Does Rman Backup benefit from Large Pool?

Comments

1. maclean says

   2011/05/23 at 22:45

   ORA-04031 “KSFQ Buffers” ksmlgpalloc

   Applies to:
   Oracle Server – Standard Edition – Version: 9.2.0.8 and later [Release: 9.2 and later ]
   Information in this document applies to any platform.
   Symptoms

   — Problem Statement:
   Getting ORA-4031 in the alert log file:

   ORA-04031: unable to allocate 532524 bytes of shared memory (“shared pool”,”unknown object”,”sga heap(1,0)”,”KSFQ Buffers”)

   …
   SO: 80355EA8, type: 4, owner: 802FB928, flag: INIT/-/-/0x00
   (session) trans: 00000000, creator: 802FB928, flag: (41) USR/- BSY/-/-/-/-/-
   DID: 0001-0095-00000022, short-term DID: 0000-0000-00000000
   txn branch: 00000000
   oct: 0, prv: 0, sql: 8F21F578, psql: 00000000, user: 0/SYS
   O/S info: user: SYSTEM, term: GEHUS5031, ospid: 5296:1468, machine: EC\GEHUS5031
   program: rman.exe
   client info: rman channel=ch1
   application name: backup archivelog: ch1, hash value=3567860541
   action name: 0000106 STARTED, hash value=3757870881
   last wait for ‘latch free’ blocking sess=0x0 seq=1234 wait_time=332
   address=8b6ca86c, number=9d, tries=0
   temporary object counter: 0
   …No current library cache object being loaded
   …No instantiation object
   …

   Analysis of callstack
   ——————————-
   Summary Stack (to Full stack) (to Function List)
   ksm_4031_dump
   1089
   ksmasg Callback function for allocating an SGA extent. The SGA is fixed size
   kghnospc KGH: There is no space available in the heap, signal an error.
   kghalo KGH: main allocation entry point
   ksmlgpalloc ksmlgpalloc – KSM LarGe Pool ALLOCation
   ksfqbalo
   ksfqbcre BUFFER MANAGER FUNCS */
   ksfqxc
   ksfqxcrx
   ksfqxcre ksfqxcre – CREate KSFQ context
   krbbpc Piece Create. This is the implementation of
   krbibpc Backup Piece Creation */
   pevm_icd_call_comm
   pfrrun PSDEVN: PL/SQL Interpreter Main Instruction Loop
   pricar ——————————– pricar ——————————– */
   pricbr PL/SQL Remote Interface – Call Back, Remote
   prient2 prient2 – common code to run given entrypoint: could be RPC or non-RPC
   prient prient – Run given (RPC) entrypoint
   kkxrpc ARGSUSED */
   kporpc kporpc.c – The oracle side function to start rpc
   opiodr OPIODR: ORACLE code request driver – route the current request
   ttcpip TTCPIP: Two Task Common PIPe read/write
   opitsk opitsk – Two Task Oracle Side Function Dispatcher
   opiino opiino – ORACLE Program Interface INitialize Opi
   opiodr OPIODR: ORACLE code request driver – route the current request
   opidrv # opidrv – ORACLE Program Interface DRiVer (IGNORE)
   sou2o # Main Oracle executable entry point
   opimai
   OracleThreadStart@
   77E64826

   Cause
   Large Pool was not configured.

   RMAN backups need Large Pool to complete the required operations.

   Solution
   — To implement the solution, please execute the following steps::
   SQL> ALTER SYSTEM SET LARGE_POOL_SIZE = 104857600 SCOPE=BOTH; — 100 MB

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2. maclean says

   2011/05/23 at 23:18

   RMAN Restore Performance from Tape is Very Poor

   Applies to:
   Oracle Server – Enterprise Edition – Version: 10.2.0.4 to 11.1.0.7
   This problem can occur on any platform.
   Symptoms
   RMAN restore from sbt backup is very slow compared to backup speed.
   Backup rate is 60-90Mb/sec.
   Restore rate is 6Mb/sec.
   Backup is hardware multiplexed but media manager does support multiplexed backups.
   Restore validate runtime is VERY fast so problem is the write back to disk.

   Cause

   RMAN is using a 128Kb buffer size for the datafiles when restoring back to disk.
   The input buffer size used during backup is 1Mb.

