gvinum raid5 vs. ZFS raidz

Scott Bennett bennett at sdf.org
Wed Aug 6 05:48:29 UTC 2014

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.  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
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