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I am trying to create a dm-cache device using a virtual machine. I have multiple disks labelled Cache_disk, Device_to_Cache and meta_data in /dev/sdb,sdc,sdd respectively, When trying to create the cache, I run the command:

dmsetup create my_cache --table '0 16775168 cache /dev/sdd1 /dev/sdb1 /dev/sdc1 512 1 writeback default 0'

as instructed in the dm-cache article Documentation page

I have enabled dm-cache in the kernel but am catching this error:

device-mapper: reload ioctl failed: Invalid or incomplete multibyte or wide character
command failed

When looking at the dmesg device-mapper: cache metadata fails its sb_check

root@msali014-VirtualBox:/home/msali014# dmesg
[ 5432.738603] device-mapper: cache-policy-mq: version 1.0.0 loaded
[ 5432.794852] device-mapper: cache metadata: sb_check failed: magic 0: wanted 1623043
[ 5432.794862] device-mapper: block manager: superblock validator check failed for block 0
[ 5432.794867] device-mapper: cache metadata: couldn't read lock superblock
[ 5432.797952] device-mapper: table: 252:0: cache: Error creating metadata object

the /var/log/syslog is similar:

Jun 28 11:17:01 msali014-VirtualBox CRON[2935]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)
Jun 28 11:33:08 msali014-VirtualBox kernel: [ 5432.738603] device-mapper: cache-policy-mq: version 1.0.0 loaded
Jun 28 11:33:08 msali014-VirtualBox kernel: [ 5432.794852] device-mapper: cache metadata: sb_check failed: magic 0: wanted 1623043
Jun 28 11:33:08 msali014-VirtualBox kernel: [ 5432.794862] device-mapper: block manager: superblock validator check failed for block 0
Jun 28 11:33:08 msali014-VirtualBox kernel: [ 5432.794867] device-mapper: cache metadata: couldn't read lock superblock
Jun 28 11:33:08 msali014-VirtualBox kernel: [ 5432.797952] device-mapper: table: 252:0: cache: Error creating metadata object
Jun 28 11:33:08 msali014-VirtualBox kernel: [ 5432.797960] device-mapper: ioctl: error adding target to table
Jun 28 11:33:08 msali014-VirtualBox udevd[619]: inotify_add_watch(6, /dev/dm-0, 10) failed: No such file or directory
Jun 28 11:33:08 msali014-VirtualBox udevd[619]: inotify_add_watch(6, /dev/dm-0, 10) failed: No such file or directory

[ 5432.797960] device-mapper: ioctl: error adding target to table

How can I change the value of sb->magic to make dm-cache successfully load? Any help would be greatly appreciated.

thisismario123
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2 Answers2

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The multibyte or wide character error message is troubling to me and I don't have any direct advice to working around that.

I assume /dev/sdd1 and /dev/sdb1 are your metadata and data storage block devices? Do they contain any data?

Have you tried zeroing out the metadata volume (dd if=/dev/zero of=/dev/sdd1)? I had issues with when setting up dm-cache a while back.

In a nutshell I do the following (on Ubuntu 13.04 + Linux 3.10 release):

dmsetup create ssd-metadata --table '0 19370 linear /dev/disk/by-id/scsi-SATA_OCZ-AGILITY2_f2d200034-part6 0'
dmsetup create ssd-blocks --table '0 189008982 linear /dev/disk/by-id/scsi-SATA_OCZ-AGILITY2_f2d200034-part6 19370'
dmsetup create home-cached --table '0 1048576000 cache /dev/mapper/ssd-metadata /dev/mapper/ssd-blocks /dev/vg0/spindle 512 1 writeback default 0'

On a side note, I ran the 3.9.6 and a few earlier 3.9 kernels without issue on Ubuntu 12.11 and 13.04.

If all else fails, I have a working solution for my setup on my blog with more details, you might want to check out for a step by step tutorial.

2bluesc
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  • The multibyte looks like an umbrella error message due to me not completely understanding the table and giving it some garbage input, I created new drives but didn't create the ssd-metadata or blocks as you did, but after doing that and getting the correct offset for the home-cached device, it works great! Thanks for all of your help. – thisismario123 Jul 01 '13 at 18:26
  • Glad you got it working. There's no reason your dedicated partitions or dedicated devices shouldn't work. Long term I may use lvm2 to manage my dm-linear regions instead of managing them myself. – 2bluesc Jul 01 '13 at 22:29
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I had the exact same problem:

[  968.960618] device-mapper: cache metadata: sb_check failed: blocknr 985712174465152: wanted 0

Copying 0 to my metadata device fixed the problem. 

dd bs=64k if=/dev/zero of=/dev/md1

Thanks @Kyle.

David B
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