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Re: botched RAID, now e2fsck or what?

On 2009-12-10, at 13:30, Lucian Șandor wrote:
2009/12/10 Andreas Dilger <adilger sun com>:

Using "od -Ax -tx4" on a regular ext3 filesystem you can see the group descriptor table starting at offset 0x1000, and the block numbers basically just "count" up. This may in fact be the easiest way to order the disks, if the group descriptor table is large enough to cover all of the disks:

# od -Ax -tx4 /dev/hda1 | more
001000 0000012c 0000012d 0000012e 02430000
001010 000001f2 00000000 00000000 00000000
001020 0000812c 0000812d 0000812e 2e422b21
001030 0000000d 00000000 00000000 00000000
001040 00010000 00010001 00010002 27630074
001050 000000b8 00000000 00000000 00000000
001060 0001812c 0001812d 0001812e 27a70b8a
001070 00000231 00000000 00000000 00000000
001080 00020000 00020001 00020002 2cc10000
001090 00000008 00000000 00000000 00000000
0010a0 0002812c 0002812d 0002812e 25660134
0010b0 00000255 00000000 00000000 00000000
0010c0 00030000 00030001 00030002 17a50003
0010d0 000001c6 00000000 00000000 00000000
0010e0 0003812c 0003812d 0003812e 27a70000
0010f0 00000048 00000000 00000000 00000000
001100 00040000 00040001 00040002 2f8b0000

See nearly regular incrementing sequence every 0x20 bytes:

0000012c, 0000812c, 00010000, 0001812c, 00020000, 0002812c, 00030000,

Each group descriptor block (4kB = 0x1000) covers 16GB of filesystem space, so 64 blocks per 1TB of filesystem size. If your RAID chunk size is not too large, and the filesystem IS large, you will be able to fully order your disks in the RAID set. You can also verify the RAID chunk size by determining how many blocks of consecutive group descriptors are present before there is a "jump" where the group descriptor blocks were written to other disks before returning to the current disk. Remember that one of the disks in the set will also need to store parity, so there will be some number of "garbage" blocks before the proper data resumes.

This seems a great idea. The 4.5 TB array is huge (should have a 1100
kB table), and likely its group descriptor table extends on all
partitions. I already found the pattern, but the job requires
programming, since it would be troubling to read megs of data over the
hundreds of permutations. I will try coding it, but I hope that
somebody else wrote it before. Isn't there any utility that will take
a group descriptor table and verify its integrity without modifying

I think you are going about this incorrectly... Run the "od" command on the raw component drives (e.g. /dev/sda, /dev/sdb, /dev/sdc, etc), not on the assembled MD RAID array (e.g. NOT /dev/md0).

The data blocks on the raw devices will be correct, with every 1/N chunks of space being used for parity information (so will look like garbage). That won't prevent you from seeing the data in the group descriptor table and allowing you to see the order in which the disks are supposed to be AND the chunk size.

Since the group descriptor table is only a few kB from the start of the disk (I'm assuming you used whole-disk devices for the MD array, instead of DOS partitions) you can just use "od ... | less" and your eyes to see what is there. No programming needed.

Cheers, Andreas
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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