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From: Dennis Zhou <dennis@kernel.org>
To: David Sterba <dsterba@suse.cz>
Cc: Dennis Zhou <dennis@kernel.org>, 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: [RFC PATCH 00/19] btrfs: async discard support
Date: Tue, 15 Oct 2019 11:41:33 -0400	[thread overview]
Message-ID: <20191015154133.GA66037@dennisz-mbp> (raw)
In-Reply-To: <20191015120831.GS2751@twin.jikos.cz>

On Tue, Oct 15, 2019 at 02:08:31PM +0200, David Sterba wrote:
> Hi,
> 
> thanks for working on this. The plain -odiscard hasn't been recommended
> to users for a long time, even with the SATA 3.1 drives that allow
> queueing the requests.
> 
> The overall approach to async discard sounds good, the hard part is not
> shoot down the filesystem by trimming live data, and we had a bug like
> that in the pastlive data, and we had a bug like that in the past. For
> correctness reasons I understand the size of the patchset.
> 
> On Mon, Oct 07, 2019 at 04:17:31PM -0400, Dennis Zhou wrote:
> > I am currently working on tuning the rate at which it discards in the
> > background. I am doing this by evaluating other workloads and drives.
> > The iops and bps rate limits are fairly aggressive right now as my
> > basic survey of a few drives noted that the trim command itself is a
> > significant part of the overhead. So optimizing for larger trims is the
> > right thing to do.
> 
> We need a sane default behaviour, without the need for knobs and
> configuration, so it's great you can have a wide range of samples to
> tune it.
> 

Yeah I'm just not quite sure what that is yet. The tricky part is that
we don't really get burned by trims until well after we've issued the
trims. The feedback loop is not really transparent with reads and
writes.

> As trim is only a hint, a short delay in processing the requests or
> slight ineffectivity should be acceptable, to avoid complications in the
> code or interfering with other IO.
> 
> > Persistence isn't supported, so when we mount a filesystem, the block
> > groups are read in as dirty and background trim begins. This makes async
> > discard more useful for longer running mount points.
> 
> I think this is acceptable.
> 
> Regarding the code, I leave comments to the block group and trim
> structures to Josef and will focus more on the low-level and coding
> style or changelogs.

Sounds good. I'll hopefully post a v2 shortly so that we can get more of
that out of the way. Then hopefully a smaller incremental later to
figure out the configuration part.

Thanks,
Dennis

      reply	other threads:[~2019-10-15 15:41 UTC|newest]

