Date: Thu, 12 May 2011 19:02:51 -0400 (EDT) From: Rick Macklem <rmacklem@uoguelph.ca> To: Bob Friesenhahn <bfriesen@simple.dallas.tx.us> Cc: freebsd-fs@freebsd.org Subject: Re: ZFS: How to enable cache and logs. Message-ID: <1700693186.266759.1305241371736.JavaMail.root@erie.cs.uoguelph.ca> In-Reply-To: <alpine.GSO.2.01.1105120842580.20825@freddy.simplesystems.org>
next in thread | previous in thread | raw e-mail | index | archive | help
> On Wed, 11 May 2011, Jeremy Chadwick wrote: > > > > Bob, please correct me if I'm wrong, but as I understand it a log > > device > > (ZIL) effectively limits the overall write speed of the pool itself. > > Consumer-level SSDs do not have extremely high write performance > > (and it > > gets worse without TRIM; again a 70% decrease in write speed in some > > cases). > > It is certainly a factor. However, large block writes (something like > 128K, I don't remember exactly) bypass the dedicated log device and > instead are written to the main store (with only a reference being > added to the dedicated device). The reason this is done is for the > exact reason you point out. The SSD has a very fast seek and zero > rotational latency but being a singular resource it suffers from > bandwidth limitations. The main store usually suffers from > multi-millisecond seeks and rotational latency but offers linearly > scalable and substantial write performance for larger writes. > > Matt Ahrens has described this a few times on the zfs-discuss list and > there is mention of it on slide 15 of the presentation found at > "http://www.slideshare.net/edigit/zfs-presentation". > > The large write feature of the ZIL is a reason why we should > appreciate modern NFS's large-write capability and avoid anchient NFS. > The size of a write for the new FreeBSD NFS server is limited to MAX_BSIZE. It is currently 64K, but I would like to see it much larger. I am going to try increasing MAX_BSIZE soon, to see what happens. This sounds like another good reason to increase it. However, a client chooses what size to use, up to the server`s limit (and, again, MAX_BSIZE for the FreeBSD client). rick
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?1700693186.266759.1305241371736.JavaMail.root>