Setting Up Separate ZFS Log Devices

Solaris 10 10/08 Release: The ZFS intent log (ZIL) is provided to satisfy 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 storage pool. In this Solaris release, you can decide if you want the ZIL blocks to continue to be allocated from the main storage pool or from a separate log device. Better performance might be possible by using separate intent log devices in your ZFS storage pool, such as with NVRAM or a dedicated disk.

Log devices for the ZFS intent log are not related to database log files.

You can set up a ZFS log device when the storage pool is created or after the pool is created. For examples of setting up log devices, see Creating a ZFS Storage Pool With Log Devices and Adding Devices to a Storage Pool.

You can attach a log device to an existing log device to create a mirrored log device. This operation is identical to attaching a device in a unmirrored storage pool.

Consider the following points when determining whether setting up a ZFS log device is appropriate for your environment:

• Any performance improvement seen by implementing a separate log device depends on the device type, the hardware configuration of the pool, and the application workload. For preliminary performance information, see this blog:

  http://blogs.sun.com/perrin/entry/slog_blog_or_blogging_on

• Log devices can be unreplicated or mirrored, but RAID-Z is not supported for log devices.

• If a separate log device is not mirrored and the device that contains the log fails, storing log blocks reverts to the storage pool.

• Log devices can be added, replaced, attached, detached, imported, and exported as part of the larger storage pool. Log devices can be removed starting in the Solaris 10 9/10 release.

• 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 in-play data that might be stored on a log device is relatively small. Log blocks are freed when the log transaction (system call) is committed.

• The maximum size of a log device should be approximately 1/2 the size of physical memory because that is the maximum amount of potential in-play 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.