Thread overview: 71+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-07 20:17 [RFC PATCH 00/19] btrfs: async discard support Dennis Zhou
2019-10-07 20:17 ` [PATCH 01/19] bitmap: genericize percpu bitmap region iterators Dennis Zhou
2019-10-07 20:26   ` Josef Bacik
2019-10-07 22:24     ` Dennis Zhou
2019-10-15 12:11       ` David Sterba
2019-10-15 18:35         ` Dennis Zhou
2019-10-07 20:17 ` [PATCH 02/19] btrfs: rename DISCARD opt to DISCARD_SYNC Dennis Zhou
2019-10-07 20:27   ` Josef Bacik
2019-10-08 11:12   ` Johannes Thumshirn
2019-10-11  9:19   ` Nikolay Borisov
2019-10-07 20:17 ` [PATCH 03/19] btrfs: keep track of which extents have been discarded Dennis Zhou
2019-10-07 20:37   ` Josef Bacik
2019-10-07 22:38     ` Dennis Zhou
2019-10-10 13:40       ` Josef Bacik
2019-10-11 16:15         ` Dennis Zhou
2019-10-08 12:46   ` Nikolay Borisov
2019-10-11 16:08     ` Dennis Zhou
2019-10-15 12:17   ` David Sterba
2019-10-15 19:58     ` Dennis Zhou
2019-10-07 20:17 ` [PATCH 04/19] btrfs: keep track of cleanliness of the bitmap Dennis Zhou
2019-10-10 14:16   ` Josef Bacik
2019-10-11 16:17     ` Dennis Zhou
2019-10-15 12:23   ` David Sterba
2019-10-07 20:17 ` [PATCH 05/19] btrfs: add the beginning of async discard, discard workqueue Dennis Zhou
2019-10-10 14:38   ` Josef Bacik
2019-10-15 12:49   ` David Sterba
2019-10-15 19:57     ` Dennis Zhou
2019-10-07 20:17 ` [PATCH 06/19] btrfs: handle empty block_group removal Dennis Zhou
2019-10-10 15:00   ` Josef Bacik
2019-10-11 16:52     ` Dennis Zhou
2019-10-07 20:17 ` [PATCH 07/19] btrfs: discard one region at a time in async discard Dennis Zhou
2019-10-10 15:22   ` Josef Bacik
2019-10-14 19:42     ` Dennis Zhou
2019-10-07 20:17 ` [PATCH 08/19] btrfs: track discardable extents for asnyc discard Dennis Zhou
2019-10-10 15:36   ` Josef Bacik
2019-10-14 19:50     ` Dennis Zhou
2019-10-15 13:12   ` David Sterba
2019-10-15 18:41     ` Dennis Zhou
2019-10-07 20:17 ` [PATCH 09/19] btrfs: keep track of discardable_bytes Dennis Zhou
2019-10-10 15:38   ` Josef Bacik
2019-10-07 20:17 ` [PATCH 10/19] btrfs: calculate discard delay based on number of extents Dennis Zhou
2019-10-10 15:41   ` Josef Bacik
2019-10-11 18:07     ` Dennis Zhou
2019-10-07 20:17 ` [PATCH 11/19] btrfs: add bps discard rate limit Dennis Zhou
2019-10-10 15:47   ` Josef Bacik
2019-10-14 19:56     ` Dennis Zhou
2019-10-07 20:17 ` [PATCH 12/19] btrfs: limit max discard size for async discard Dennis Zhou
2019-10-10 16:16   ` Josef Bacik
2019-10-14 19:57     ` Dennis Zhou
2019-10-07 20:17 ` [PATCH 13/19] btrfs: have multiple discard lists Dennis Zhou
2019-10-10 16:51   ` Josef Bacik
2019-10-14 20:04     ` Dennis Zhou
2019-10-07 20:17 ` [PATCH 14/19] btrfs: only keep track of data extents for async discard Dennis Zhou
2019-10-10 16:53   ` Josef Bacik
2019-10-07 20:17 ` [PATCH 15/19] btrfs: load block_groups into discard_list on mount Dennis Zhou
2019-10-10 17:11   ` Josef Bacik
2019-10-14 20:17     ` Dennis Zhou
2019-10-14 23:38       ` David Sterba
2019-10-15 15:42         ` Dennis Zhou
2019-10-07 20:17 ` [PATCH 16/19] btrfs: keep track of discard reuse stats Dennis Zhou
2019-10-10 17:13   ` Josef Bacik
2019-10-07 20:17 ` [PATCH 17/19] btrfs: add async discard header Dennis Zhou
2019-10-10 17:13   ` Josef Bacik
2019-10-07 20:17 ` [PATCH 18/19] btrfs: increase the metadata allowance for the free_space_cache Dennis Zhou
2019-10-10 17:16   ` Josef Bacik
2019-10-07 20:17 ` [PATCH 19/19] btrfs: make smaller extents more likely to go into bitmaps Dennis Zhou
2019-10-10 17:17   ` Josef Bacik
2019-10-11  7:49 ` [RFC PATCH 00/19] btrfs: async discard support Nikolay Borisov
2019-10-14 21:05   ` Dennis Zhou
2019-10-15 12:08 ` David Sterba
2019-10-15 15:41   ` Dennis Zhou [this message]

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