gvinum raid5 vs. ZFS raidz

[Paul Kraus]

On Aug 2, 2014, at 6:25, Warren Block <wblock at wonkity.com> wrote:

> On Sat, 2 Aug 2014, Scott Bennett wrote:
>>    On Tue, 29 Jul 2014 12:01:36 -0400 Paul Kraus <paul at 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.

Do NOT try to understand the spreadsheet on it’s 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=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.

Look at the names of the sheets in the document. They are referred to back in the blog entry.

--
Paul Kraus
paul at kraus-haus.org