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?
Correct. Cheers, -- Saso _______________________________________________ zfs-discuss mailing list firstname.lastname@example.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss