Date: Sat, 30 Oct 2010 15:17:08 -0500 From: "James R. Van Artsdalen" <james-freebsd-fs2@jrv.org> To: =?ISO-8859-1?Q?Peter_Ankerst=E5l?= <peter@pean.org> Cc: freebsd-fs@freebsd.org Subject: Re: Raid + zfs performace. Message-ID: <4CCC7D44.4080704@jrv.org> In-Reply-To: <D2954020-C3A0-46EC-8C64-EB57EA1E9B21@pean.org> References: <D2954020-C3A0-46EC-8C64-EB57EA1E9B21@pean.org>
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Peter Ankerst=E5l wrote: > I have a question about raid and zfs. I Have a hardware-raid running. A= mirror thats the only storage in my zfs pool. Im going to > add another mirror to the machine and my question is, what is the best = option performace-wise? > Is it to add the other mirror to the same pool or create another separa= te pool for that mirror?=20 > Btw. Today my disk are quite saturated r/w wise. You should see if your disks are evenly saturated with reads & writes, or heavily reads with few writes etc. Whether the new mirror should be added to the existing pool or placed in a new pool depends on your unstated goals: do you want to reduce existing I/O saturation or add storage that does not compete with existing I/O traffic? If you are concerned about recovery scenarios then it is better to put the RAID controller in JBOD mode using ZFS for a number of reasons: 1. When you replace a disk ZFS only rebuilds the areas of the disk that are used. The RAID controller must rebuild the entire disk, resulting in extra I/O. 2. If a failure is not in the disk but rather somewhere else - perhaps a disconnected cable - then ZFS can quickly rebuild the disk when reconnected, often very quickly. Last year I accidentally yanked out the cable to an enclosure with 4 2TB disks (each in a different mirror), and when I reconnected the enclosure ZFS took about *two seconds* to rebuild everything. A RAID controller would have had to rebuild all 8 TB= =2E 3. With ZFS the different disks in a mirror need not be on the same controller. For a while I ran one "side" of each mirror on an LSI SAS controller using the mpt driver and other "side" of each mirror on a 3124 SATA controller using the siis driver. With hardware RAID you've generally got the RAID controller and device driver as single points of failure. ZFS mirrors can be many-way mirrors. I have not tested this but it seems likely ZFS load balances reads in a mirror. If the I/O load is heavily slanted towards reads then adding more "ways" to the mirror might add performance as well as redundancy.
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