On 16.5.2016 20:55, Mike Snitzer wrote:
On Fri, May 13 2016 at 6:39am -0400,
Gionatan Danti <g.danti@assyoma.it> wrote:
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.
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
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