On Oct 20, 2009, at 10:24 PM, 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.
I am surprised by this assertion and cannot find any confirmation from
Intel.
Rather, the cache flush command is specifically mentioned as supported
in
Section 6.1.1 of the Intel X25-E SATA Solid State Drive Product Manual.
http://download.intel.com/design/flash/nand/extreme/extreme-sata-ssd-datasheet.pdf
I suspect that this is confusion relating to the various file systems,
OSes,
or virtualization platforms which may or may not by default ignore cache
flushes.
Since NTFS uses the cache flush commands, I would be very surprised if
Intel would intentionally ignore it.
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 !!
The ZIL works fine for devices which support the cache flush command.
-- richard
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