   To confirm that you are hitting the same problem, query v$backup_async IO and check buffer_size for the backup eg
   SQL>select device_type, type, buffer_size from v$backup_async_io where
   open_time > to_date(’30-jun-09 15:14:00′, ‘dd-mon-rr hh24:mi:ss’) and
   close_time select device_type, type, buffer_size from v$backup_async_io where
   open_time > to_date(’30-jun-09 14:50:00′, ‘dd-mon-rr hh24:mi:ss’) and
   close_time alter system set “_BACKUP_KSFQ_BUFSZ”=1048576;

   Users restoring to ASM may want to set this to the ASM stripe size.

   Problem can affect any release prior to 11.2.

   Please note:

   Setting _BACKUP_KSFQ_BUFSZ=1048576 will benefit restore performance overall where you can see that a buffer size of 128Kb is being used. However, if restore validate is also taking a long time then setting this hidden parameter alone may not be enough, particularly if you have hardware multiplexed the original backup (allocated > 1 channel per physical tape device). If this is the case then refer to Note 740911.1: RMAN Restore Performance.

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3. maclean says

   2011/05/23 at 23:25

   Hdr: 12422903 10.2.0.5.0 RDBMS 10.2.0.5.0 RMAN PRODID-5 PORTID-226
   Abstract: RMAN BACKUP FAILING WITH ORA-600 [1862]

   PROBLEM:
   ——–
   when setting if _backup_ksfq_bufsz > 1048576 then rman backup fails with:

   RMAN-8011: including current control file in backupset
   RMAN-8038: channel ORA_SBT_TAPE_1: starting piece 1 at 03-05-2011 17:54:59
   RMAN-571: ===========================================================
   RMAN-569: =============== ERROR MESSAGE STACK FOLLOWS ===============
   RMAN-571: ===========================================================
   RMAN-3009: failure of backup command on ORA_SBT_TAPE_1 channel at 05/03/2011
   17:55:02
   ORA-600: internal error code, arguments: [1862], [8388608], [], [], [], [],
   [], []

   DIAGNOSTIC ANALYSIS:
   ——————–
   /home/oracle/admin/PCAP/udump/pcap1_ora_22266.trc
   Oracle Database 10g Enterprise Edition Release 10.2.0.5.0 – 64bit Production
   With the Partitioning, Real Application Clusters, Oracle Label Security,
   OLAP,
   Data Mining Scoring Engine and Real Application Testing options
   ORACLE_HOME = /u01/app/oracle/10.2.0
   System name: Linux
   Node name: fr-srv-orahm01
   Release: 2.6.18-194.el5
   Version: #1 SMP Tue Mar 16 21:52:39 EDT 2010
   Machine: x86_64
   Instance name: PCAP1
   Redo thread mounted by this instance: 1
   Oracle process number: 439
   Unix process pid: 22266, image: oracle@fr-srv-orahm01 (TNS V1-V3)

   *** NAME:(0000018 STARTED16) 2011-05-03 14:24:43.976
   *** NAME:(backup full datafile) 2011-05-03 14:24:43.976
   *** NAME:(SYS$USERS) 2011-05-03 14:24:43.976
   *** ID:(6242.2499) 2011-05-03 14:24:43.976
   *** 14:24:43.976
   ksedmp: internal or fatal error
   ORA-600: internal error code, arguments: [1862], [67108864], [], [], [],
   [], [], []
   —– Call Stack Trace —–
   ssd_unwind_bp ksedst ksedmp ksfdmp kgeriv kgesiv ksesic1 ksfqxc
   ksfqxcrx ksfqxcre krbbcThread krbbpc krbibpc pevm_icd_call_commo
   pfrinstr_ICAL pfrrun_no_tool pfrrun plsql_run pricar pricbr prient2
   prient kkxrpc kporpc opiodr ttcpip opitsk opiino opiodr opidrv sou2o
   opimai_real main libc_start_main start

   user: 0/SYS
   service name: SYS$USERS
   O/S info: user: oracle, term: pts/1, ospid: 21905, machine: fr-srv-orahm01
   program: rman@fr-srv-orahm01 (TNS V1-V3)
   client info: rman channel=ORA_SBT_TAPE_1
   application name: backup full datafile, hash value=2287030075
   action name: 0000018 STARTED16, hash value=109080347

