zfsonlinux/zfs — pull request #783 by dechamps (2012-06-12 09:51:48+00:00)
Title: Make zil_slog_limit a tunable module parameter

`zil_slog_limit` specifies the maximum commit size to be written to the separate log device. Larger commits bypass the separate log device and go directly to the data devices.

The optimal value for `zil_slog_limit` directly depends on the latency and throughput characteristics of both the separate log device and the data disks. Small synchronous writes are faster on low-latency separate log devices (e.g. SSDs) whereas large synchronous writes are faster on high-latency data disks (e.g. spindles) because of higher throughput, especially with a large array. The point is, the line between "small" and "large" synchronous writes in this scenario is heavily dependent on the hardware used. That's why it should be made configurable.