2012-07-29 19:50, Sašo Kiselkov wrote:
On 07/29/2012 04:07 PM, Jim Klimov wrote:
For several times now I've seen statements on this list implying
that a dedicated ZIL/SLOG device catching sync writes for the log,
also allows for more streamlined writes to the pool during normal
healthy TXG syncs, than is the case with the default ZIL located
within the pool.
Is this understanding correct? Does it apply to any generic writes,
or only to sync-heavy scenarios like databases or NFS servers?
Yes, it is correct. It applies to all writes. If the log is allocated on
a slog devices, then the synchronous log records don't fragment the
pool. As far as I understand it, txgs happen sequentially even with no
slog device present, but the log entries don't - they occur as is needed
to fulfill the sync write request with minimum latency.
Thanks, I thought similarly, but the persistent on-list mention
(or words that could be interpreted that way) that with SLOG
devices writes ought to be better coalesced and less fragmented,
I started getting confused. :)
So, I guess, if the sync-write proportion on a particular system
is negligible (and that can be measured with dtrace scripts),
then a slog won't help much with fragmentation of generic async
writes, right?
//Jim
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