Date: Sat, 02 Aug 2014 13:39:04 +0100 From: Arthur Chance <freebsd@qeng-ho.org> To: Warren Block <wblock@wonkity.com>, Scott Bennett <bennett@sdf.org> Cc: freebsd-questions@freebsd.org, Paul Kraus <paul@kraus-haus.org> Subject: Re: gvinum raid5 vs. ZFS raidz Message-ID: <53DCDBE8.8060704@qeng-ho.org> In-Reply-To: <alpine.BSF.2.11.1408020356250.1128@wonkity.com> References: <201408020621.s726LsiA024208@sdf.org> <alpine.BSF.2.11.1408020356250.1128@wonkity.com>
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On 02/08/2014 11:25, Warren Block wrote: > On Sat, 2 Aug 2014, Scott Bennett wrote: >> On Tue, 29 Jul 2014 12:01:36 -0400 Paul Kraus <paul@kraus-haus.org> > >>> ZFS parity is handled slightly differently than for traditional >>> raid-5 (as well as the striping of data / parity blocks). So you >>> cannot just count on loosing 1, 2, or 3 drives worth of space to >>> parity. See Matt Ahren?s Blog entry here >>> http://blog.delphix.com/matt/2014/06/06/zfs-stripe-width/ for >>> (probably) more data on this than you want :-) And here >>> https://docs.google.com/a/delphix.com/spreadsheets/d/1tf4qx1aMJp8Lo_R6gpT689wTjHv6CGVElrPqTA0w_ZY/edit?pli=1#gid=2126998674 >>> is his spreadsheet that relates space lost due to parity to number of >>> drives in raidz vdev and data block size (yes, the amount of space >>> lost to parity caries with data block, not configured filesystem >>> block size!). There is a separate tab for each of RAIDz1, RAIDz2, and >>> RAIDz3. >>> >> Anyway, using lynx(1), it is very hard to make any sense of the >> spreadsheet. > > Even with a graphic browser, let's say that spreadsheet is not a paragon > of clarity. It's not clear what "block size in sectors" means in that > context. Filesystem blocks, presumably, but are sectors physical or > virtual disk blocks, 512 or 4K? What is that number when using a > standard configuration of a disk with 4K sectors and ashift=12? It > could be 1, or 8, or maybe something else. > > As I read it, RAIDZ2 with five disks uses somewhere between 67% and 40% > of the data space for redundancy. The first seems unlikely, but I can't > tell. Better labels or rearrangement would help. > > A second chart with no labels at all follows the first. It has only the > power-of-two values in the "block size in sectors" column. A > restatement of the first one... but it's not clear why. > > My previous understanding was that RAIDZ2 with five disks would leave > 60% of the capacity for data. Quite right. If you have N disks in a RAIDZx configuration, the fraction used for data is (N-x)/N and the fraction for parity is x/N. There's always overhead for the file system bookkeeping of course, but that's not specific to ZFS or RAID.
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