Backup superblock for thin provision?
Hi all,
using thin provisioning in production machines (using it mostly for its fast snapshot support, rather than for thin provision / storage overcommit by itself), I wonder what to do if a critical metadata corruption, as the loss of the superblock, should happen.
Filesystems generally have some backup copy of the superblock; should the primary one fail, another copy can be used.
So I have the following questions:
- how about thin LVM? Has it a backup superblock somewhere?
- how can the metadata be reliable backupped without shutting down the volume?
- more generally, how to deal with metadata backup? Does vgcfgrestore works for thin volumes?
Thank you all.
Signature · 5 lines
Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: [email protected] - [email protected] GPG public key ID: FF5F32A8
Quoted 9 lines · Gionatan Danti
Hi all,
using thin provisioning in production machines (using it mostly for its fast snapshot support, rather than for thin provision / storage overcommit by itself), I wonder what to do if a critical metadata corruption, as the loss of the superblock, should happen.Filesystems generally have some backup copy of the superblock; should the primary one fail, another copy can be used.
Quoted 8 lines
So I have the following questions:
- how about thin LVM? Has it a backup superblock somewhere?
- how can the metadata be reliable backupped without shutting down the volume?
- more generally, how to deal with metadata backup? Does vgcfgrestore works for thin volumes?Thank you all.
There is more to the thinp metadata than just the metadata superblock.
The DM thin-pool's metadata device was purposely split out from the data device to allow for additional metadata fault protection using RAID.
I'll defer to the LVM developers for if/how LVM can be used to have thinp metadata redundancy even if you don't have multiple devices to be able to use a conventional RAID device.
Quoted 21 lines · Mike Snitzer
Quoted 9 lines · Gionatan Danti
Hi all,
using thin provisioning in production machines (using it mostly for its fast snapshot support, rather than for thin provision / storage overcommit by itself), I wonder what to do if a critical metadata corruption, as the loss of the superblock, should happen.Filesystems generally have some backup copy of the superblock; should the primary one fail, another copy can be used.
Quoted 8 lines
So I have the following questions:
- how about thin LVM? Has it a backup superblock somewhere?
- how can the metadata be reliable backupped without shutting down the volume?
- more generally, how to deal with metadata backup? Does vgcfgrestore works for thin volumes?Thank you all.
Quoted 8 lines
There is more to the thinp metadata than just the metadata superblock.
The DM thin-pool's metadata device was purposely split out from the data device to allow for additional metadata fault protection using RAID.
I'll defer to the LVM developers for if/how LVM can be used to have thinp metadata redundancy even if you don't have multiple devices to be able to use a conventional RAID device.
There is always the option to take 'metadata' snapshot and just thin_dump content of metadata to a file (located in some 'safe' place)
However validation of 'restore' if there are some 'snapshots' is questionable as the b-tree describing mapped blocks may change significantly so 'rescued' content may than reference lots of bad blocks.
If you want to just protect 'superblock' against disk fault - usage
of 'raid' could be an option - but ATM there are some 'related' costs with management of 'stacked' device tree.
Regards
Zdenek
Quoted 5 lines
There is always the option to take 'metadata' snapshot and just
thin_dumpcontent of metadata to a file (located in some 'safe' place)
Something as "dmsetup reserve_metadata_snap", thin_dump, "dmsetup release_metadata_snap" ? If so, I'm already using for testing purpose ;)
Quoted 4 lines
However validation of 'restore' if there are some 'snapshots' is questionable as the b-tree describing mapped blocks may change significantly
so 'rescued' content may than reference lots of bad blocks.
Sure, with snapshots the situation is surely more complex, as due to the CoW the old metadata can point to stale data, right?
Quoted 5 lines
If you want to just protect 'superblock' against disk fault - usage of 'raid' could be an option - but ATM there are some 'related' costs with management of 'stacked' device tree.
Both my tmeta and tdata reside on a RAID6 array provided by a BBU-protected RAID card, so I should be safe here.
Signature · 5 lines
Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: [email protected] - [email protected] GPG public key ID: FF5F32A8
Quoted 10 lines · Mike Snitzer
There is more to the thinp metadata than just the metadata superblock.
The DM thin-pool's metadata device was purposely split out from the data device to allow for additional metadata fault protection using RAID.
I'll defer to the LVM developers for if/how LVM can be used to have thinp metadata redundancy even if you don't have multiple devices to be able to use a conventional RAID device.
Sure - I'm using thinp on a RAID6 volume, with two disks redundancy.
But what about an user error, or a lvm-related bug, which invalidate the superblock? In this case, RAID is of no help. On the other side, a superblock backup copy can be used to restore the system without major data loss.
More in general, how do you deal with metadata backups? Thanks.
Signature · 5 lines
Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: [email protected] - [email protected] GPG public key ID: FF5F32A8