BLOCK BUFFER
`
`UNWRITTEN
`
`ADMINISTRATION
`
`INFORMATION
`
`UNLOCK FILE
`
`COMPLETION
`
`WRITE
`
`UPDATE FILE ADMINISTRA(cid:173)
`
`TION INFORMATION
`
`INTO BLOCK
`WRITE DATA
`
`NEWLY ALLOCATE
`
`BLOCK
`
`64
`
`63
`
`62
`
`WRITE --.l LOCK FILE ADMINISTRA-
`
`TION INFORMATION
`
`-1
`
`REQUEST
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 30
`
`

`

`1
`REDUNDANT CLIENT SERVER SYSTEM
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`The present invention relates to a client server alternation
`control system, and in particular to a client alternation
`control system reduced in influence cauded by alternation
`control of a server conducted when a fault has occurred.
`2. Description of the Related Art
`First of all, a client server system which includes a server
`of execution system and a server of standby system and
`which is the subject of the present invention will now be
`described by referring to FIG. 22. The client server system
`shown in FIG. 22 includes a client 203, a server of execution
`system 201 and a server of standby system 202. A disk
`device 206 such as a hard disk is connected to the server of
`execution system 201. Via a network 204, the server of
`execution system 201 conducts processing for a "request"
`received from the client 203. The server of standby system
`202 is connected to the server of execution system 201 via
`an exclusive communication line 205 and conducts process(cid:173)
`ing instead of the server of execution system 201 when a
`fault has occurred in the server of execution system 201.
`Each of the client 203, the execution server 201 and the
`server of standby system 202 is an information device such
`as a personal computer or a work station. As the network
`204, the Ethernet (trademark of Xerox corporation) or the
`FDDI (Fiber Distributed Data Interface) is typically
`employed. As the exclusive communication line 205, the
`RS-232C is typically employed.
`Via the network 204, the client 203 transmits a "request"
`containing the transmission source, destination and process(cid:173)
`ing contents to the server of execution system 201. Under
`normal conditions, the server of execution system 201
`conducts processing for that "request." The subject of the
`"request" is the disk device 206. The "request" orders data
`recorded in the disk device 206 to be read/written. In the
`case where a fault has occurred in the server of execution
`system 201, the server of standby system 202 is informed of
`the fault via the exclusive communication line 205. From
`that time on, therefore, the server of standby system 202
`conducts processing instead of the server of execution
`system 201.
`FIG. 23 shows the configuration of the server of execution
`system and the server of standby system illustrated in FIG.
`22. The server of execution system 201 includes a server
`processor 301 for processing the "request" supplied from the
`client 203 and a monitor 302 for monitoring a fault of the
`server processor 301 and informing the server of standby
`system 202 of the fault. To be concrete, the server processor
`301 is software which operates on the server and mainly
`conducts processing concerning the disk device 206. Under
`normal conditions, the server processor 301 and the monitor
`302 included in the server of execution system 201 as well
`as a monitor 304 included in the server of standby system
`202 are in the execution state, whereas a server processor
`303 included in the server of standby system 202 is in the
`standby state.
`Processing conducted in the conventional server of execu(cid:173)
`tion system 201 and server of standby system 202 will now
`be described by referring to FIGS. 24 through 26.
`FIG. 24 shows processing conducted in the normal con(cid:173)
`dition. First of all, the client 203 transmits the "request" to
`the server of execution system 201. The server processor
`301 which is included in the server of execution system 201
`
`30
`
`35
`
`40
`
`45
`
`5,845,061
`
`10
`
`2
`and which has received the "request" reads/writes data
`strored in the disk device 206, as processing corresponding
`to the "request." Simultaneously at this time, the monitor
`302 in the server of execution system 201 monitors the
`5 server processor 301 to check whether any fault has
`occurred in the server processor 301.
`With reference to FIG. 25, if any fault has occurred in the
`server processor 301 included in the server of execution
`system 201, the monitor 302 in the server of execution
`system 201 transmits a notice of occurrence of the fault to
`the server of standby system 202 via the exclusive commu-
`nication line 205 between the servers. Upon receiving the
`notice of the fault, the monitor 304 in the server of standby
`system 202 starts the server processor 303 in the server of
`standby system 202. In addition, the server of execution
`15 system 201 hands over processing to the server of standby
`system 202. Contents handed over at this time include
`information of processing concerning the disk device 206
`conducted by the server processor 301 in the server of
`execution system 201 at the time of occurrence of a fault and
`20 communication address definition information set in the
`server 201 of execution system. By such handing over of the
`processing information and definition information as here(cid:173)
`tofore described, alternation processing from the server of
`execution system 201 to the server of standby system 202 is
`25 completed. The server of original standby system functions
`as a server of new execution system 404 and the server of
`original execution system 201 functions as a server of new
`standby system 401.
`With reference to FIG. 26, roles are exchanged between
`the server of execution system 201 and the server of standby
`system 202 and thereafter the client 203 transmits a
`"request" to the server 404 of new execution system which
`is the server of the ex-standby system 202. The server
`Processor 405 in the server of new execution system 404
`conducts processing correspondng to the "request." Simul-
`taneously at this time, the monitor 406 in the server of new
`execution system 404 monitors the server processor 405 to
`check whether a fault has occurred in the server processor.
`On the other hand, the server of new standby system 401
`which is the server of ex-execution system 201 conducts
`recovery processing for the faulty server processor 301 by
`starting up the server machine again and then gets into the
`standby state in preparation for a fault notice.
`The above described conventional client server system
`has two problems described below.
`(1) In the case where a fault has occurred in the server
`processor in the server of execution system, the server of
`execution system hands over processing information of the
`50 interrupted server processing and definition information
`concerning the communication processing to the server of
`standby system. The client must change the address of a
`"request" from the server of execution system to the server
`of standby system. Therefore, the client must be always
`55 conscious of a server which should process the "request."
`(2) Even in the case where a fault occurs only in an
`extremely small part of processing conducted by the server
`processor in the server of execution system, the fault actu(cid:173)
`ally occurs in the disk input/output portion in most cases.
`60 Since all processing is handed over to the server of standby
`system, the time required for the handing over, i.e., the
`interruption time of server processing for the "request" is
`prolonged. When the subject of alternation processing is a
`disk device having a large capacity, therefore, the above
`65 described handing over requires much time.
`FIG. 31 shows the configuration of a remote file system
`which is an example of the client server system. A server 10a
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1024, p. 31
`
`

