Sequential write behind

If all 4 pages of a cluster are dirty, then as soon as a page in the next partition is modified, the 4 dirty pages of the cluster are scheduled to go to disk. Without this feature, pages would remain in memory until the syncd daemon runs, which could cause I/O bottlenecks and fragmentation of the file.

By default, a JFS file is partitioned into 16 KB partitions or 4 pages. Each of these partitions is called a cluster.

The number of clusters that the VMM uses as a threshold is tunable. The default is one cluster. You can delay write behind by increasing the numclust parameter using the ioo -o numclust command.

For Enhanced JFS, the ioo -o j2_nPagesPerWriteBehindCluster command is used to specify the number of pages to be scheduled at one time, rather than the number of clusters. The default number of pages for an Enhanced JFS cluster is 32, implying a default size of 128 KB for Enhanced JFS.