On 2/2/18 4:35 PM, Dave Chinner wrote:
> On Fri, Feb 02, 2018 at 04:45:33PM -0500, Todd Gill wrote:
>>
>> Hi,
>>
>> I'd like to be able to take a snapshot of an XFS filesystem on top of
>> dm-thin.
>>
>> The goal for the snapshot is:
>>
>> - fast to complete (couple seconds or better)
>> - independent of origin (UUID updated?)
>> - read/write
>>
>> To satisfy the independent goal, I'm updating the UUID on the snapshot.
>> Currently "xfs_admin -U xxx" takes in the ballpark of 14 seconds on my
>> test system.
>
> There's a very good chance that this is caused by dm-thinp doing
> COW for the storage underlying the superblocks modified by the UUID
> change.
It's actually because we rewrite the whole log, because it has the
user-facing superblock stamped in it.
I looked at this a bit, and didn't immediately see an easy way around
xfs_db's need to rewrite the whole log.
uuid_f()
/* clear the log (setting uuid) if it's not dirty */
if (!sb_logzero(&uu))
return 0;
so I think that's taking the bulk of the time. I haven't looked closely
enough at all the mount-time log parsing to see if we could maybe write
less of it and still be valid.
> Best to start by measuring how much IO is going on when you change
> the UUID. e.g. `iostat -x -d -m 1`. An strace showing how long the
> xfs_db read() and write() calls to the dm-thinp device would lso be
> instructive.
>
>> Is there a better approach to achieve the goals?
>
> No, you are doing the right thing.
>
>> Or can the UUID update
>> be changed to be faster?
>
> Not sure - there's a fixed amount of modification we need to make
> to change the UUID so I'm not sure there is any optimisations we
> can make here.
In retrospect I think we (I) should have left the "metadata" uuid as
the one to match the log, not the user-facing one.
I'm not sure there's a nice backwards-compatible way to un-ring
that bell, though.
-eric
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