   WORKAROUND:
   ———–
   Dont set _backup_ksfq_bufsz > 1048576

   STACK TRACE:
   ————
   ssd_unwind_bp ksedst ksedmp ksfdmp kgeriv kgesiv ksesic1 ksfqxc
   ksfqxcrx ksfqxcre krbbcThread krbbpc krbibpc pevm_icd_call_commo
   pfrinstr_ICAL pfrrun_no_tool pfrrun plsql_run pricar pricbr prient2
   prient kkxrpc kporpc opiodr ttcpip opitsk opiino opiodr opidrv sou2o
   opimai_real main libc_start_main start

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4. maclean says

   2011/05/23 at 23:27

   ORA-4030 “Out Of Process Memory Error” During RMAN Restore

   Applies To
   Oracle Server – Enterprise Edition – Version: 8.1.7.4
   This problem can occur on any platform.
   Symptoms
   During RMAN restore following error occurs

   run
   {
   allocate channel c1 type disk;
   restore database ;
   }

   RMAN-03002: failure during compilation of command
   RMAN-03013: command type: restore
   RMAN-03006: non-retryable error occurred during execution of command: IRESTORE
   RMAN-07004: unhandled exception during command execution on channel c1
   RMAN-10035: exception raised in RPC: ORA-04030: out of process memory when
   trying to allocate
   1048632 bytes (pga heap,KSFQ Buffers)
   RMAN-10031: ORA-19583 occurred during call to
   DBMS_BACKUP_RESTORE.RESTOREBACKUPPIECE
   Recovery Manager complete.
   Cause
   The RMAN failed to restore because of ORA-4030 “out of process memory”
   Fix
   Increase data(kbytes), stack(kbytes), memory(kbytes) parameter to unlimited in ulimit -a

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5. maclean says

   2011/05/23 at 23:28

   Why ORA-04031 on Shared Pool during RMAN back up even when Large Pool is set?

   Applies to:
   Oracle Server – Enterprise Edition – Version: 10.2.0.1 and later [Release: 10.2 and later ]
   Information in this document applies to any platform.
   Goal
   Following error occurs while performing RMAN back up:

   RMAN-03009: failure of backup command on ch0 channel at 07/23/2010 10:39:21
   ORA-04031: unable to allocate 7836 bytes of shared memory (“shared pool”,”unknown object”,”sga heap(1,0)”,”ksfqpar”)

   The LARGE_POOL_SIZE was set.

   But the Large Pool was not used and ORA-4031 error occurred.

   Solution
   The LARGE POOL is only utilized when following are set:

   1. DBWR_IO_SLAVES and/or BACKUP_TAPE_IO_SLAVES to a non-zero value
   2. Set DISK_ASYNCH_IO = FALSE

   Otherwise shared pool is used. You can test this as below:

   When DBWR_IO_SLAVES is set to zero and DISK_ASYNCH_IO is set to true then Large pool is not used:

   RMAN> allocate channel for maintenance type disk;

   SQL> select * from v$sgastat where name like ‘%KSFQ

   POOL NAME BYTES
   ———— ————————– ———-
   shared pool X$KSFQP ANCHOR 52
   shared pool KSFQ buffer pool 2376

   SQL> select * from v$sgastat where pool=’large pool’;

   POOL NAME BYTES
   ———— ————————– ———-
   large pool PX msg pool 262144
   large pool free memory 33292288

   When DBWR_IO_SLAVES is set to non-zero and DISK_ASYNCH_IO is set to false:

   RMAN> allocate channel for maintenance type disk;

   SQL> select * from v$sgastat where pool=’large pool’;

   POOL NAME BYTES
   ———— ————————– ———-
   large pool PX msg pool 262144
   large pool free memory 29081600
   large pool KSFQ Buffers 4210688

   SQL> select * from v$sgastat where name like ‘%KSFQ%’;

   POOL NAME BYTES
   ———— ————————– ———-
   shared pool X$KSFQP ANCHOR 52
   shared pool KSFQ buffer pool 2376
   shared pool KSFQ Buffer Structures 208
   large pool KSFQ Buffers 4210688

   Now, we see the large pool is in use.

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6. maclean says

   2011/05/23 at 23:32

   Bug 10635701 – RMAN backup many files consumes lots of PGA / fails with ORA-4030

   RMAN backups of large numbers of files can fail with ORA-4030
   due to excessive memory consumption.

