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From: Dennis Zhou <dennis@kernel.org>
To: Josef Bacik <josef@toxicpanda.com>
Cc: 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: Mon, 14 Oct 2019 16:17:46 -0400	[thread overview]
Message-ID: <20191014201746.GF40077@dennisz-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <20191010171137.xxuhjvmqzgifuixd@macbook-pro-91.dhcp.thefacebook.com>

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. 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. If others are in favor
of just going the fitrim route for users, I'm happy to drop this patch,
but I do like the fact that this makes the whole system consistent
without user intervention. Does anyone else have an opinion?

On a side note, the find_free_extent() allocator tries pretty hard
before allocating subsequent block groups. So maybe it's right to just
deprioritize these block groups instead of just not loading them.

Thanks, 
Dennis

  reply	other threads:[~2019-10-14 20:17 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 [this message]
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

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