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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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