Sequential and random write behind

There are two types of write behind: sequential and random.

The AIX file system code logically divides each file into 16 KB clusters for JFS and 128 KB clusters for Enhanced JFS for the following reasons:

• Increase write performance
• Limit the number of dirty file pages in memory
• Reduce system overhead
• Minimize disk fragmentation

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 code 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 re-accesses any of the pages before their frames are reused, no I/O is required.

If a large number of dirty file pages remain in memory and do not get reused, the sync daemon writes them to disk, which might result in abnormal disk utilization. To distribute the I/O activity more evenly, you can turn on write behind 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 size of the write behind partitions and the write behind threshold can be changed with the ioo command. See Sequential and random write behind performance tuning for more information.