What makes you say that the X25-E's cache can't be disabled or flushed? The net seems to be full of references to people who are disabling the cache, or flushing it frequently, and then complaining about the performance!

T

Frédéric VANNIERE wrote:
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 
system.

For more informations : 
http://www.c0t0d0s0.org/archives/5993-Somewhat-stable-Solid-State.html

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