   Rediscovery Notes:
   RMAN backup fails with ORA-4030.
   RMAN tracing shows that the error occurred in function
   dbms_backup_restore.validationvalidate
   with a trace line such as:
   krmxrpc – channel t1 kpurpc2 err=4030 db=target proc=SYS.DBMS_BACKUP_RESTORE.VALIDATIONVALIDATE

   Hdr: 10635701 11.2.0.2 RDBMS 11.2.0.2 RMAN PRODID-5 PORTID-226
   Abstract: BACKUP OF FRA CONSUMES 15GB OF PGA AND FAIL WITH ORA-4030

   PROBLEM:
   ——–
   Exadata Quarter rack: 3 storage cells.
   using 11.2.0.2 on RDBMS.
   using Veritas NetBackup for Oracle – Release 7.0

   The backup of flash recovery area allocates up to 15gb of PGA and it fails
   with error ORA-4030

   The script used to run the backup:

   connect target ‘/’

   run {
   allocate channel t1 type ‘SBT_TAPE’;
   backup recovery area backupset all delete input filesperset=4;
   release channel t1;
   }

   DIAGNOSTIC ANALYSIS:
   ——————–
   DIAGNOSTIC
   ———-

   The incident trace file BGC0TT1_ora_21853_i1344592.trc has this:

   Dump continued from file:
   /u01/app/oracle/admin/diag/rdbms/bgc0tta/BGC0TT1/trace/BGC0TT1_ora_21853.trc
   ORA-4030: out of process memory when trying to allocate 1052696 bytes (KSFQ
   heap,KSFQ Buffers)

   ========= Dump for incident 1344592 (ORA-4030) ========
   —– Beginning of Customized Incident Dump(s) —–
   =======================================
   TOP 10 MEMORY USES FOR THIS PROCESS
   —————————————
   94% 14 GB, 14727 chunks: “KSFQ Buffers ”
   KSFQ heap ds=0x2af17258bd70 dsprt=0xb7d3bc0

   There is a massive allocation of memory with comments KSFQ Buffers from the
   KSFQ heap.

   The callstack:
   kghalf <- ksfqbalo <- ksfqbcre <- ksfqxc <- ksfqxcrx <- ksfqxcre <- krbrvv

   The limitation of FILESPERSET does not change the results, still error
   ORA-4030.

   We have asked the customer to run the backup and get output from
   v$backup_async_io, to identify the number of buffers and size of buffers
   allocated during the backup.

   Call Stack of the failing process:
   kghalf <- ksfqbalo <- ksfqbcre <- ksfqxc <- ksfqxcrx
   <- ksfqxcre <- krbrvv

   The heap where memory is allocated KSFQ heap with chunks
   with comments "KSFQ Buffers". That information is dumped
   into the trace file generated by error ORA-4030

   Workaround:
   None

   RELEASE NOTES:
   RMAN backups of large numbers of files can fail with ORA-4030
   due to excessive memory consumption.
   REDISCOVERY INFORMATION:
   RMAN backup fails with ORA-4030. RMAN tracing shows that the error occurred
   in function dbms_backup_restore.validationvalidate, with a trace line such
   as:

   krmxrpc – channel t1 kpurpc2 err=4030 db=target proc=SYS.DBMS_BACKUP_RESTORE.VA
   LIDATIONVALIDATE
   WORKAROUND:
   None

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7. maclean says

   2011/05/23 at 23:47

   Hdr: 9523300 10.2.0.4 USERDOC 10.2.0.4 PRODID-5 PORTID-212
   Abstract: TAPE_ASYNCH_IO AND BACKUP_TAPE_IO_SLAVES CLARIFICATION

   PROBLEM:
   ——–
   From the online documentation :

   Oracle® Database Administrator’s Reference
   10g Release 2 (10.2) for UNIX-Based Operating Systems
   Part Number B15658-06

   href660

   ” A.2.4 I/O Slaves
   …..

   There are times when you must turn off asynchronous I/O. For example, if
   instructed to do so by Oracle Support for debugging. You can use the
   DISK_ASYNCH_IO and TAPE_ASYNCH_IO parameters to switch off asynchronous I/O
   for disk or tape devices. ”

   Customers find this whole section confusing as it does not distinguish
   between disk and tape io slaves and gives the impression that tape async io
   is enabled by default (as tape_asynch_io defaults to true) and that
   backup_tape_io_slaves therefore need never be set.

   As I understand it: tape async io can never be asynchronous unless both
   tape_asynch_io and backup_tape_io_slaves are BOTH set to true and the default
   settings for these parameters means tape io will always by default be
   synchronous.
   Asynchronous IO to tape can only be simulated via the use of tape IO slaves
   and setting tape_asynch_io=true alone is not enough.

