On 04/17/10 07:59, Dave Vrona wrote:
1) Mirroring. Leaving cost out of it, should ZIL and/or L2ARC SSDs
be mirrored ?
L2ARC cannot be mirrored -- and doesn't need to be. The contents are
checksummed; if the checksum doesn't match, it's treated as a cache miss
and the block is re-read from the main pool disks.
The ZIL can be mirrored, and mirroring it improves your ability to
recover the pool in the face of multiple failures.
2) ZIL write cache. It appears some have disabled the write cache on
the X-25E. This results in a 5 fold performance hit but it
eliminates a potential mechanism for data loss. Is this valid?
With the ZIL disabled, you may lose the last ~30s of writes to the pool
(the transaction group being assembled and written at the time of the
crash).
With the ZIL on a device with a write cache that ignores cache flush
requests, you may lose the tail of some of the intent logs, starting
with the first block in each log which wasn't readable after the
restart. (I say "may" rather than "will" because some failures may not
result in the loss of the write cache). Depending on how quickly your
ZIL device pushes writes from cache to stable storage, this may narrow
the window from ~30s to less than 1s, but doesn't close the window entirely.
If I can mirror ZIL, I imagine this is no longer a concern?
Mirroring a ZIL device with a volatile write cache doesn't eliminate
this risk. Whether it reduces the risk depends on precisely *what*
caused your system to crash and reboot; if the failure also causes loss
of the write cache contents on both sides of the mirror, mirroring won't
help.
- Bill
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