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From: David Sterba <dsterba@suse.cz>
To: Dennis Zhou <dennis@kernel.org>
Cc: David Sterba <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 00/12] btrfs: async discard follow up
Date: Mon, 6 Jan 2020 18:49:01 +0100	[thread overview]
Message-ID: <20200106174901.GP3929@twin.jikos.cz> (raw)
In-Reply-To: <20200106172847.GB16428@dennisz-mbp>

On Mon, Jan 06, 2020 at 12:28:47PM -0500, Dennis Zhou wrote:
> On Mon, Jan 06, 2020 at 05:30:01PM +0100, David Sterba wrote:
> > Is it expected to leave the counters in a state where are discardable
> > extents but not process after a long period of time? I found
> > 
> > discard_bitmap_bytes:316833792
> > discard_bytes_saved:59390722048
> > discard_extent_bytes:26122764288
> > discardable_bytes:44863488
> > discardable_extents:883
> > iops_limit:10
> > kbps_limit:0
> > max_discard_size:67108864
> > 
> > there was activity when the number of extents wen from about 2000 to
> > that value (833), so this could bea nother instance of the -1 accounting
> > bug.
> 
> There is no guarantee each invocation of the work item will find
> something to discard. This was designed to prevent any edge case from
> consuming the cpu.
> 
> If free space is added back while a block_group has it's cursor being
> moved (unless it's fully free), it will not go back and trim those
> extents. So we may leave stuff untrimmed until the next time around.
> This is also to prevent a pathological case of just resetting in the
> same block_group. Therefore, we may be in a situation where we have
> discardable extents, but we aren't actively discarding it. The premise
> is some filesystem usage will eventually occur and kick it back onto the
> list. This also works because btrfs tries to reuse block groups before
> allocating another one.

Ok I see, thanks. Removing all the data again followed by balance
reached the state with 1 extent some small size (corresponds to 'used'
of data block gruops), which could be explained by the above as well.

      reply	other threads:[~2020-01-06 17:49 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-02 21:26 [PATCH 00/12] btrfs: async discard follow up Dennis Zhou
2020-01-02 21:26 ` [PATCH 01/12] btrfs: calculate discard delay based on number of extents Dennis Zhou
2020-01-03 14:38   ` David Sterba
2020-01-02 21:26 ` [PATCH 02/12] btrfs: add bps discard rate limit for async discard Dennis Zhou
2020-01-03 14:40   ` David Sterba
2020-01-02 21:26 ` [PATCH 03/12] btrfs: limit max discard size " Dennis Zhou
2020-01-03 14:41   ` David Sterba
2020-01-02 21:26 ` [PATCH 04/12] btrfs: make max async discard size tunable Dennis Zhou
2020-01-03 14:44   ` David Sterba
2020-01-02 21:26 ` [PATCH 05/12] btrfs: have multiple discard lists Dennis Zhou
2020-01-02 21:26 ` [PATCH 06/12] btrfs: only keep track of data extents for async discard Dennis Zhou
2020-01-02 21:26 ` [PATCH 07/12] btrfs: keep track of discard reuse stats Dennis Zhou
2020-01-02 21:26 ` [PATCH 08/12] btrfs: add async discard header Dennis Zhou
2020-01-02 21:26 ` [PATCH 09/12] btrfs: increase the metadata allowance for the free_space_cache Dennis Zhou
2020-01-02 21:26 ` [PATCH 10/12] btrfs: make smaller extents more likely to go into bitmaps Dennis Zhou
2020-01-02 21:26 ` [PATCH 11/12] btrfs: ensure removal of discardable_* in free_bitmap() Dennis Zhou
2020-01-02 21:26 ` [PATCH 12/12] btrfs: add correction to handle -1 edge case in async discard Dennis Zhou
2020-01-03 14:42   ` David Sterba
2020-01-05 20:35   ` Nikolay Borisov
2020-01-06 13:44     ` David Sterba
2020-01-03 14:51 ` [PATCH 00/12] btrfs: async discard follow up David Sterba
2020-01-03 17:43   ` Dennis Zhou
2020-01-06 15:25 ` David Sterba
2020-01-06 17:14   ` Dennis Zhou
2020-01-06 17:37     ` David Sterba
2020-01-06 16:30 ` David Sterba
2020-01-06 17:28   ` Dennis Zhou
2020-01-06 17:49     ` David Sterba [this message]

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