Date: Sat, 2 Aug 2014 11:58:47 -0400 From: Paul Kraus <paul@kraus-haus.org> To: Warren Block <wblock@wonkity.com> Cc: Scott Bennett <bennett@sdf.org>, freebsd-questions@freebsd.org Subject: Re: gvinum raid5 vs. ZFS raidz Message-ID: <27521B45-98E0-4FDD-978F-83DA0CFA993E@kraus-haus.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 Aug 2, 2014, at 6:25, Warren Block <wblock@wonkity.com> wrote: > On Sat, 2 Aug 2014, Scott Bennett wrote: >> On Tue, 29 Jul 2014 12:01:36 -0400 Paul Kraus = <paul@kraus-haus.org> >=20 >>> 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_R6gpT6= 89wTjHv6CGVElrPqTA0w_ZY/edit?pli=3D1#gid=3D2126998674 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. >>>=20 >> Anyway, using lynx(1), it is very hard to make any sense of the = spreadsheet. >=20 > Even with a graphic browser, let's say that spreadsheet is not a = paragon of clarity. Do NOT try to understand the spreadsheet on it=92s own, it is part of = the Blog entry. Read the blog and look at the spreadsheet as Matt refers = to it. > 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=3D12? It could be 1, = or 8, or maybe something else. >=20 > 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. >=20 > 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. Look at the names of the sheets in the document. They are referred to = back in the blog entry. -- Paul Kraus paul@kraus-haus.org
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