Commands Reference, Volume 3

When -r is used in combination without a NewValue, the nextboot value for tunable displays. When -p is used in combination without a NewValue, a value displays only if the current and next boot values for he Tunableare the same. Otherwise NONE displays as the value.

When used with -a or -o without specifying a new value, the values display only if the current and next boot values for a parameter are the same. Otherwise NONE displays as the value.

When used with -a or -o without specifying a new value, next boot values for tunables display instead of current values.

Name   Current  Default  Reboot   Minimum  Maximum  Unit  Type  Dependencies
       value    value    value    value    value
----------------------------------------------------------------------------

param1 5        2        4        1        10       MB/s     I  param2
                                                                param3

Parameters can be of the following types:

• D for Dynamic if tunable can be changed at any time.
• S for Static if tunable can never be changed.
• R for Reboot if tunable can only be changed during reboot sequence.
• B for Bosboot if tunable can only be changed by running bosboot and rebooting machine.
• M for Mount if tunable is only effective for future filesystems or directory mountings.
• I for Incremental if tunable can only be incremented, except at boot time

Any change (with -o, -d or -D) to a parameter of type Mount will result in a message being displayed to warn the user that the change is only effective for future mountings.

Any attempt to change (with -o, -d or -D) a parameter of type Bosboot or Reboot without -r, will result in an error message.

Any attempt to change (with -o, -d or -D but without -r) the current value of a parameter of type Incremental with a new value smaller than the current value, will result in an error message.

Purpose:

Specifies the total number of pbufs that the LVM uses.

Values:

• Default: For the 32-bit kernel, the default is (4 + number of physical disks with an open logical volume) * 64. For the 64-bit kernel, the default is (4 + number of physical disks with an open logical valume) * 128.
• Range:
• Type: Incremental

Diagnosis:

Value of hd_pendqblked is non-zero, which indicates that LVM had to wait for pbufs. To see value of hd_pendqblked, use the vmstat -v command.

Tuning:

Useful to increase if there are a substantial number of simultaneous I/Os and value of hd_pendqblked is non-zero.

Refer To:

Miscellaneous I/O Tuning Parameters

Purpose:

Specifies the number of LVM buffers for raw physical I/Os.

Values:

• Default: 9
• Range: 1 to 64
• Type: Dynamic

Diagnosis:

Applications performing large writes to striped raw logical volumes are not obtaining the desired throughput rate.

Tuning:

LVM splits large raw I/Os into multiple buffers of 128 K a piece. Default value of 9 means that about 1 MB I/Os can be processed without waiting for more buffers. If a system is configured to have striped raw logical volumes and is doing writes greater than 1.125 MB, increasing this value may help the throughput of the application. If performing larger than 1 MB raw I/Os, it might be useful to increase this value.

Refer To:

Miscellaneous I/O Tuning Parameters

Purpose:

Specifies the maximum number of pages to be read ahead when processing a sequentially accessed file.

Values:

• Default: 8 (the default should be a power of two and should be greater than or equal to minpgahead)
• Range: 0 to 4096
• Type: Dynamic

Diagnosis:

Observe the elapsed execution time of critical sequential-I/O-dependent applications with the time command.

Tuning:

Because of limitations in the kernel, do not exceed 512 as the maximum value used. The difference between minfree and maxfree should always be equal to or greater than maxpgahead. If execution time decreases with higher maxpgahead, observe other applications to ensure that their performance has not deteriorated.

Refer To:

Sequential Read-Ahead

Purpose:

Specifies a threshold (in 4 KB pages) for random writes to accumulate in RAM before subsequent pages are flushed to disk by the write-behind algorithm. The random write-behind threshold is on a per-file basis.

Values:

• Default: 0
• Range: 0 to largest_file_size_in_pages
• Type: Dynamic

Diagnosis:

vmstat -n shows page out and I/O wait peaks on regular intervals (usually when the sync daemon is writing pages to disk).

Tuning:

Useful to set this value to 1 or higher if too much I/O occurs when syncd runs. Default is to have random writes stay in RAM until a sync operation. Setting maxrandwrt ensures these writes get flushed to disk before the sync operation has to occur. However, this could degrade performance because the file is then being flushed each time. Tune this option to favor interactive response time over throughput. After the threshold is reached, all subsequent pages are then immediately flushed to disk. The pages up to the threshold value stay in RAM until a sync operation. A value of 0 disables random write-behind.

Refer To:

VMM Write-Behind

Purpose:

Specifies the number of pages with which sequential read-ahead starts.

Values:

• Default: 2
• Range: 0 to 4096 (should be a power of two)
• Type: Dynamic

Diagnosis:

Observe the elapsed execution time of critical sequential-I/O-dependent applications with time command.

