Re: swidth with mdadm and RAID6

[Shailendra Tripathi <stripathi@agami.com>]

Hi Steve,
            I checked the code and it appears that XFS is not *aware* of 
RAID6. Basically, for all md devices, it gets the volume info by making 
a an ioctl call. I can see that XFS only take care of level 4 and level 
5. It does not account for level 6.
Only extra line need to be added here as below:

if (md.level == 6)
     md.nr_disks -= 2; /* RAID 6 has  2  parity disks */
 You can try with this change if you can. Do let mew know if it solves 
your problem.

This code is in function: md_get_subvol_stripe in <xf_progs>/libdisk/md.c


               /* Deduct a disk from stripe width on RAID4/5 */
                if (md.level == 4 || md.level == 5)
                        md.nr_disks--;

                /* Update sizes */
                *sunit = md.chunk_size >> 9;
                *swidth = *sunit * md.nr_disks;

                return 1;
        }

Regards,
Shailendra
Steve Cousins wrote:

>Hi Shailendra,
>
>Here is the info:
>
>1. [root@juno ~]# cat /proc/mdstat 
>Personalities : [raid6] 
>md0 : active raid6 sdb[0] sdl[10](S) sdk[9] sdj[8] sdi[7] sdh[6] sdg[5]
>sdf[4] sde[3] sdd[2] sdc[1]
>      3907091968 blocks level 6, 64k chunk, algorithm 2 [10/10]
>[UUUUUUUUUU]
>      
>unused devices: <none>
>
>2. mdadm --create /dev/md0 --chunk=64 --level=6 --raid-devices=10
>--spare-devices=1 /dev/sd[bcdefghijkl]
>
>3.  [root@juno ~]# xfs_db -r /dev/md*
>xfs_db> sb
>xfs_db> p
>magicnum = 0x58465342
>blocksize = 4096
>dblocks = 976772992
>rblocks = 0
>rextents = 0
>uuid = 04b32cce-ed38-496f-811f-2ccd51450bf4
>logstart = 536870919
>rootino = 256
>rbmino = 257
>rsumino = 258
>rextsize = 144
>agblocks = 30524160
>agcount = 32
>rbmblocks = 0
>logblocks = 32768
>versionnum = 0x3d84
>sectsize = 4096
>inodesize = 256
>inopblock = 16
>fname = "\000\000\000\000\000\000\000\000\000\000\000\000"
>blocklog = 12
>sectlog = 12
>inodelog = 8
>inopblog = 4
>agblklog = 25
>rextslog = 0
>inprogress = 0
>imax_pct = 25
>icount = 36864
>ifree = 362
>fdblocks = 669630878
>frextents = 0
>uquotino = 0
>gquotino = 0
>qflags = 0
>flags = 0
>shared_vn = 0
>inoalignmt = 2
>unit = 16
>width = 144
>dirblklog = 0
>logsectlog = 12
>logsectsize = 4096
>logsunit = 4096
>features2 = 0
>xfs_db> 
>
>
>Thanks for the help.
>
>Steve
>
>______________________________________________________________________
> Steve Cousins, Ocean Modeling Group    Email: cousins@umit.maine.edu
> Marine Sciences, 452 Aubert Hall       http://rocky.umeoce.maine.edu
> Univ. of Maine, Orono, ME 04469        Phone: (207) 581-4302
>
>On Mon, 18 Sep 2006, Shailendra Tripathi wrote:
>
>  
>
>>Can you list the output of
>>1. cat /proc/mdstat
>>2. the command to create 8+2 RAID6 with one spare ?
>>3. and output of following:
>>    xfs_db -r /dev/md*
>>    xfs_db> sb
>>    xfs_db> p
>>
>>-shailendra
>>
>>Steve Cousins wrote:
>>    
>>
>>>>I have a RAID6 array of 11 500 GB drives using mdadm.  There is one
>>>>hot-spare so the number of data drives is 8.  I used mkfs.xfs with
>>>>defaults to create the file system and it seemed to pick up the chunk size
>>>>I used correctly (64K) but I think it got the swidth wrong.  Here is what
>>>>xfs_info says:
>>>>
>>>>===========================================================================
>>>>meta-data=/dev/md0               isize=256    agcount=32, agsize=30524160
>>>>blks
>>>>         =                       sectsz=4096  attr=0
>>>>data     =                       bsize=4096   blocks=976772992, imaxpct=25
>>>>         =                       sunit=16     swidth=144 blks, unwritten=1
>>>>naming   =version 2              bsize=4096
>>>>log      =internal               bsize=4096   blocks=32768, version=2
>>>>         =                       sectsz=4096  sunit=1 blks
>>>>realtime =none                   extsz=589824 blocks=0, rtextents=0
>>>>===========================================================================
>>>>
>>>>So, sunit*bsize=64K, but swidth=144 and swidth/sunit=9 so it looks like it
>>>>thought there were 9 data drives instead of 8.  
>>>>
>>>>Am I diagnosing this correctly?  Should I recreate the array and
>>>>explicitly set sunit=16 and swidth=128?
>>>>
>>>>Thanks for your help.
>>>>
>>>>Steve
>>>>______________________________________________________________________
>>>> Steve Cousins, Ocean Modeling Group    Email: cousins@umit.maine.edu
>>>> Marine Sciences, 452 Aubert Hall       http://rocky.umeoce.maine.edu
>>>> Univ. of Maine, Orono, ME 04469        Phone: (207) 581-4302
>>>>
>>>>
>>>>        
>>>>
>
>  
>