Performance of a USB ZIL for ZFS

[Dan Nelson]

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 at 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 at allantgroup.com