zfs_prefetch_disable Parameter

Description

This parameter determines a file-level prefetching mechanism called zfetch. This mechanism looks at the patterns of reads to files and anticipates on some reads, thereby reducing application wait times. The current behavior suffers from two drawbacks:

• Sequential read patterns made of small reads very often hit in the cache. In this case, the current behavior consumes a significant amount of CPU time trying to find the next I/O to issue, whereas performance is governed more by the CPU availability.

• The zfetch code has been observed to limit scalability of some loads. CPU profiling can be done by using the lockstat ‐I command or er_kernel. For detailed descriptions, see the following sources:

  • lockstat(8)

  • Oracle Developer Studio documentation (https://docs.oracle.com/en/operating-systems/studio.html)

  • Oracle Developer Studio Documentation (https://www.oracle.com/application-development/technologies/developerstudio-documentation.html)

  Although never recommended, you can disable prefetching by setting zfs_prefetch_disable in a file the /etc/system.d directory. For instructions, see /etc/system.d/ Directory Files.

  Device-level prefetching is disabled when zfs_vdev_cache_size is disabled. This means that tuning vdev cache shift is no longer necessary if zfs_vdev_cache_size is disabled.

Data Type

Boolean

Default

0 (enabled)

Range

0 (enabled) or 1 (disabled)

Dynamic?

Yes

Validation

No

When to Change

If the results of er_kernel show significant time in zfetch_* functions, or if lock profiling with lockstat shows contention around zfetch locks, and if the affected workload is critical, then disabling file level prefetching should be considered. Other workloads that are running concurrently can be degraded.

Commitment Level

Unstable