[linux-lvm] lvconvert --uncache takes hours

[Roger Heflin]

On Wed, Mar 1, 2023 at 4:50 PM Roy Sigurd Karlsbakk <roy at karlsbakk.net> wrote:
>
> Hi all
>
> Working with a friend's machine, it has lvmcache turned on with writeback. This has worked well, but now it's uncaching and it takes *hours*. The amount of cache was chosen to 100GB on an SSD not used for much else and the dataset that is being cached, is a RAID-6 set of 10x2TB with XFS on top. The system mainly works with file serving, but also has some VMs that benefit from the caching quite a bit. But then - I wonder - how can it spend hours emptying the cache like this? Most write caching I know of last only seconds or perhaps in really worst case scenarios, minutes. Since this is taking hours, it looks to me something should have been flushed ages ago.
>
> Have I (or we) done something very stupid here or is this really how it's supposed to work?
>
> Vennlig hilsen
>
> roy

A spinning raid6 array is slow on writes (see raid6  write penalty).
Because of that the array can only do about 100 write operattions/sec.

If the disk is doing other work then it only has the extra capacity so
it could destage slower.

A lot depends on how big each chunk is.     The lvmcache indicates the
smallest chunksize is 32k.

100G / 32k = 3 million, and at 100seeks/sec that comes to at least an hour.

Lvm bookkeeping has to also be written to the spinning disks I would
think, so 2 hours if the array were idle.

Throw in a 50% baseload on the disks and you get 4 hours.

Hours is reasonable.