   This sentence also needs clarifying:

   “Set the DBWR_IO_SLAVES parameter to greater than 0 only if the
   DISK_ASYNCH_IO or TAPE_ASYNCH_IO parameter is set to false”

   This again does not distinguish between disk and tape io slaves and should
   be:

   Set DBWR_IO_SLAVES parameter to greater than 0 only if DISK_ASYNCH_IO=FALSE.
   For tape io slaves, set BACKUP_TAPE_IO_SLAVES=TRUE and leave
   TAPE_ASYNCH_IO=TRUE (default).

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8. maclean says

   2011/05/23 at 23:51
   • What is the purpose of I/O slaves?

   I/O Slaves are specialized processes whose only function is to perform I/O . In Oracle7, all File I/O OSDs are blocking. The only exception is the OSD used by the direct loader. In this case, the direct loader asks for asynchronous I/O. In case the platform does not support asynchronous I/O, the direct loader has to revert to synchronous I/O.

   In Oracle8, the file I/O interface was reworked to provide non-blocking I/O calls for its clients. I/O slaves can be used to provide non-blocking I/O requests even if the platform does not support asynchronous I/O. Many platforms that do support asynchronous I/O for disk devices, may not do so for tape devices. Even in this case, I/O slaves can be used to do non-blocking I/O to tape devices. Under heavy loads, the Database Writer may need additional processes to do some of the I/O. In Oracle7, DBWR would use multiple processes to do I/Os on its behalf. In Oracle8, DBWR would use I/O slaves instead. I/O slaves can be deployed by the DBWR, LGWR, ARCH or the processes doing Backup.

   • Why the change from Oracle7?

   In Oracle7, “I/O slaves” were implemented at the OSD level specifically for DBWR in the form of multiple db_writers. The need existed to make the feature more general. This prompted the design of Oracle8 I/O slaves, which can be used theoretically by any server process. However, access to I/O slaves is restricted to DBWR, LGWR, ARCH and processes performing Backups, since it is not clear if any other process can take advantage of the I/O slaves.

   • What are initialization parameters that control I/O slave deployment?

   In Oracle8, the following parameters are significant:

   Parameter	Possible Values	Default
   dbwr_io_slaves	0..>100	0
   lgwr_io_slaves	0..>100	0
   arch_io_slaves	0..>100	0
   backup_disk_io_slaves	0..>100	0
   backup_tape_io_slaves	TRUE/FALSE	FALSE
   _io_slaves_disabled	TRUE/FALSE	FALSE
   disk_asynch_io	TRUE/FALSE	TRUE
   tape_asynch_io	TRUE/FALSE	TRUE

   • What is the purpose of the parameters disk_asynch_io and tape_asynch_io?

   It may be necessary to turn off the OS provided asynchronous I/O facility. For example, if the platform’s asynchronous I/O code has bugs or is not efficient. Asynchronous I/O can be disabled on a “per device type” basis. These parameters are now generic parameters, and replace all platform-specific parameters from Oracle7, such as use_async_io, async_read, and async_write.

   • Are slaves allocated dynamically as needed, or at instance startup?

   I/O slaves for DBWR are allocated immediately following database open when the first I/O request is made. For example, if the parameter dbwr_io_slaves=4, four I/O slaves will be spawned for the DBWR. Other I/O slaves (for LGWR, ARCH, and backup processes) are dynamically allocated on an as needed basis. This deployment method may change in the future, such that I/O slaves may be deployed upon Instance startup.

   • If dynamic, once allocated, do they ever exit?

   I/O slaves for DBWR and LGWR do not exit. Inactive I/O slaves for ARCH and backup processes exit.

   • How is a slave determined to be “inactive”? Is there a timeout value?

   An I/O slave determines that it is inactive if it has been idle for 60 seconds.

   • Are they sent a signal from another process, or do they exit on their own?

   If an I/O slave finds that it has reached the threshold of inactive period, then it exits on its own by signalling OER(448).

   • What is the max number of slaves?
   • Whose responsibility is it to spawn a slave?

   Whoever is issuing the I/O. For example, DBWR, or a foreground process.

   • How does the file I/O subsystem “recognize” the need for a slave?

