HP OpenVMS System Manager's Manual, Volume 1:...

Using BACKUP

Understanding BACKUP File Formats

Using Disks and Tapes

Setting Software Parameters for Efficient Backups  

Most save operations are limited by the time required to open a file and read its extents into memory. Hardware or software caches cannot help to improve disk performance because the data is read only once. Therefore, how files are laid out on disk is important.

File Sizes and Extents

Input file processing in BACKUP results in a minimum of two 1-block read I/Os to open the file and to read the file attributes. This overhead is the same for small as for large files. Therefore, saving the same amount of data from large files can be as much as three times more efficient than saving data from small files.

In issuing one I/O for each file extent or file fragment on disk, BACKUP must break down I/Os for larger extents to fit the internal buffer size that the /BLOCKSIZE parameter specifies. The maximum I/O that BACKUP can issue is 127 blocks, or 63.5K. Files with just one extent (contiguous file) can be saved most efficiently. The more extents per file, the longer it takes to save the file.

BACKUP reads files from disk in alphabetic order by directory path and file name. You can obtain the best performance by placing files and all their extents on disk in alphabetic order and make the files contiguous. You can accomplish this by using BACKUP/IMAGE when you do an image save-and-restore. Most defragmentation utilities do not arrange files in alphabetic order, and, in some cases, these utilities decrease the performance gains that result from consolidating files into larger extents.

The Effect of File Size on BACKUP Performance

File size can have a great effect on BACKUP performance when creating save sets, copies, and image backups, but file size has not effect on physical backups. The graph in XXXXX illustrates that, under certain circumstances, a set of small files can take up to 40 times longer to back up than a set of large files containing the same amount of data. This is due to file system overhead. The four different curves in the figure show that the effect varies with the type of BACKUP and the components involved.

Figure 1  Effect of File Size on BACKUP Performance

File Layout: Fragmentation

Fragfmentation can also slow down BACKUP considerably. On HSJ50s, fragmented files can take over twice as long to back up as non-fragmented files. The effect on performance of EVAs and other storage arrays has not been quantified but should, nevertheless, be taken into consideration when you look at performance that fails to meet expectations.

CPU Consumption

With advances in disk and tape drives, backups can run fast enough to consume substantial CPU, especially when creating save sets. This load can be considerable if multiple backups are run simultaneously on the same machine. Under certain circumstances, tests with 8 parallel backup processes can saturate a 4-CPU machine.

Software Tuning

You can gain some performance improvements with the current design of OpenVMS BACKUP by adjusting system and process parameters. This is not, however, a simple, straightforward task, and you can expect a performance improvement of, typically, less than 15%. Changing system and process parameters can also result in worse performance. Disks with different file fragmentation and file sizes might require different process quotas to save these files efficiently.

Software tuning parameters are only for save operations; they have no impact on restore performance.

Qualifiers That Can Affect Performance

The qualifiers listed in BACKUP Qualifiers That Affect Performance can influence BACKUP performance.

Table 4   BACKUP Qualifiers That Affect Performance

Qualifier	Description
/BLOCKSIZE	To write a save set to a tape device, always use /BLOCKSIZE = 65,024, the largest block size you can use with save sets on tape. Note: To be able to copy save sets from tape to disk, use a maximum block size of 32,768.
/GROUP	Today's tape and disk drive technology makes the XOR group feature of BACKUP obsolete. Use /GROUP=0. (If you do not specify the /GROUP qualifier, the default of 10 is used, which adds 10% more data to a save set).
/CRC	Keep the default of /CRC, which does not add extra data to the save set. A 32-bit field is always reserved in a save set whether you enable CRC or not. However, adding a small amount of CPU time helps to ensure the integrity of a backup save set.
/FAST	This qualifier does not imply that your backup will go faster! /FAST forces BACKUP to read [000000]INDEXF.SYS to build a decision table to speed up the file selection process. If you select only a few files, reading all of INDEXF.SYS results in unnecessary disk I/Os; in this case, do not use /FAST. Time a save operation with and without using /FAST. The /IMAGE qualifier implicitly includes /FAST.

Disk Settings That Affect BACKUP Performance

The disk settings in Disk Settings That Affect BACKUP Performance can influence BACKUP performance when writing a save set to disk.

