On 07/29/2012 06:01 PM, Jim Klimov wrote:
> 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?


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