Questions on RAID alignment
Hello list
a few questions
1- Does XFS support alignment to concatenation RAIDs (e.g. linear over raid1) similarly to striped ones? I am guessing probably su and sw should not be specified and the agcount should be multiple of the number of the concatenated devices, is that correct?
2- Can mkfs parameters be altered after creation, e.g. after growing a RAID and then growing the XFS fs, can I also alter su, sw, agcount somehow?
3- I seem to remember there was an xattr that one could attach to /home (if /home was under XFS) so that every user (subdir of /home) would get a different AG in a different RAID device. I cannot find that information anymore on the internet; could you refresh my mind?
Thank you
Spelic
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Le Thu, 20 Sep 2012 13:11:38 +0200
Spelic <[email protected]> écrivait:
Quoted 4 lines
1- Does XFS support alignment to concatenation RAIDs (e.g. linear over raid1) similarly to striped ones? I am guessing probably su and sw should not be specified and the agcount should be multiple of the number of the concatenated devices, is that correct?
No, it's better to set your sw/su accordingly to your RAID setup (example: 8 drives RAID-10 256k stripe -> su=256k,sw=4) and let mkfs.xfs determine itself how many ag it wants, to adapt it both to the striping and filesystem size.
Quoted 3 lines
2- Can mkfs parameters be altered after creation, e.g. after growing a RAID and then growing the XFS fs, can I also alter su, sw, agcount somehow?
agcount will be adjusted by xfs_growfs AFAIK. For the other parameters, not so much.
Quoted 6 lines
3- I seem to remember there was an xattr that one could attach to /home (if /home was under XFS) so that every user (subdir of /home) would get a different AG in a different RAID device. I cannot find that information anymore on the internet; could you refresh my mind?
I really have no idea on this.
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http://oss.sgi.com/mailman/listinfo/xfsQuoted 5 lines · Spelic
Hello list
a few questions1- Does XFS support alignment to concatenation RAIDs (e.g. linear over raid1) similarly to striped ones?
No. It's being considered and I know how to do it, but it's lower priority than other stuff I'm working on at the moment.
Quoted 3 lines
I am guessing probably su and
sw should not be specified and the agcount should be multiple of the number of the concatenated devices, is that correct?
If you are concatenating RAID0/5/6, then you shoul duse su/sw as per the alignment of the volumes being concatenated.
Quoted 3 lines
2- Can mkfs parameters be altered after creation, e.g. after growing a RAID and then growing the XFS fs, can I also alter su, sw, agcount somehow?
You can alter su/sw as a mount option if the filesystem was made with them initially.
Quoted 4 lines
3- I seem to remember there was an xattr that one could attach to /home (if /home was under XFS) so that every user (subdir of /home) would get a different AG in a different RAID device. I cannot find that information anymore on the internet; could you refresh my mind?
Nothing of the sort. if you are using the inode64 allocator, then each new directory is placed i a different AG. That's as good as it gets right now.
Cheers,
Dave.
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Dave Chinner [email protected] _______________________________________________ xfs mailing list [email protected] http://oss.sgi.com/mailman/listinfo/xfs
Quoted 5 lines · Spelic
Hello list
a few questions1- Does XFS support alignment to concatenation RAIDs (e.g. linear over raid1) similarly to striped ones?
XFS has a stripe alignment feature for writeout of metadata and data. It is used to optimize writeout to striped devices. If you concatenate striped arrays then you can use XFS stripe alignment, AS LONG AS EACH ALLOCATION GROUP IS WHOLLY CONTAINED WITHIN A CONCAT MEMBER ARRAY, and doesn't cross concat member boundaries.
Thus, more consideration and planning must go into designing such a storage stack. In this situation you will need to specify agcount manually so AGs don't spill over boundaries. Drive size and array configuration will dictate how many AGs you need to use.
Quoted 3 lines
I am guessing probably su and sw
should not be specified and the agcount should be multiple of the number of the concatenated devices, is that correct?
If your concat member devices are mirrors, then you don't specify su/sw.
If they are striped arrays, you would specify su/sw as you would if
using just one of those striped arrays. Again, in the latter case your AGs must not spill. For example:
24x 300GB SAS drive FC SAN box, 4x 6 drive RAID6, 1.2TB/concat member. Max agsize is 1TB, not evenly divisible into 1.2TB. Specify su/sw to match the RAID6 parms. Specify agcount of 16. Each AG is fully contained within a concat device.
12x 3TB drive JBOD on RAID card, 2x 6 drive RAID6, 12TB/concat member. Max agsize is 1TB, evenly divisible into 12TB. Specify su/sw to match RAID6 parms. Specify agcount of 24. Each AG is fully contained within a concat device.
Quoted 2 lines
2- Can mkfs parameters be altered after creation, e.g. after growing a RAID and then growing the XFS fs, can I also alter su, sw, agcount somehow?
mkfs parms can't be altered, but the XFS writeout geometry can with mount options. See 'man mount'. However, if you're using striped arrays as concat members, you must expand the concat with identical striped array members so your geometry remains the same. Thus you won't modify su/sw because they haven't changed.
xfs_growfs will create new AGs in the new space made available by adding a new device (array) to the concat. NOTE that xfs_growfs doesn't allow manual control of agcount as mkfs.xfs does. However, if you expand your concat only one array at a time, it shouldn't matter how many AGs are created, as they will all be contained within that concat member array, and thus su/sw parms are still correct. If you add 2 or more arrays to the concat the grow operation may spill AGs across member boundaries.
Quoted 4 lines
3- I seem to remember there was an xattr that one could attach to /home (if /home was under XFS) so that every user (subdir of /home) would get a different AG in a different RAID device. I cannot find that information anymore on the internet; could you refresh my mind?
What you're referring to is the inode64 mount option. Every directory created in the XFS will go into the next AG, round robin, until you hit the last AG, then it wraps back to AG0, rinse, repeat. There is no way to control what dirs get created on what RAID devices within a concat, but for the techniques I described above.
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