The ZFS intent log (ZIL) satisfies POSIX requirements for synchronous transactions. For example, databases often require their transactions to be on stable storage devices when returning from a system call. NFS and other applications can also use fsync() to ensure data stability.
By default, the ZIL is allocated from blocks within the main pool. However, you can obtain better performance by using separate intent log devices such as NVRAM or a dedicated disk.
Log devices for the ZFS intent log are not related to database log files. Deploying separate log devices can improve performance but the improvement also depends on the device type, the hardware configuration of the pool, and the application workload.
You can configure mirrored log devices only for redundancy and not RAID-Z log devices. If an unmirrored log device fails, storing log blocks reverts to the storage pool. Further, you can add, replace, remove, attach, detach, import, and export log devices as part of the larger storage pool.
Consider the following points about ZFS log devices:
The minimum size of a log device is the same as the minimum size of each device in a pool, which is 64 MB. The amount of logged data that might be stored on a log device is relatively small. Log blocks are freed when the log transaction or system call is committed.
The maximum size of a log device should be approximately half the size of physical memory, which is the maximum amount of potential logged data that can be stored. For example, if a system has 16 GB of physical memory, consider a maximum log device size of 8 GB.
To create a storage pool with log devices, use the log keyword. The following example shows how to configure a mirrored storage pool called datap with a mirrored log device.
# zpool create datap mirror c0t5000C500335F95E3d0 c0t5000C500335F907Fd0 \
mirror c0t5000C500335BD117d0 c0t5000C500335DC60Fd0 \
log mirror c0t5000C500335E106Bd0 c0t5000C500335FC3E7d0
# zpool status datap
pool: datap
state: ONLINE
scrub: none requested
config:
NAME STATE READ WRITE CKSUM
datap ONLINE 0 0 0
mirror-0 ONLINE 0 0 0
c0t5000C500335F95E3d0 ONLINE 0 0 0
c0t5000C500335F907Fd0 ONLINE 0 0 0
mirror-1 ONLINE 0 0 0
c0t5000C500335BD117d0 ONLINE 0 0 0
c0t5000C500335DC60Fd0 ONLINE 0 0 0
logs
mirror-2 ONLINE 0 0 0
c0t5000C500335E106Bd0 ONLINE 0 0 0
c0t5000C500335FC3E7d0 ONLINE 0 0 0
errors: No known data errors