RAID6 chunk size change?

RAID6 chunk size change?

[Mark Knecht]

Hi,
   I've got an existing (and well backed up as of a couple of minutes
ago) 5-drive RAID6 shown here:

c2stable ~ # mdadm --detail /dev/md7
/dev/md7:
        Version : 1.2
  Creation Time : Sun Jan 16 17:02:24 2011
     Raid Level : raid6
     Array Size : 395387904 (377.07 GiB 404.88 GB)
  Used Dev Size : 131795968 (125.69 GiB 134.96 GB)
   Raid Devices : 5
  Total Devices : 5
    Persistence : Superblock is persistent

    Update Time : Sun Jan 23 10:25:41 2011
          State : clean
 Active Devices : 5
Working Devices : 5
 Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 512K

           Name : c2stable:7  (local to host c2stable)
           UUID : ded5e0c6:1a5a73ad:01949557:bb4e015a
         Events : 17

    Number   Major   Minor   RaidDevice State
       0       8        7        0      active sync   /dev/sda7
       1       8       23        1      active sync   /dev/sdb7
       2       8       39        2      active sync   /dev/sdc7
       3       8       50        3      active sync   /dev/sdd2
       4       8       66        4      active sync   /dev/sde2
c2stable ~ #

   I would like to change this RAID to a smaller chunk size. (16K for
now) What is the right command to use?

   From Google and the mdadm man pages I'm coming up with something like:

mdadm --grow /dev/md7 --level=6 --chunk=16 --backup-file=/root/backup-md7

Maybe I don't need --level because the level isn't changing? Neither
is the number of drives.

Thanks in advance,
Mark

Re: RAID6 chunk size change?

[Mark Knecht]

On Sun, Jan 23, 2011 at 10:53 AM, Mark Knecht <markknecht@gmail.com> wrote:
> Hi,
>   I've got an existing (and well backed up as of a couple of minutes
> ago) 5-drive RAID6 shown here:
>
> c2stable ~ # mdadm --detail /dev/md7
> /dev/md7:
>        Version : 1.2
>  Creation Time : Sun Jan 16 17:02:24 2011
>     Raid Level : raid6
>     Array Size : 395387904 (377.07 GiB 404.88 GB)
>  Used Dev Size : 131795968 (125.69 GiB 134.96 GB)
>   Raid Devices : 5
>  Total Devices : 5
>    Persistence : Superblock is persistent
>
>    Update Time : Sun Jan 23 10:25:41 2011
>          State : clean
>  Active Devices : 5
> Working Devices : 5
>  Failed Devices : 0
>  Spare Devices : 0
>
>         Layout : left-symmetric
>     Chunk Size : 512K
>
>           Name : c2stable:7  (local to host c2stable)
>           UUID : ded5e0c6:1a5a73ad:01949557:bb4e015a
>         Events : 17
>
>    Number   Major   Minor   RaidDevice State
>       0       8        7        0      active sync   /dev/sda7
>       1       8       23        1      active sync   /dev/sdb7
>       2       8       39        2      active sync   /dev/sdc7
>       3       8       50        3      active sync   /dev/sdd2
>       4       8       66        4      active sync   /dev/sde2
> c2stable ~ #
>
>   I would like to change this RAID to a smaller chunk size. (16K for
> now) What is the right command to use?
>
>   From Google and the mdadm man pages I'm coming up with something like:
>
> mdadm --grow /dev/md7 --level=6 --chunk=16 --backup-file=/root/backup-md7
>
> Maybe I don't need --level because the level isn't changing? Neither
> is the number of drives.
>
> Thanks in advance,
> Mark
>

OK, I found a blog of Neil's in which I had read the top but hadn't
gone through the comments:

http://neil.brown.name/blog/20090817000931
It seems he answers my question further down in the comments.

Looks like

mdadm --grow /dev/md7 --chunk=16

would be enough.

Cheers,
Mark
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Re: RAID6 chunk size change?

