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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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