Table 5   Disk Settings That Affect BACKUP Performance

Disk Setting	Description
SET RMS/BUFFER=127/ EXTEND=5000	This setting allows BACKUP to use more larger buffers when writing save set blocks to disk. The larger extend value reduces the impact of extending the save set file during a save operation. Enter this command before the BACKUP command line.
SET VOLUME/ NOHIGHWATER_MARKING	Disabling highwater marking reduces the overhead of extending a save set file. Enter this command once for each volume.

Using WSQUOTA

WSQUOTA is the most sensitive parameter to influence the save performance of BACKUP. While tuning,, set a high value for WSQUOTA (set it to WSMAX) for the account running the backups. To change WSQUOTA, use the DCL command SET WORKING_SET command before the BACKUP command line. This is a more convenient way to change WSQUOTA than to change the quota using the AUTHORIZE utility.

WSQUOTA and /BLOCKSIZE, together, create the in-memory buffers at the start of a save operation. Refer to the output of /LIST for the actual number of buffers used. Although you might assume that more buffers are better, keep in mind that BACKUP scans the input disk for files to be saved and maps these files to the available buffer space. The more buffers you have, the longer this operation takes. At the same time, the output tape drive or disk drive is idle. The fewer buffers created (by using smaller WSQUOTA values) tends to result in better overlap of input and output I/Os and, hence, better performance, especially when the save set is written to a tape device.

Recommended Process Quotas

Process Quotas Recommended for Efficient Backups indicates how to set process quotas for efficient backups.

Table 6    Process Quotas Recommended for Efficient Backups

Process Quotas	Tuning Impact	Pooled Quota?	Recommended Setting
WSQUOTA	High	No	Initial value of 32,768 pagelets Vary in increments of 5,000 Values over 100,000 typically result in worst performance Set PQL_MWSQUOTA to less than or equal to WSQUOTA
FILLM	Low	Yes	Equal to 128 (usually sufficient) Less than CHANNELCNT - 20 Vary in increments of 10 Use larger values if input disk contains smaller files or is highly fragmented Effectiveness limited by WSQUOTA Set PQL_MFILLM to less than or equal to FILLM
DIOLM	Low	No	Equal to 100 (usually sufficient) Vary in increments of 10 Larger values can cause disk I/O subsystem hangs or resets without a performance advantage Set PQL_MDIOLM to less than or equal to DIOLM
WSEXTENT	None	No	Equal to WSMAX
PGFLQUOTA	None	Yes	Equal to or greater than WSQUOTA + 25,000
ASTLM	None	No	Equal to or greater than DIOLM + 100
BIOLM	None	No	Equal to or greater than FILLM + 100
BYTLM	None	Yes	Equal to or greater than 256 * FILLM + 6 * DIOLM + 10,000
ENQLM	None	Yes	Equal to or greater than FILLM + 100

How to Perform This Task

To set process quotas for efficient backups, perform the following actions:

1. Use the Authorize utility (AUTHORIZE) to determine the current quota values for the account you will use for backups. For example, if you are using the SYSTEM account for backups, enter the following commands:

   $ SET DEFAULT SYS$SYSTEM
   $ RUN AUTHORIZE
   UAF> SHOW SYSTEM

2. Using the System Management utility (SYSMAN), determine the value of the system parameter WSMAX, as follows:

   $ RUN SYS$SYSTEM:SYSMAN
   SYSMAN> PARAMETERS SHOW WSMAX

   %SYSMAN-I-USEACTNOD, a USE ACTIVE has been defaulted on node DIEM
   Node DIEM:   Parameters in use: ACTIVE
   Parameter Name          Current   Default   Minimum   Maximum Unit  Dynamic
   --------------          -------   -------   -------   ------- ----  -------
   WSMAX                    100000      4096      1024 134217728 Pagelets 
    
   SYSMAN> EXIT
   $ 

   In this case, the value for WSMAX, as shown in the column marked Current, is 100000. Use this value to help set the correct values for the process quotas.
3. Use AUTHORIZE to compare the values for the process quotas to the recommended values for efficient backups, as shown in Process Quotas Recommended for Efficient Backups.
4. If necessary, change process quotas using the AUTHORIZE command MODIFY. If you change process quotas, you must log out and log in again for the changes to take effect. For information about changing these process quotas, see Modifying a User Account.
   
