Date: Sun, 26 Jun 2011 02:41:45 -0500 From: Dan Nelson <dnelson@allantgroup.com> To: Joshua Isom <jrisom@gmail.com> Cc: Damien Fleuriot <ml@my.gd>, "freebsd-questions@freebsd.org" <freebsd-questions@freebsd.org> Subject: Re: Performance of a USB ZIL for ZFS Message-ID: <20110626074141.GB44024@dan.emsphone.com> In-Reply-To: <4E06C2D1.4050601@gmail.com> References: <4E06180F.7090409@gmail.com> <16AE025C-EA55-45F3-8419-542228CE5AA4@my.gd> <4E06C2D1.4050601@gmail.com>
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In the last episode (Jun 26), Joshua Isom said: > On 6/25/2011 9:32 PM, Damien Fleuriot wrote: > > On 25 Jun 2011, at 19:17, Joshua Isom<jrisom@gmail.com> wrote: > >> I was wondering if anyone had tried using a decent USB flash drive for > >> the ZIL. I know it'd be hard finding one fast enough, but some from > >> patriot seem like they might be suitable for home use. Part of the > >> idea is to just minimize hard drive thrashing and the wear and tear > >> associated with it. If it helps prevent the drives from going bad, and > >> doesn't hurt performance too bad all the better. But if it's going to > >> hurt performance too much or not help prevent thrashing there isn't a > >> point. > > > > I stopped reading at the title. > > The answer is no. > > > > Grab a SSD for $80-120ish. > > Perhaps it would have helped to read the email. Part of the concern is > making sure the drives don't fail and not just throughput. > > Given that Kingston sells an SATA SSD for $40 that only gets writes at > 30mb/s write, and some USB drives might get up to 20mb/s. If I get two > drives and put them on different controllers, mirrored, I might get > acceptable performance. I may still loose performance, but if my drives > last a year longer, I can probably accept it. I'm ok with loosing some > performance, but I just don't want it dragging down the system. And if it > won't help the drives last longer there's no point. A seaparate ZIL isn't meant to extend the lifetime of the hard drives; it's meant to accelerate the speed of sync writes. Those are pretty infrequent themselves, unless you're an NFS server. You'll see a couple syncs per commit on a database server, but compared to the amount of regular reads and writes on your average system, you'll save under 1% of the writes by adding a fast ZIL. And remember, the ZIL is just a write log. Everything that gets written to it will get flushed to disk when zfs writes the next transaction group. -- Dan Nelson dnelson@allantgroup.com
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