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From: Dennis Zhou <dennis@kernel.org>
To: dsterba@suse.cz, David Sterba <dsterba@suse.com>,
	Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
	Omar Sandoval <osandov@osandov.com>,
	kernel-team@fb.com, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH v6 00/22] btrfs: async discard support
Date: Thu, 19 Dec 2019 15:22:28 -0600
Message-ID: <20191219212228.GB38803@dennisz-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <20191219200607.GQ3929@twin.jikos.cz>

On Thu, Dec 19, 2019 at 09:06:07PM +0100, David Sterba wrote:
> On Wed, Dec 18, 2019 at 08:03:37PM -0600, Dennis Zhou wrote:
> > > > This happened also when I deleted everything from the filesystem and ran
> > > > full balance.
> > 
> > Also were these both on fresh file systems so it seems reproducible for
> > you?
> 
> Yes the filesystem was freshly created before the test.
> 
> No luck reproducing it, I tried to repeat the steps as before but the
> timing must make a difference and the numbers always ended up as 0
> (bytes) 0 (extents).
> 
> > > I'll report back if I continue having trouble reproing it.
> > 
> > I spent the day trying to repro against ext/dzhou-async-discard-v6
> > without any luck... I've been running the following:
> > 
> > $ mkfs.btrfs -f /dev/nvme0n1
> > $ mount -t btrfs -o discard=async /dev/nvme0n1 mnt
> > $ cd mnt
> > $ bash ../age_btrfs.sh .
> > 
> > where age_btrfs.sh is from [1].
> > 
> > If I delete arbitrary subvolumes, sync, and then run balance:
> > $ btrfs balance start --full-balance .
> > It all seems to resolve to 0 after some time. I haven't seen a negative
> > case on either of my 2 boxes. I've also tried unmounting and then
> > remounting, deleting and removing more free space items.
> > 
> > I'm still considering how this can happen. Possibly bad load of free
> > space cache and then freeing of the block group? Because being off by
> > just 1 and it not accumulating seems to be a real corner case here.
> > 
> > Adding asserts in btrfs_discard_update_discardable() might give us
> > insight to which callsite is responsible for going below 0.
> 
> Yeah more asserts would be good.

I'll add a few assert patches and some code to ensure that life can
still move on properly if we do hit the -1 case. I think it probably has
something to do with free space cache removal as it can't be a simple
corner case, otherwise we'd see the -1 accumulating much more easily.
What does puzzle me is it's a single nodesize that I'm off by and not
some other random number.

Thanks,
Dennis

  reply index

Thread overview: 49+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-14  0:22 Dennis Zhou
2019-12-14  0:22 ` [PATCH 01/22] bitmap: genericize percpu bitmap region iterators Dennis Zhou
2019-12-14  0:22 ` [PATCH 02/22] btrfs: rename DISCARD opt to DISCARD_SYNC Dennis Zhou
2019-12-14  0:22 ` [PATCH 03/22] btrfs: keep track of which extents have been discarded Dennis Zhou
2019-12-14  0:22 ` [PATCH 04/22] btrfs: keep track of cleanliness of the bitmap Dennis Zhou
2019-12-14  0:22 ` [PATCH 05/22] btrfs: add the beginning of async discard, discard workqueue Dennis Zhou
2019-12-14  0:22 ` [PATCH 06/22] btrfs: handle empty block_group removal Dennis Zhou
2019-12-14  0:22 ` [PATCH 07/22] btrfs: discard one region at a time in async discard Dennis Zhou
2019-12-14  0:22 ` [PATCH 08/22] btrfs: add removal calls for sysfs debug/ Dennis Zhou
2019-12-18 11:45   ` Anand Jain
2019-12-14  0:22 ` [PATCH 09/22] btrfs: make UUID/debug have its own kobject Dennis Zhou
2019-12-18 11:45   ` Anand Jain
2019-12-14  0:22 ` [PATCH 10/22] btrfs: add discard sysfs directory Dennis Zhou
2019-12-18 11:45   ` Anand Jain
2019-12-14  0:22 ` [PATCH 11/22] btrfs: track discardable extents for async discard Dennis Zhou
2019-12-14  0:22 ` [PATCH 12/22] btrfs: keep track of discardable_bytes Dennis Zhou
2019-12-14  0:22 ` [PATCH 13/22] btrfs: calculate discard delay based on number of extents Dennis Zhou
2019-12-30 16:50   ` David Sterba
2020-01-02 16:45     ` Dennis Zhou
2019-12-14  0:22 ` [PATCH 14/22] btrfs: add bps discard rate limit Dennis Zhou
2019-12-30 17:58   ` David Sterba
2020-01-02 16:46     ` Dennis Zhou
2019-12-14  0:22 ` [PATCH 15/22] btrfs: limit max discard size for async discard Dennis Zhou
2019-12-30 18:00   ` David Sterba
2019-12-30 18:08   ` David Sterba
2020-01-02 16:48     ` Dennis Zhou
2019-12-14  0:22 ` [PATCH 16/22] btrfs: make max async discard size tunable Dennis Zhou
2019-12-30 18:05   ` David Sterba
2020-01-02 16:50     ` Dennis Zhou
2019-12-14  0:22 ` [PATCH 17/22] btrfs: have multiple discard lists Dennis Zhou
2019-12-14  0:22 ` [PATCH 18/22] btrfs: only keep track of data extents for async discard Dennis Zhou
2019-12-30 17:39   ` David Sterba
2020-01-02 16:55     ` Dennis Zhou
2019-12-14  0:22 ` [PATCH 19/22] btrfs: keep track of discard reuse stats Dennis Zhou
2019-12-30 17:33   ` David Sterba
2020-01-02 16:57     ` Dennis Zhou
2019-12-14  0:22 ` [PATCH 20/22] btrfs: add async discard header Dennis Zhou
2019-12-14  0:22 ` [PATCH 21/22] btrfs: increase the metadata allowance for the free_space_cache Dennis Zhou
2019-12-14  0:22 ` [PATCH 22/22] btrfs: make smaller extents more likely to go into bitmaps Dennis Zhou
2019-12-17 14:55 ` [PATCH v6 00/22] btrfs: async discard support David Sterba
2019-12-18  0:06   ` Dennis Zhou
2019-12-19  2:03     ` Dennis Zhou
2019-12-19 20:06       ` David Sterba
2019-12-19 21:22         ` Dennis Zhou [this message]
2019-12-19 20:34     ` David Sterba
2019-12-19 21:17       ` Dennis Zhou
2019-12-30 18:13 ` David Sterba
2019-12-30 18:49   ` Dennis Zhou
2020-01-02 13:22     ` David Sterba

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