Tuning:

Useful to increase if there are lots of large sequential accesses. Observe other applications to ensure that their performance has not deteriorated. Value of 0 may be useful if I/O pattern is purely random.

Refer To:

Sequential Read-Ahead

Purpose:

Specifies the number of 16 k clusters processed by the sequential write-behind algorithm of the VMM.

Values:

• Default: 1
• Range: 0 to any positive integer
• Type: Dynamic

Diagnosis:

N/A

Tuning:

Useful to increase if there is a need to keep more pages in RAM before scheduling them for I/O when the I/O pattern is sequential. May be appropriate to increase if striped logical volumes or disk arrays are being used.

Refer To:

VMM Write-Behind

Purpose:

Specifies the number of file system bufstructs.

Values:

• Default: 186 (value is dependent on the size of the bufstruct)
• Range:
• Type: Mount

Diagnosis:

N/A

Tuning:

If the VMM must wait for a free bufstruct, it puts the process on the VMM wait list before the start I/O is issued and will wake it up once a bufstruct has become available. May be appropriate to increase if striped logical volumes or disk arrays are being used.

Refer To:

Miscellaneous I/O Tuning Parameters

Purpose:

Specifies the number of pages that should be deleted in one chunk from RAM when a file is deleted.

Values:

• Default: 65536
• Range: 1 to largest filesize_in_pages
• Type: Dynamic

Diagnosis:

Real-time applications that experience sluggish response time while files are being deleted.

Tuning:

Tuning this option is only useful for real-time applications. If real-time response is critical, adjusting this option may improve response time by spreading the removal of file pages from RAM more evenly over a workload.

Refer To:

Miscellaneous I/O Tuning Parameters

Purpose:

If set, will cause a sync() to flush all I/O to a file without holding the i-node lock, and then use the i-node lock to do the commit.

Values:

• Default: 0 (off)
• Range 0 or 1
• Type: Dynamic

Diagnosis:

I/O to a file is blocked when the syncd daemon is running.

Tuning:

Default value of 0 means that the i-node lock is held while all dirty pages of a file are flushed.

Refer To:

Tuning File Syncs

Purpose:

Specifies the maximum number of pages to be read ahead when processing a sequentially accessed file on Enhanced JFS.

Values:

• Default: 8
• Range:
• Type: Dynamic

Diagnosis:

N/A

Tuning:

The difference between minfree and maxfree should always be equal to or greater than j2_maxPageReadAhead. If run time decreases with higher a j2_maxPageReadAhead value, observe other applications to ensure that their performance has not deteriorated.

Purpose:

Specifies a threshold for random writes to accumulate in RAM before subsequent pages are flushed to disk by the Enhanced JFS's write-behind algorithm. The random write-behind threshold is on a per-file basis.

Values:

• Default: 0
• Range:
• Type: Dynamic

Diagnosis:

N/A

Tuning:

Useful if too many pages are flushed out by syncd.

Purpose:

Specifies the minimum number of pages to be read ahead when processing a sequentially accessed file on Enhanced JFS.

Values:

• Default: 2
• Range:
• Type: Dynamic

Diagnosis:

N/A

Tuning:

Useful to increase if there are lots of large sequential accesses. Observe other applications to ensure that their performance has not deteriorated. Value of 0 may be useful if I/O pattern is purely random.

Purpose:

Specifies the number of file system bufstructs for Enhanced JFS.

Values:

• Default: 512
• Range:
• Type: Mount

Diagnosis:

Using vmstat -v, the value of xpagerbufwaitcnt increases fast.

Tuning:

If the kernel must wait for a free bufstruct, it puts the process on a wait list before the start I/O is issued and will wake it up once a bufstruct has become available. May be appropriate to increase if striped logical volumes or disk arrays are being used.

Purpose:

Specifies the number of pages per cluster processed by Enhanced JFS's write behind algorithm.

Values:

• Default: 32
• Range:
• Type: Dynamic

Diagnosis:

N/A

Tuning:

Useful to increase if there is a need to keep more pages in RAM before scheduling them for I/O when the I/O pattern is sequential. May be appropriate to increase if striped logical volumes or disk arrays are being used.

Purpose:

Specifies the distance apart (in clusters) that writes have to exceed in order for them to be considered as random by the Enhanced JFS's random write behind algorithm.

Values:

• Default: 0
• Range:
• Type: Dynamic

Diagnosis:

N/A

Tuning:

Useful to increase if there is a need to keep more pages in RAM before scheduling them for I/O when the I/O pattern is random and random write behind is enabled (j2_maxRandomWrite).