Re: Highly reflinked and fragmented considered harmful?

[Chris Dunlop <chris@onthe.net.au>]

On Wed, May 11, 2022 at 12:52:35PM +1000, Dave Chinner wrote:
> On Wed, May 11, 2022 at 12:16:57PM +1000, Chris Dunlop wrote:
>> Out of interest, would this work also reduce the time spent mounting in 
>> my case? I.e. would a lot of the work from my recovery mount be punted 
>> off to a background thread?
>
> No. log recovery will punt the remaining inodegc work to background
> threads so it might get slightly parallelised, but we have a hard
> barrier between completing log recovery work and completing the
> mount process at the moment. Hence we wait for inodegc to complete
> before log recovery is marked as complete.
>
> In theory we could allow background inodegc to bleed into active
> duty once log recovery has processed all the unlinked lists, but
> that's a change of behaviour that would require a careful audit of
> the last part of the mount path to ensure that it is safe to be
> running concurrent background operations whilst complete mount state
> updates.
>
> This hasn't been on my radar at all up until now, but I'll have a
> think about it next time I look at those bits of recovery. I suspect
> that probably won't be far away - I have a set of unlinked inode
> list optimisations that rework the log recovery infrastructure near
> the top of my current work queue, so I will be in that general
> vicinity over the next few weeks...

I'll keep an eye out.

> Regardless, the inodegc work is going to be slow on your system no
> matter what we do because of the underlying storage layout. What we
> need to do is try to remove all the places where stuff can get
> blocked on inodegc completion, but that is somewhat complex because
> we still need to be able to throttle queue depths in various
> situations.

That reminds of a something I've been wondering about for obvious reasons: 
for workloads where metadata operations are dominant, do you have any 
ponderings on allowing AGs to be put on fast storage whilst the bulk data 
is on molasses storage?

Cheers,

Chris