Hello,
An array that I created using a custom --size parameter has failed and needs to be recovered. I am very comfortable recovering arrays using --assume-clean but due to a typo at creation time I don't know the device size that was originally used. I am hoping this value can be recalculate from data in the superblocks but this calculation is not obvious to me.
Here's what I know... I'm using metadata version 1.0 with an internal bitmap on all my arrays. I ran some experiments in the lab with 3TB drives and found that when I specified a device size of 2929687500 when creating an array, 'mdadm -D' reported a 'Used Dev Size' of 5859374976. The value specified on the command line is in kilobytes so I was expecting 3,000,000,000,000 bytes to be used on each device. The value reported by mdadm is in sectors (512 bytes) so turning this into bytes I get 2,999,999,987,712 bytes. This is off by 12,288 bytes (12kb) which I assume is used by the v1.0 superblock and/or the internal bitmap. I also tried creating an array with 2TB drives (Requested Size: 1953125000, Used Dev Size: 3906249984) and got a difference of 8kb (2,000,000,000,000 vs 1,999,999,991,808 bytes) so clearly the amount of extra space used depends on the size of the device in some way.
The array that I'm trying to recover reports a 'Used Dev Size' of 5858574976. This is just 800,000 sectors less than I got when requesting an even 3 trillion bytes so I know the size to use on the command line is close to 2929687500. But I don't know how to calculate the exact size... Can someone help me?
Once I know the size I will recreate the array using the following:
size=???
md11='/dev/sdc /dev/sdf /dev/sdi /dev/sdl /dev/sdo /dev/sdr /dev/sdu /dev/sdx missing missing /dev/sdag /dev/sdaj /dev/sdam /dev/sdap /dev/sdas'
mdadm --create /dev/md11 --metadata=1.0 --size=$size --bitmap=internal --auto=yes -l 6 -n 15 --assume-clean $md11
Just incase it helps, here's the full output from mdadm -D for the array I'm trying to recover and mdadm -E for the first device in that array:
mdadm -E /dev/sdc
/dev/sdc:
Magic : a92b4efc
Version : 1.0
Feature Map : 0x1
Array UUID : 560bd0d9:a8d4758c:9849143c:a2ef5b8e
Name : sm345:11 (local to host sm345)
Creation Time : Sat Dec 17 07:22:56 2011
Raid Level : raid6
Raid Devices : 15
Avail Dev Size : 5860532896 (2794.52 GiB 3000.59 GB)
Array Size : 76161474688 (36316.62 GiB 38994.68 GB)
Used Dev Size : 5858574976 (2793.59 GiB 2999.59 GB)
Super Offset : 5860533152 sectors
State : clean
Device UUID : f3ad57be:c0835578:4f242111:fb465c0a
Internal Bitmap : -176 sectors from superblock
Update Time : Sat Jun 16 05:45:17 2012
Checksum : 4936d9a7 - correct
Events : 187674
Chunk Size : 64K
Array Slot : 0 (empty, 1, 2, 3, 4, 5, 6, 7, failed, failed, 10, 11, 12, 13, 14)
Array State : _uuuuuuu__uuuuu 2 failed
mdadm -D /dev/md11
/dev/md11:
Version : 01.00
Creation Time : Sat Dec 17 07:22:56 2011
Raid Level : raid6
Array Size : 38080737344 (36316.62 GiB 38994.68 GB)
Used Dev Size : 5858574976 (5587.17 GiB 5999.18 GB)
Raid Devices : 15
Total Devices : 15
Preferred Minor : 11
Persistence : Superblock is persistent
Intent Bitmap : Internal
Update Time : Sat Jun 16 05:45:17 2012
State : active, degraded
Active Devices : 12
Working Devices : 15
Failed Devices : 0
Spare Devices : 3
Chunk Size : 64K
Name : sm345:11 (local to host sm345)
UUID : 560bd0d9:a8d4758c:9849143c:a2ef5b8e
Events : 187674
Number Major Minor RaidDevice State
0 0 0 0 removed
1 8 80 1 active sync /dev/sdf
2 8 128 2 active sync /dev/sdi
3 8 176 3 active sync /dev/sdl
4 8 224 4 active sync /dev/sdo
5 65 16 5 active sync /dev/sdr
6 65 64 6 active sync /dev/sdu
7 65 112 7 active sync /dev/sdx
8 0 0 8 removed
9 0 0 9 removed
10 66 0 10 active sync /dev/sdag
11 66 48 11 active sync /dev/sdaj
12 66 96 12 active sync /dev/sdam
13 66 144 13 active sync /dev/sdap
14 66 192 14 active sync /dev/sdas
0 8 32 - spare /dev/sdc
15 65 160 - spare /dev/sdaa
16 65 208 - spare /dev/sdad
Thanks,
Tim
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