ZFS and cache devices

My guess is that the ZFS Intent Log (ZIL) is limiting the write performance. A fast SSD used as a log device could help. Some hints:

• ZIL Accelerator: DRAM or Flash? explains the issues in detail.
• You may want to test with a disabled ZIL. See my other posting.
• Checkout ZFS Features in Oracle Solaris Express if you're using a recent ZFS release because the way how to disable the ZIL has changed. There's a new sync filesystem property now.
• Until recently it was impossible to remove a log device from a ZFS pool. Of course, this caused problems if the single log devices failed. It was therefore recommended to use a mirror for the log device. However, recent ZFS releases fixed this limitation and it's now possible to remove the single log device with zpool remove. Make sure you use a version that supports this feature if you really going to install the SSD.

I asked Richard Elling, ex-sun ZFS engineer this question. He told me L2ARC is uncompressed, just like the ARC is uncompressed.

Sorry, I can't provide documentation or specifications. My only proof is that one of the guys who helped design ZFS told me in person when I met him last week. :)