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Date:      Sat, 2 Aug 2014 04:25:02 -0600 (MDT)
From:      Warren Block <wblock@wonkity.com>
To:        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:  <alpine.BSF.2.11.1408020356250.1128@wonkity.com>
In-Reply-To: <201408020621.s726LsiA024208@sdf.org>
References:  <201408020621.s726LsiA024208@sdf.org>

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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.



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