Random write behind
The write behind feature provides a mechanism such that when the number of dirty pages in memory for a given file exceeds a defined threshold, the subsequent pages written are then scheduled to be written to disk.
There may be applications that perform a lot of random I/O, that is, the I/O pattern does not meet the requirements of the write behind algorithm and thus all the pages stay resident in memory until the syncd daemon runs. If the application has modified many pages in memory, this could cause a very large number of pages to be written to disk when the syncd daemon issues a sync() call.
You can tune the threshold by using the ioo command with the JFS maxrandwrt parameter. The default value is 0, indicating that random write behind is disabled. Increasing this value to 128 indicates that once 128 memory-resident pages of a file are dirty, any subsequent dirty pages are scheduled to be written to the disk. The first set of pages will be flushed after a sync() call.
For Enhanced JFS, ioo command options j2_nRandomCluster (-z flag) and j2_maxRandomWrite (-J flag) are used to tune random write behind. Both options have a default of 0. The j2_maxRandomWrite option has the same function for enhanced JFS as maxrandwrt does for JFS. That is, it specifies a limit for the number of dirty pages per file that can remain in memory. The j2_nRandomCluster option specifies the number of clusters apart two consecutive writes must be in order to be considered random.