Intent Log
The ZFS Intent Log (ZIL) satisfies POSIX requirements for synchronous transactions. For instance, 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 intent log is allocated from blocks within the main pool. However, it might be possible to get better performance using separate intent log devices such as NVRAM or a dedicated disk. For example:
# zpool create pool sda sdb log sdc
[B]Multiple log devices can also be specified, and they can be mirrored.[/B] See the EXAMPLES section for an example of mirroring multiple log devices.
Log devices can be added, replaced, attached, detached, and imported and exported as part of the larger pool. Mirrored log devices can be removed by specifying the top-level mirror for the log.
Cache Devices
Devices can be added to a storage pool as "cache devices." These devices provide an additional layer of caching between main memory and disk. For read-heavy workloads, where the working set size is much larger than what can be cached in main memory, using
cache devices allow much more of this working set to be served from low latency media. Using cache devices provides the greatest performance improvement for random read-workloads of mostly static content.
To create a pool with cache devices, specify a "cache" vdev with any number of devices. For example:
# zpool create pool sda sdb cache sdc sdd
[B]Cache devices cannot be mirrored or part of a raidz configuration.[/B] If a read error is encountered on a cache device, that read I/O is reissued to the original storage pool device, which might be part of a mirrored or raidz configuration.
The content of the cache devices is considered volatile, as is the case with other system caches.
