sunit & swidth on RAID-1?

sunit & swidth on RAID-1?

[mike dentifrice]

Hello,

I would like to know if it makes any sense to specify sunit & swidth
parameters as mount options to XFS filesystems sitting on a Linux
software RAID-1 array, as advertised here [1] or there [2].

I'm asking, because it seems that chunk_size does not matter in a RAID-1
context, nor do stripe units; or do they?

Any tip would be appreciated,

[1] http://www.socalsysadmin.com/2008/10/15/optimizing-xfs-on-raid-arrays/
[2] http://feedblog.org/2008/06/18/howto-configure-raid-strides-in-xfs/

-- 
mike dentifrice <fluor@poivron.org>

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Re: sunit & swidth on RAID-1?

[Justin Piszcz]

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On Wed, 11 Feb 2009, mike dentifrice wrote:

> Hello,
>
> I would like to know if it makes any sense to specify sunit & swidth
> parameters as mount options to XFS filesystems sitting on a Linux
> software RAID-1 array, as advertised here [1] or there [2].
>
> I'm asking, because it seems that chunk_size does not matter in a RAID-1
> context, nor do stripe units; or do they?
>
> Any tip would be appreciated,
>
> [1] http://www.socalsysadmin.com/2008/10/15/optimizing-xfs-on-raid-arrays/
> [2] http://feedblog.org/2008/06/18/howto-configure-raid-strides-in-xfs/
>
> -- 
> mike dentifrice <fluor@poivron.org>
>
> _______________________________________________
> xfs mailing list
> xfs@oss.sgi.com
> http://oss.sgi.com/mailman/listinfo/xfs

mkfs.xfs will auto-optimize for software-raid arrays for the most part, 
with HW raid on a 3ware controller, tweaking mkfs.xfs does not help very 
much.  with SW raid, at least for raid1 - I use the defaults.

Justin.

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Re: sunit & swidth on RAID-1?

[Eric Sandeen]

mike dentifrice wrote:
> Hello,
> 
> I would like to know if it makes any sense to specify sunit & swidth
> parameters as mount options to XFS filesystems sitting on a Linux
> software RAID-1 array, as advertised here [1] or there [2].
> 
> I'm asking, because it seems that chunk_size does not matter in a RAID-1
> context, nor do stripe units; or do they?

I don't think it does matter.  And for what it's worth, mkfs.xfs queries
an MD raid, but ignores raid1/10/0:

                /*
                 * Ignore levels we don't want aligned (e.g. linear)
                 * and deduct disk(s) from stripe width on RAID4/5/6
                 */
                switch (md.level) {
                case 6:
                        md.raid_disks--;
                        /* fallthrough */
                case 5:
                case 4:
                        md.raid_disks--;
                        /* fallthrough */
                case 1:
                case 0:
                case 10:
                        break;
                default:
                        return 0;

In general, mkfs.xfs on md should just "do the right thing"

-Eric


> Any tip would be appreciated,
> 
> [1] http://www.socalsysadmin.com/2008/10/15/optimizing-xfs-on-raid-arrays/
> [2] http://feedblog.org/2008/06/18/howto-configure-raid-strides-in-xfs/
> 

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Re: sunit & swidth on RAID-1?

[Justin Piszcz]

On Wed, 11 Feb 2009, mike dentifrice wrote:

> Justin Piszcz a dit:
>> mkfs.xfs will auto-optimize for software-raid arrays for the most
>> part,  with HW raid on a 3ware controller, tweaking mkfs.xfs does not
>> help very  much.  with SW raid, at least for raid1 - I use the
>> defaults.
>
> Thanks for your reply. However, I wonder about the defaults:
>
> 	xfs_info /dev/md1 | grep -E '(sunit|swidth)'
>
> Outputs:
>
>        sunit=0      swidth=0 blks, unwritten=1
>        sectsz=512   sunit=0 blks
>
> What's the consequence of 0 values in this case?
>
> Greetings,
>
> -- 
> mike dentifrice <fluor@poivron.org>
>

# xfs_info /dev/md2 | grep -E '(sunit|swidth)'
          =                       sunit=0      swidth=0 blks
          =                       sectsz=512   sunit=0 blks, lazy-count=0

Mine is similar, I guess you would need to benchmark to find out if you 
were worried about it.  I ran multiple copies from a RAID1 (dual raptors)
and was able to get the performance of both of the drives combine, so I am 
not really worried about this setting.

Justin.

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Re: sunit & swidth on RAID-1?

[Michael Monnerie]

On Mittwoch 11 Februar 2009 mike dentifrice wrote:
>         sunit=0      swidth=0 blks, unwritten=1
>         sectsz=512   sunit=0 blks
>
> What's the consequence of 0 values in this case?

XFS then knows that 512 byte (aka sector) alignment is sufficient, as 
RAID-1 will only write 512b to each disk and does not need full stripe 
unit writes like RAID-4/5/6 need. Therefore performance is better with 
the values you have.

mfg zmi
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