   Sample Process Quotas for Efficient Backups lists a set of process quota values that are appropriate for many configurations. If your disks are highly fragmented or if your backups will be performed during periods of heavy system use, you should reduce the values shown for WSQUOTA and FILLM.

   Table 7   Sample Process Quotas for Efficient Backups

   Process Quota	Suggested Value
   WSQUOTA	32768
   FILLM	128
   DIOLM	100
   WSEXTENT	50000
   PGFLQUOTA	100000
   ASTLM	1000
   BIOLM	1000
   BYTLM	100000
   ENQLM	1000

   
   

Example

The following steps show the commands that you would use to run the Authorize utility and set process quotas for the SYSTEM account (if you plan to run backups from a different account, determine the process quotas for that account):

1. Determine the current quota values:

   $ SET DEFAULT SYS$SYSTEM
   $ RUN AUTHORIZE
   UAF> SHOW SYSTEM
   
   Username: SYSTEM                           Owner:  SYSTEM MANAGER
   Account:  SYSTEM                           UIC:    [1,4] ([SYSTEM])
   CLI:      DCL                              Tables: DCLTABLES
   Default:  SYS$SYSROOT:[SYSMGR]
                                    .
                                    .
                                    .
   Maxjobs:         0  Fillm:        40  Bytlm:        32768
   Maxacctjobs:     0  Shrfillm:      0  Pbytlm:           0
   Maxdetach:       0  BIOlm:        18  JTquota:       1024
   Prclm:          10  DIOlm:        18  WSdef:          256
   Prio:            4  ASTlm:        24  WSquo:          512
   Queprio:         0  TQElm:        20  WSextent:      2048
   CPU:        (none)  Enqlm:       200  Pgflquo:      20480
                                    .
                                    .
                                    .
   UAF> EXIT
   
   %UAF-I-NOMODS, no modifications made to system authorization file
   %UAF-I-NAFNOMODS, no modifications made to network authorization file
   %UAF-I-RDBNOMODS, no modifications made to rights database 
   $

   In this example, SYSTEM has the following quotas:

   WSQUOTA	512
   WSEXTENT	2048
   PGFLQUOTA	20480
   FILLM	40
   DIOLM	18
   ASTLM	24
   BIOLM	18
   BYTLM	32768
   ENQLM	200

   
   

2. Using the System Management utility (SYSMAN), determine the value of the system parameter WSMAX, as follows:

   $ RUN SYS$SYSTEM:SYSMAN
   SYSMAN> PARAMETERS SHOW WSMAX
   %SYSMAN-I-USEACTNOD, a USE ACTIVE has been defaulted on node DIEM
   Node DIEM:   Parameters in use: ACTIVE
   Parameter Name          Current   Default   Minimum   Maximum Unit  Dynamic
   --------------          -------   -------   -------   ------- ----  -------
   WSMAX                    100000      4096      1024 134217728 Pagelets 
    
   SYSMAN> EXIT
   $ 

   In this case, the value for WSMAX, as shown in the column marked Current, is 100000.
3. Compare the values for SYSTEM to the values in Process Quotas Recommended for Efficient Backups, and set the appropriate values:

   $ SET DEFAULT SYS$SYSTEM
   $ RUN AUTHORIZE
   UAF> MODIFY SYSTEM/WSQUOTA=32768
   UAF> MODIFY SYSTEM/FILLM=128
   UAF> MODIFY SYSTEM/DIOLM=100
   UAF> MODIFY SYSTEM/WSEXTENT=100000
   UAF> MODIFY SYSTEM/PGFLQUOTA=200000
   UAF> MODIFY SYSTEM/ASTLM=1000
   UAF> MODIFY SYSTEM/BIOLM=1000
   UAF> MODIFY SYSTEM/BYTLM=100000
   UAF> MODIFY SYSTEM/ENQLM=1000
   UAF>  EXIT

4. Log out and then log in again so that these process quotas take effect.

Understanding BACKUP File Formats

Using Disks and Tapes