On Sun, 10 Jan 2010, Lutz Schumann wrote:
Talking about read performance. Assuming a reliable ZIL disk (cache
flush = working): The ZIL can guarantee data integrity, however if
the backend disks (aka pool disks) do not properly implement cache
flush - a reliable ZIL device does not "workaround" the bad backend
disks rigth ???
(meaning: having a reliable ZIL + some MLC SSD with write cache
enabled is not reliable at the end)
As soon as there is more than one disk in the pool, it is necessary
for cache flush to work or else the devices may contain content from
entirely different transaction groups, resulting in a scrambled pool.
If you just had one disk which tended to ignore cache flush requests,
then you should be ok as long as the disk writes the data in order.
In that case any unwritten data would be lost, but the pool should not
be lost. If the device ignores cache flush requests and writes data
in some random order, then the pool is likely to eventually fail.
I think that zfs mirrors should be safer than raidz when faced with
devices which fail to flush (should be similar to the single-disk
case), but only if there is one mirror pair.
A scary thing about SSDs is that they may re-write old data while
writing new data, which could result in corruption of the old data if
the power fails while it is being re-written.
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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