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Re: sunit & swidth on RAID-1?

To: mike dentifrice <fluor@xxxxxxxxxxx>
Subject: Re: sunit & swidth on RAID-1?
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Wed, 11 Feb 2009 10:46:26 -0600
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20090211155638.GA3864@dentifrice>
References: <20090211155638.GA3864@dentifrice>
User-agent: Thunderbird 2.0.0.19 (Macintosh/20081209)
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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