Re: [PATCH v6 00/22] btrfs: async discard support

[Dennis Zhou <dennis@kernel.org>]

On Tue, Dec 17, 2019 at 03:55:41PM +0100, David Sterba wrote:
> On Fri, Dec 13, 2019 at 04:22:09PM -0800, Dennis Zhou wrote:
> > Hello,
> > 
> > Dave reported a lockdep issue [1]. I'm a bit surprised as I can't repro
> > it, but it obviously is right. I believe I fixed the issue by moving the
> > fully trimmed check outside of the block_group lock.  I mistakingly
> > thought the btrfs_block_group lock subsumed btrfs_free_space_ctl
> > tree_lock. This clearly isn't the case.
> > 
> > Changes in v6:
> >  - Move the fully trimmed check outside of the block_group lock.
> 
> v6 passed fstests, with some weird test failures that don't seem to be
> related to the patchset.

Yay!

> 
> Meanwhile I did manual test how the discard behaves. The workload was
> a series of linux git checkouts of various release tags (ie. this should
> provide some freed extents and coalesce them eventually to get larger
> chunks to discard), then a simple large file copy, sync, remove, sync.
> 
> The discards going down to the device were followin the maximum default
> size (64M) but I observed that only one range was discarded per 10
> seconds, while the other stats there are many more extents to discard.
> 
> For the large file it took like 5-10 cycles to send all the trimmed
> ranges, the discardable_extents decreased by one each time until it
> reached ... -1. At this point the discardable bytes were -16384, so
> thre's some accounting problem.
> 
> This happened also when I deleted everything from the filesystem and ran
> full balance.
> 

Oh no :(. I've been trying to repro with some limited checking out and
syncing, then subsequently removing everything and calling balance. It
is coming out to be 0 for me. I'll try harder to repro this and fix it.

> Regarding the slow io submission, I tried to increase the iops value,
> default was 10, but 100 and 1000 made no change. Increasing the maximum
> discard request size to 128M worked (when there was such long extent
> ready). I was expecting a burst of like 4 consecutive IOs after a 600MB
> file is deleted.  I did not try to tweak bps_limit because there was
> nothing to limit.
> 

Ah so there's actually a max time between discards set to 10 seconds as
the maximum timeout is calculated over 6 hours. So if we only have 6
extents, we'd discard 1 per hour(ish given it decays), but this is
clamped to 10 seconds.

At least on our servers, we seem to discard at a reasonable rate to
prevent performance penalties during a large number of reads and writes
while maintaining reasonable write amplification performance. Also,
metadata blocks aren't tracked, so on freeing of a whole metadata block
group (minus relocation), we'll trickle discards slightly slower than
expected.


> So this is something to fix but otherwise the patchset seems to be ok
> for adding to misc-next. Due to the timing of the end of the year and
> that we're already at rc2, this will be the main feature in 5.6.

I'll report back if I continue having trouble reproing it.

Thanks v5.6 sounds good to me!
Dennis