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From: Dennis Zhou <dennis@kernel.org>
To: David Sterba <dsterba@suse.cz>
Cc: Josef Bacik <josef@toxicpanda.com>,
	David Sterba <dsterba@suse.com>, Chris Mason <clm@fb.com>,
	Omar Sandoval <osandov@osandov.com>,
	kernel-team@fb.com, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 15/19] btrfs: load block_groups into discard_list on mount
Date: Tue, 15 Oct 2019 11:42:25 -0400	[thread overview]
Message-ID: <20191015154225.GB66037@dennisz-mbp> (raw)
In-Reply-To: <20191014233825.GR2751@twin.jikos.cz>

On Tue, Oct 15, 2019 at 01:38:25AM +0200, David Sterba wrote:
> On Mon, Oct 14, 2019 at 04:17:46PM -0400, Dennis Zhou wrote:
> > On Thu, Oct 10, 2019 at 01:11:38PM -0400, Josef Bacik wrote:
> > > On Mon, Oct 07, 2019 at 04:17:46PM -0400, Dennis Zhou wrote:
> > > > Async discard doesn't remember the discard state of a block_group when
> > > > unmounting or when we crash. So, any block_group that is not fully used
> > > > may have undiscarded regions. However, free space caches are read in on
> > > > demand. Let the discard worker read in the free space cache so we can
> > > > proceed with discarding rather than wait for the block_group to be used.
> > > > This prevents us from indefinitely deferring discards until that
> > > > particular block_group is reused.
> > > > 
> > > > Signed-off-by: Dennis Zhou <dennis@kernel.org>
> > > 
> > > What if we did completely discard the last time, now we're going back and
> > > discarding again?  I think by default we just assume we discarded everything.
> > > If we didn't then the user can always initiate a fitrim later.  Drop this one.
> > > Thanks,
> > > 
> > 
> > Yeah this is something I wasn't sure about.
> > 
> > It makes me a little uncomfortable to make the lack of persistence a
> > user problem. If in some extreme case where someone frees a large amount
> > of space and then unmounts.
> 
> Based on past experience, umount should not be slowed down unless really
> necessary.
> 
> > We can either make them wait on unmount to
> > discard everything or retrim the whole drive which in an ideal world
> > should just be a noop on already free lba space.
> 
> Without persistence of the state, we can't make it perfect and I think,
> without any hard evidence, that trimming already trimmed blocks is no-op
> on the device. We all know that we don't know what SSDs actually do, so
> it's best effort and making it "device problem" is a good solution from
> filesystem POV.

That makes sense and sounds good to me. I've dropped this patch.

Thanks,
Dennis

  reply	other threads:[~2019-10-15 15:42 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 [this message]
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

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