AIX 5L Problem Determination - IBM Redbooks

The VMM tries to anticipate the future need for pages of a sequential file by
observing the pattern in which a program is accessing the file. When the program
accesses two successive pages of the file, the VMM assumes that the program
will continue to access the file sequentially, and the VMM schedules additional
sequential reads of the file. This is called Sequential-Access Read Ahead. These
reads are overlapped with the program processing, and will make the data
available to the program sooner than if the VMM had waited for the program to
access the next page before initiating the I/O. The number of pages to be read
ahead is determined by two VMM thresholds:
minpgahead Number of pages read ahead when the VMM first detects
the sequential access pattern. If the program continues to
access the file sequentially, the next read ahead will be
for 2 times minpgahead, the next for 4 times minpgahead,
and so on until the number of pages reaches
maxpgahead.
maxpgahead Maximum number of pages the VMM will read ahead in a
sequential file.
If the program deviates from the sequential-access pattern and accesses a page
of the file out of order, sequential read ahead is terminated. It will be resumed
with minpgahead pages if the VMM detects a resumption of sequential access by
the program. The values of minpgahead and maxpgahead can be set with the
vmtune command.
To increase write performance, limit the number of dirty file pages in memory,
reduce system overhead, and minimize disk fragmentation. The file system
divides each file into 16-KB partitions. The pages of a given partition are not
written to disk until the program writes the first byte of the next 16-KB partition. At
that point, the file system forces the four dirty pages of the first partition to be
written to disk. The pages of data remain in memory until their frames are
reused, at which point no additional I/O is required. If a program accesses any of
the pages before their frames are reused, no I/O is required.
If a large number of dirty file pages remains in memory and does not get reused,
the sync daemon writes them to disk, which might result in abnormal disk
utilization. To distribute the I/O activity more efficiently across the workload,
write-behind can be turned on to tell the system how many pages to keep in
memory before writing them to disk. The write-behind threshold is on a per-file
basis, which causes pages to be written to disk before the sync daemon runs.
The I/O is spread more evenly throughout the workload.
There are two types of write-behind: Sequential and random. The size of the
write-behind partitions and the write-behind threshold can be changed with the
vmtune command.
Chapter 10. Performance problem determination 259