`

`5,845,061
`
`10
`
`4
`update file administration information into the block buffer
`cache 21 (step 64). Thereafter, the file system 20 conducts
`unlock processing for declaring that it abandons the right of
`processing of the file administration information (step 65).
`The file system informs the process which issued the write
`request of completion of write processing. Thereafter, upon
`occurrence of a cache flash event (which indicates the timing
`reflecting, to the HDD 120, information which is written into
`the block buffer cache 21 and which is not updated), held
`data is written in a plurality of installments on the basis of
`the command block shown in FIG. 33. At the time point
`when the write completion is informed of, the data is not
`written into the HDD 120.
`Input/output processing in the UNIX is described in detail
`15 in "Design and Mounting of UNIX4.3BSD" written by S. J.
`Leffler et al., translated by Akira, Nakamura et al., and
`published by Maruzen. Data transfer on the SCSI bus is
`described in detail in ANSI SCSI Specification.
`In the conventional client server system, usefuness is
`improved by disposing, in a computer system of deccentral(cid:173)
`ized processing type having a LAN as a medium, a plurality
`of server machines capable of providing equal services.
`Since consideration as to input/output processing in the
`UNIX file system is insufficient, however, the following
`25 problems are pointed out.
`(1) The data to be written into the HDD is stored in the
`main storage for a fixed period by the block buffer cache in
`each server machine. From the time when the write is
`completed on the process until it is reflected to the HDD,
`30 therefore, data matching cannot be assured.
`(2) In writing via the SCSI bus, only consecutive data
`storage areas can be specified as the subject of update at a
`time. This is a problem specific to SCSI buses. In actual file
`35 writing processing in which a plurality of nonconsecutive
`data storage areas are updated, writing is performed inde(cid:173)
`pendently for respective data storage areas requiring update.
`If the server machine is alternated during writing processing
`of non-consecutive areas in the same file, therefore, a
`40 contradiction occurs in the data storage structure on the
`HDD.
`(3) In the case where input/output processing conducted
`in each server machine is being logged in the case of (2), the
`data storage structure on the HDD can be reconstructed.
`However, it takes a considerable time