Date: Wed, 06 Aug 2014 00:48:14 -0500 From: Scott Bennett <bennett@sdf.org> To: freebsd-questions@freebsd.org, wblock@wonkity.com Cc: Paul Kraus <paul@kraus-haus.org> Subject: Re: gvinum raid5 vs. ZFS raidz Message-ID: <201408060548.s765mEQr007372@sdf.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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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> > > >> 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 Sounds like that documents the situation no better than the gcache(8) man page regarding the use of gcache(8) with graid3(8). :-( > 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. I wish I knew a way to get these drives to admit to the operating system that they really use 4k sectors, rather than wasting kernel time supervising eight 512-byte I/O operations for each real 4096-byte I/O operations. :-{ > > My previous understanding was that RAIDZ2 with five disks would leave > 60% of the capacity for data. That was the way I had understood it, too. I have nowhere found any explanation of his reference to "padding" either. Scott Bennett, Comm. ASMELG, CFIAG ********************************************************************** * Internet: bennett at sdf.org *xor* bennett at freeshell.org * *--------------------------------------------------------------------* * "A well regulated and disciplined militia, is at all times a good * * objection to the introduction of that bane of all free governments * * -- a standing army." * * -- Gov. John Hancock, New York Journal, 28 January 1790 * **********************************************************************
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