[Roman Mamedov]

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On Sun, 23 Jan 2011 11:10:40 -0800
Mark Knecht <markknecht@gmail.com> wrote:

> On Sun, Jan 23, 2011 at 10:53 AM, Mark Knecht <markknecht@gmail.com> wrote:
> > Hi,
> >   I've got an existing (and well backed up as of a couple of minutes
> > ago) 5-drive RAID6 shown here:
> >
> > c2stable ~ # mdadm --detail /dev/md7
> > /dev/md7:
> >        Version : 1.2
> >  Creation Time : Sun Jan 16 17:02:24 2011
> >     Raid Level : raid6
> >     Array Size : 395387904 (377.07 GiB 404.88 GB)
> >  Used Dev Size : 131795968 (125.69 GiB 134.96 GB)
> >   Raid Devices : 5
> >  Total Devices : 5
> >    Persistence : Superblock is persistent
> >
> >    Update Time : Sun Jan 23 10:25:41 2011
> >          State : clean
> >  Active Devices : 5
> > Working Devices : 5
> >  Failed Devices : 0
> >  Spare Devices : 0
> >
> >         Layout : left-symmetric
> >     Chunk Size : 512K
> >
> >           Name : c2stable:7  (local to host c2stable)
> >           UUID : ded5e0c6:1a5a73ad:01949557:bb4e015a
> >         Events : 17
> >
> >    Number   Major   Minor   RaidDevice State
> >       0       8        7        0      active sync   /dev/sda7
> >       1       8       23        1      active sync   /dev/sdb7
> >       2       8       39        2      active sync   /dev/sdc7
> >       3       8       50        3      active sync   /dev/sdd2
> >       4       8       66        4      active sync   /dev/sde2
> > c2stable ~ #
> >
> >   I would like to change this RAID to a smaller chunk size. (16K for
> > now) What is the right command to use?
> >
> >   From Google and the mdadm man pages I'm coming up with something like:
> >
> > mdadm --grow /dev/md7 --level=6 --chunk=16 --backup-file=/root/backup-md7
> >
> > Maybe I don't need --level because the level isn't changing? Neither
> > is the number of drives.
> >
> > Thanks in advance,
> > Mark
> >
> 
> OK, I found a blog of Neil's in which I had read the top but hadn't
> gone through the comments:
> 
> http://neil.brown.name/blog/20090817000931
> It seems he answers my question further down in the comments.
> 
> Looks like
> 
> mdadm --grow /dev/md7 --chunk=16

I suggest that you reconsider going with 16K, it is a suboptimal chunk size.
The older default of 64K seems to be the sweet spot, at least for RAID5/6:
http://louwrentius.com/blog/2010/05/linux-raid-level-and-chunk-size-the-benchmarks/

-- 
With respect,
Roman

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Re: RAID6 chunk size change?

[Mark Knecht]

On Sun, Jan 23, 2011 at 11:52 AM, Roman Mamedov <rm@romanrm.ru> wrote:
> On Sun, 23 Jan 2011 11:10:40 -0800
> Mark Knecht <markknecht@gmail.com> wrote:
<SNIP>
>>
>> OK, I found a blog of Neil's in which I had read the top but hadn't
>> gone through the comments:
>>
>> http://neil.brown.name/blog/20090817000931
>> It seems he answers my question further down in the comments.
>>
>> Looks like
>>
>> mdadm --grow /dev/md7 --chunk=16
>
> I suggest that you reconsider going with 16K, it is a suboptimal chunk size.
> The older default of 64K seems to be the sweet spot, at least for RAID5/6:
> http://louwrentius.com/blog/2010/05/linux-raid-level-and-chunk-size-the-benchmarks/
>
> --
> With respect,
> Roman
>
Thanks for the link. That's good information.

I have no strong preference for 16K other than I have two RAID6's on
this set of drives and the one that's 16K tests in iozone as about 2
times faster than the 512K one that I want to change.

I can certainly try 64K, see how it works out, and then do the reshape
again if necessary.

My work model for this drive is basically running VMware virtual
machines. The bulk of the files stored on this RAID are 2GB files. I
have no idea how VMware (or Virtualbox which I also us) makes use
those files other than they represent the virtual hard drive.

Again, thanks for the info.

Cheers,
Mark