   The I/O issuing process looks for an idle I/O slave. If one is available, that I/O slave will get a post. If there are no idle slaves, then the I/O issuer will spawn one. If the allowed number of slaves have been spawned, then the issuer waits and tries again to find an idle slave.

   • Given a DBWR slave, does the DBWR process do any work, or just the slave(s)?

   DBWR continues to do all the DBWR related work. Further, the DBWR process initiates the I/O. The DBWR’s I/O slave simply does the I/O on DBWR’s behalf.

   • What is the slave process naming convention?

   The slave process name is ora_iNnn_<SID> where:

   iNnn	Description	Possible Values
   i	Denotes this is an I/O slave	“i”
   N	I/O adaptor number	1..F
   nn	Slave number	01..0z, 10..zz

   Once slave number 0z is spawned, the next slaves are 10, 11, 12, … As an example, if dbwr_io_slaves=15, the following process will be started:

   ora_i101_SID ora_i102_SID ora_i103_SID ora_i104_SID
   ora_i105_SID ora_i106_SID ora_i107_SID ora_i108_SID
   ora_i109_SID ora_i10a_SID ora_i10b_SID ora_i10c_SID
   ora_i10d_SID ora_i10e_SID ora_i10f_SID

   • Will a process like DBWR always use the same adaptor number? Or rephrased, given a process ora_i031_<SID>, what process is this slave doing I/O for?

   A process may not necessarily always use the same adaptor number.

   • What mechanism do the slaves use to communicate with the master?

   If a process wants to use I/O slaves to do its I/O, then the I/O buffers must be in shared memory.

   • Is there a need for I/O slaves if async I/O is available?

   The short answer is no. However, if the system is very large, and the DBWR is falling behind, then one could consider using the I/O slaves.

   Note, I/O slaves can do asynchronous I/O. In other words, the deployment of I/O slaves is orthogonal to availability of asynchronous I/Os in the system.

   • What concerns are there running I/O slaves even if async I/O is available?

   • You end up using shared memory, when you could have used process private memory
   • Communication overheads
   • Processing overheads (plus context switches)

   • Will each slave use async I/O individually?

   Yes

   • If a platform supports async I/O to raw devices only, will I/O to raw be done async, and I/O to the filesystem be done sync?

   IO slaves use the same algorithm as direct IO adaptor. If the platform supports asynchronous IO to the target device (or file) then IO will be done asynchronously.

   • Can I/O slaves be used with List I/O?

   Yes.

   • What part of the SGA is used for backup/restore slaves?

   The variable part of the SGA.

   • I’m most interested in the “separate ‘pool’ of SGA memory”. How large will this pool be and how will it affect the size of the SGA?

   Backup process will use PGA memory if it is not using any IO slaves. However, the backup process will use SGA, if it were to use IO slaves. In either case, the size of the memory used is proportional to:

   • number of files that are opened at any time
   • number of files being backed up

   • Does it depend on the number of possible I/O slaves?

   No.

   • Is this separate pool allocated when the slave(s) is allocated, or at instance startup?

   The pool is allocated when the backup process is ready to do IO.

   • Are there any concerns in using I/O slaves if using MTS?

   No.

   • What’s the recommended settings for <adaptor>_io_slaves?

   The general recommendation is: set the value to 2, if the I/O activity is expected to be moderate. Set it to 4, if you expect a lot of I/O activity. Simulating asynchronous I/O behavior is not free. So, deployment of I/O slaves should be done carefully. I cannot stress this point enough.

   • How can the performance of I/O slaves be monitored?

   Use v$sesstat, looking at physical writes, physical reads, logical reads, write requests. Use v$sess_io and v$filestat to correlate these numbers.

   • How do backup_io_slaves work with multiple channels?

   Each channel will get its own set of slaves, because each channel has its own Oracle session. Within each channel, there may be separate ksfq contexts allocated for reading and writing, both of which will share the same ksfh context and the same set of I/O slaves.

   • How are they used to improve backup performance?

   If the platform does not provide async I/O, then slaves are used to simulate async I/O. Rather than blocking on each file read, backup can process the buffers for one file while the slaves are filling the buffers for other files. If there are enough files and enough slaves, then we should be able to keep a tape streaming. If we have neither slaves nor async I/O, then there is no performance benefit to multi-plexing datafiles.

   • Are they used for restore and application of incrementals?

   Yes, for both restore and application. Both reading to tape and writing to disk will share the same set of I/O slaves.

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