The ZIL is a write-only log that is only read after a power failure. Several GB
is large enough for most workloads.
You can't use the Intel X25-E because it has a 32 or 64 MB volatile cache that
can't be disabled neither flushed by ZFS.
Imagine your server has a power failure while writing data to the pool. In
normal situation, with ZIL on a reliable device, ZFS will read the ZIL and come
back to a stable state at reboot. You may have lost some data (30 seconds) but
the zpool works. With the Intel X25-E as ZIL some log data has been lost with
the power failure (32/64MB max) which lead to a corrupted log and so ... you
loose your zpool and all your data !!
For the ZIL you need 2 reliable mirrored SSD devices with a supercapacitor that
can flush the write cache to NAND when a power failure occurs.
A hard-disk has a write cache but it can be disabled or flush by the operating
For more informations :
This message posted from opensolaris.org
zfs-discuss mailing list