On Jun 21, 2010, at 11:17 AM, Arne Jansen wrote: > Roy Sigurd Karlsbakk wrote: >> Hi all >> I plan to setup a new system with four Crucial RealSSD 256MB SSDs for both >> SLOG and L2ARC. The plan is to use four small slices for the SLOG, striping >> two mirrors. I have seen questions in here about the theoretical benefit of >> doing this, but I haven't seen any answers, just some doubt about the effect. >> Does anyone know if this will help gaining performance? Or will it be bad? > > I'm planning to do something similar, though I only want to install 2 devices. > Some thoughts I had so far: > > - mirroring l2arc won't gain anything, as it doesn't contain any information > that cannot be rebuilt if a device is lost.
This is not properly stated. There is nothing in the L2ARC that is not also in the pool, so it is simply a cache. You are correct in that the L2ARC is not rebuilt, but you miss the reason why the rebuild is not necessary (or desired) > Further, if a device is lost, > the system just uses the remaining devices. So I wouldn't waste any space > mirroring l2arc, I'll just stripe them. Yes, this is the most economical solution. > - the purpose of a zil device is to reduce latency. The purpose of a separate device is to reduce latency for synchronous writes. > Throughput is probably not > an issue, especially if you configure your pool so that large writes go to > the main pool. As 2 devices don't have a lower latency than one, I see no > real point in striping slog devices. Striping separate log devices can help for high transaction rate environments or where the latency is constrained by bandwidth. > - For slog you need SSD with supercap which are significantly more expensive > than without. I'll try the OCZ Vertex 2 Pro in the next few days and can > give a report how it performs. For L2ARC cheap MLC SSDs will do. For a separate log, choose a device which honors cache flush commands. Nonvolatile caches are nice, but rare -- HDDs do act well as ZIL devices, but almost never have nonvolatile caches. -- richard -- Richard Elling rich...@nexenta.com +1-760-896-4422 ZFS and NexentaStor training, Rotterdam, July 13-15, 2010 http://nexenta-rotterdam.eventbrite.com/ _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
