Date: Tue, 28 Aug 2012 08:27:27 -0700 From: Freddie Cash <fjwcash@gmail.com> To: Niki Hammler <mailinglists@nobaq.net> Cc: freebsd-fs@freebsd.org Subject: Re: zvol + raidz issue? Message-ID: <CAOjFWZ6a6h_o1sq6KGN-N5Jnq9NSj6KvjEyfvEXXYrznw%2Bj8fA@mail.gmail.com> In-Reply-To: <503C8AEE.1090703@nobaq.net> References: <503A6F9F.7070801@nobaq.net> <CAOjFWZ4Ep5ZONO%2B7UNqt36stFN_OXoMhK=83UwPpv51P8OTjfg@mail.gmail.com> <503C8AEE.1090703@nobaq.net>
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On Tue, Aug 28, 2012 at 2:10 AM, Niki Hammler <mailinglists@nobaq.net> wrote: > Am 26.08.2012 22:13, schrieb Freddie Cash: >> (Sorry for top-post, sending from phone.) >> >> Please show the command-line used to create the zvol. Especially the >> recordsize option. When using zvols, you have to make sure to match the >> recordsize of the zvol to that of the filesystem used above it. >> Otherwise, performance will be atrocious. > > Sorry for my third posting on this. > Now I strictly followed your suggestion and used > > zfs create -b 128k -V 500g plvl5i0/zvtest > > (with 128k being the recordsize of the dataset in the zpool). > > Suddenly the write performance increased from the 2.5 MB/s to 250 MB/s > (or 78MB/s when using bs=4096 with dd) > > 1.) How can this explained? > 2.) Is there any problem when choosing -b 128k (can I always blindly > choose -b 128k)? > > Remember again that the problem ONLY occurs with raidz1+zvol+force 4096 > block alignment and in no other case! Most likely it has to do with the raidz stripe size and the constant block size of the zvol causing alignment or similar issues. I've also seen indications in the zfs-discuss mailing list about optimal and sub-optimal disk configurations for the various raidz types (wrong number of disks in the vdev leads to horrible performance). -- Freddie Cash fjwcash@gmail.com
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