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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 04/19] btrfs: keep track of cleanliness of the bitmap
Date: Fri, 11 Oct 2019 12:17:09 -0400
Message-ID: <20191011161709.GC29672@dennisz-mbp> (raw)
In-Reply-To: <20191010141629.xzlwkf6tn57dsdnv@macbook-pro-91.dhcp.thefacebook.com>

On Thu, Oct 10, 2019 at 10:16:30AM -0400, Josef Bacik wrote:
> On Mon, Oct 07, 2019 at 04:17:35PM -0400, Dennis Zhou wrote:
> > There is a cap in btrfs in the amount of free extents that a block group
> > can have. When it surpasses that threshold, future extents are placed
> > into bitmaps. Instead of keeping track of if a certain bit is trimmed or
> > not in a second bitmap, keep track of the relative state of the bitmap.
> > 
> > With async discard, trimming bitmaps becomes a more frequent operation.
> > As a trade off with simplicity, we keep track of if discarding a bitmap
> > is in progress. If we fully scan a bitmap and trim as necessary, the
> > bitmap is marked clean. This has some caveats as the min block size may
> > skip over regions deemed too small. But this should be a reasonable
> > trade off rather than keeping a second bitmap and making allocation
> > paths more complex. The downside is we may overtrim, but ideally the min
> > block size should prevent us from doing that too often and getting stuck
> > trimming
> > pathological cases.
> > 
> > BTRFS_FSC_TRIMMING_BITMAP is added to indicate a bitmap is in the
> > process of being trimmed. If additional free space is added to that
> > bitmap, the bit is cleared. A bitmap will be marked BTRFS_FSC_TRIMMED if
> > the trimming code was able to reach the end of it and the former is
> > still set.
> > 
> > Signed-off-by: Dennis Zhou <dennis@kernel.org>
> 
> I went through and looked at the end result and it appears to me that we never
> have TRIMMED and TRIMMING set at the same time.  Since these are the only two
> flags, and TRIMMING is only set on bitmaps, it makes more sense for this to be
> more like
> 
> enum btrfs_trim_state {
> 	BTRFS_TRIM_STATE_TRIMMED,
> 	BTRFS_TRIM_STATE_TRIMMING,
> 	BTRFS_TRIM_STATE_UNTRIMMED,
> };
> 
> and then just have enum btrfs_trim_state trim_state in the free space entry.
> This makes things a bit cleaner since it's really just a state indicator rather
> than a actual flags.  Thanks,
> 

That makes sense. I've gone ahead and done this for both this state and
the block group discard state. FWIW, at the time I didn't know what I
would need and flags was just easier to iterate on. I agree this is
nicer.

Thanks,
Dennis

  reply index

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 [this message]
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

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