Re: [zfs-discuss] slog/L2ARC on a hard drive and not SSD?
On Jul 21, 2010, at 7:56 AM, Hernan F wrote: Hi, Out of pure curiosity, I was wondering, what would happen if one tries to use a regular 7200RPM (or 10K) drive as slog or L2ARC (or both)? My rule of thumb is that if the latency of the slog (write latency) or L2ARC (random read) is 10x better than the pool devices, then go for it. There are some cases where a fast HDD can have lower read or write latency than a slow HDD (or Marty's USB disk :-) -- richard -- Richard Elling rich...@nexenta.com +1-760-896-4422 Enterprise class storage for everyone www.nexenta.com ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] slog/L2ARC on a hard drive and not SSD?
From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- boun...@opensolaris.org] On Behalf Of Hernan F Hi, Out of pure curiosity, I was wondering, what would happen if one tries to use a regular 7200RPM (or 10K) drive as slog or L2ARC (or both)? I tested it once, for the same reasons as you. Curiosity. I never said anything about it because it wasn't interesting. In some cases, it accelerates some. In some cases, it slows things down. It basically depends on the characteristics of your work load, but overall, it was a net zero gain. Or perhaps a net loss. Generally speaking, people won't sacrifice the hardware or disk slots, unless there's a clear gain. So that's the conclusion. Obvious though it may be. There is no clear gain, so just don't do it. Offhand, I think the only situation where there was a gain was ... If the primary pool (6 disks) is being absolutely hammered by async operations, and then you benchmark some sync writes simultaneously, with and without the extra disk for dedicated log. The end result is: Since the dedicated log is idle, the sync write can immediately write to it, and then become just another async write going along with the crowd. Without the dedicated log, the sync write competes against the async operations for access to the main pool, slowing itself down and the async operations. But with the dedicated log, you've got 6 disks active with async operations and the 7th disk is idle during normal async operations ... But when you mix the async and sync operations, then suddenly you're able to leverage all 7 disks and have a net gain because your primary pool is not dorking around with tiny little sync operations. Like I said. Unimpressive, not interesting. No general purpose, and generally no gain. Don't do it. Just by the SSD instead. ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] slog/L2ARC on a hard drive and not SSD?
Hi, Out of pure curiosity, I was wondering, what would happen if one tries to use a regular 7200RPM (or 10K) drive as slog or L2ARC (or both)? I have done both with success. At one point my backup pool was a collection of USB attached drives (please keep the laughter down) with dedup=verify. Solaris' slow USB performance coupled with slow drives and dedup reads gave abysmal write speeds, so much so that at times it had trouble keeping the snapshots synchronized. To help it along, I took an unused fast, small SCSI disk and made it L2ARC, which significantly improved write performance on the pool. During testing of some iSCSI applications, I ran into a scenario where a client was performing many small, synchronous writes to a zvol in a wide RAIDZ3 stripe. Since synchronous writes can double the write activity (once for the zil and once for the actual pool), actual throughput from the client was below 2MB/s, even though the pool would sustain 200MB/s on sequential writes. As above, I added a mirrored slog which was two small, fast SCSI drives. While I expected the throughput to double, it actually went up by a factor of 4, to 8MB/s. Even though 8MB/s wasn't mind-numbing, it was enough that it was close to saturating the client's 100Mb ethernet link, so it was ok. I think the reason that the slog improved things so much is that the slog disks were not bothered with other i/o and were doing very little seeking. -- This message posted from opensolaris.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] slog/L2ARC on a hard drive and not SSD?
On Wed, 2010-07-21 at 07:56 -0700, Hernan F wrote: Hi, Out of pure curiosity, I was wondering, what would happen if one tries to use a regular 7200RPM (or 10K) drive as slog or L2ARC (or both)? I know these are designed with SSDs in mind, and I know it's possible to use anything you want as cache. So would ZFS benefit from it? Would it be the same? Would it slow down? I guess it would slow things down, because it would be trying to read/write from a single spindle instead of a multidisk array, right? I havent found any articles discussing this, only ones talking about SSD-based slogs/caches. Thanks, Hernan I think yes, it would probably slow things down, at least for typical usage. However, there is a small change it might improve things by offloading this functionality from the main spindle(s) to separate ones. But I think you'd be better off expanding a stripe than using a disk in this way. - Garrett ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] slog/L2ARC on a hard drive and not SSD?
Another data point - I used three 15K disks striped using my RAID controller as a slog for the zil, and performance went down. I had three raidz sata vdevs holding the data, and my load was VMs, i.e. a fair amount of small, random IO (60% random, 50% write, ~16k in size). Scott -- This message posted from opensolaris.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
