[v10,10/21] btrfs: try more times to alloc metadata reserve space
diff mbox

Message ID 1459492512-31435-11-git-send-email-quwenruo@cn.fujitsu.com
State New
Headers show

Commit Message

Qu Wenruo April 1, 2016, 6:35 a.m. UTC
From: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>

In btrfs_delalloc_reserve_metadata(), the number of metadata bytes we try
to reserve is calculated by the difference between outstanding_extents and
reserved_extents.

When reserve_metadata_bytes() fails to reserve desited metadata space,
it has already done some reclaim work, such as write ordered extents.

In that case, outstanding_extents and reserved_extents may already
changed, and we may reserve enough metadata space then.

So this patch will try to call reserve_metadata_bytes() at most 3 times
to ensure we really run out of space.

Such false ENOSPC is mainly caused by small file extents and time
consuming delalloc functions, which mainly affects in-band
de-duplication. (Compress should also be affected, but LZO/zlib is
faster than SHA256, so still harder to trigger than dedupe).

Signed-off-by: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
---
 fs/btrfs/extent-tree.c | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

Comments

David Sterba May 17, 2016, 1:20 p.m. UTC | #1
On Fri, Apr 01, 2016 at 02:35:01PM +0800, Qu Wenruo wrote:
> @@ -5815,6 +5817,23 @@ out_fail:
>  	}
>  	if (delalloc_lock)
>  		mutex_unlock(&BTRFS_I(inode)->delalloc_mutex);
> +	/*
> +	 * The number of metadata bytes is calculated by the difference
> +	 * between outstanding_extents and reserved_extents. Sometimes though
> +	 * reserve_metadata_bytes() fails to reserve the wanted metadata bytes,
> +	 * indeed it has already done some work to reclaim metadata space, hence
> +	 * both outstanding_extents and reserved_extents would have changed and
> +	 * the bytes we try to reserve would also has changed(may be smaller).
> +	 * So here we try to reserve again. This is much useful for online
> +	 * dedupe, which will easily eat almost all meta space.
> +	 *
> +	 * XXX: Indeed here 3 is arbitrarily choosed, it's a good workaround for
> +	 * online dedupe, later we should find a better method to avoid dedupe
> +	 * enospc issue.
> +	 */
> +	if (unlikely(ret == -ENOSPC && loops++ < 3))
> +		goto again;
> +

This does not seem right and needs to be addressed properly before I
consider adding the patchset to for-next. I don't have idea how to fix
it.
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Qu Wenruo May 18, 2016, 12:57 a.m. UTC | #2
David Sterba wrote on 2016/05/17 15:20 +0200:
> On Fri, Apr 01, 2016 at 02:35:01PM +0800, Qu Wenruo wrote:
>> @@ -5815,6 +5817,23 @@ out_fail:
>>  	}
>>  	if (delalloc_lock)
>>  		mutex_unlock(&BTRFS_I(inode)->delalloc_mutex);
>> +	/*
>> +	 * The number of metadata bytes is calculated by the difference
>> +	 * between outstanding_extents and reserved_extents. Sometimes though
>> +	 * reserve_metadata_bytes() fails to reserve the wanted metadata bytes,
>> +	 * indeed it has already done some work to reclaim metadata space, hence
>> +	 * both outstanding_extents and reserved_extents would have changed and
>> +	 * the bytes we try to reserve would also has changed(may be smaller).
>> +	 * So here we try to reserve again. This is much useful for online
>> +	 * dedupe, which will easily eat almost all meta space.
>> +	 *
>> +	 * XXX: Indeed here 3 is arbitrarily choosed, it's a good workaround for
>> +	 * online dedupe, later we should find a better method to avoid dedupe
>> +	 * enospc issue.
>> +	 */
>> +	if (unlikely(ret == -ENOSPC && loops++ < 3))
>> +		goto again;
>> +
>
> This does not seem right and needs to be addressed properly before I
> consider adding the patchset to for-next. I don't have idea how to fix
> it.
>
>
Right, the code is bad, just as Josef pointed out.

In fact this behavior really hides a lot of ENOSPC problem and shows 
quite a lot original metadata reserve limitation of current code.
(Mostly hidden by the default 128M max extent size)

We are actively debugging and testing new root fix for the problem.

Thanks,
Qu


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Mark Fasheh June 1, 2016, 10:14 p.m. UTC | #3
On Tue, May 17, 2016 at 03:20:16PM +0200, David Sterba wrote:
> On Fri, Apr 01, 2016 at 02:35:01PM +0800, Qu Wenruo wrote:
> > @@ -5815,6 +5817,23 @@ out_fail:
> >  	}
> >  	if (delalloc_lock)
> >  		mutex_unlock(&BTRFS_I(inode)->delalloc_mutex);
> > +	/*
> > +	 * The number of metadata bytes is calculated by the difference
> > +	 * between outstanding_extents and reserved_extents. Sometimes though
> > +	 * reserve_metadata_bytes() fails to reserve the wanted metadata bytes,
> > +	 * indeed it has already done some work to reclaim metadata space, hence
> > +	 * both outstanding_extents and reserved_extents would have changed and
> > +	 * the bytes we try to reserve would also has changed(may be smaller).
> > +	 * So here we try to reserve again. This is much useful for online
> > +	 * dedupe, which will easily eat almost all meta space.
> > +	 *
> > +	 * XXX: Indeed here 3 is arbitrarily choosed, it's a good workaround for
> > +	 * online dedupe, later we should find a better method to avoid dedupe
> > +	 * enospc issue.
> > +	 */
> > +	if (unlikely(ret == -ENOSPC && loops++ < 3))
> > +		goto again;
> > +
> 
> This does not seem right and needs to be addressed properly before I
> consider adding the patchset to for-next. I don't have idea how to fix
> it.

Agreed, and this sort of issue is a reason why I strongly feel we don't want
to merge this series piecemeal until we know that after everything is
complete, we can end up with a fully baked in-band dedupe implementation.

Luckily Qu says he's on it so if he posts a workable fix here my whole point
can become moot. Until then though this is exactly the type of 'fix later'
coding we need to be avoiding.
	--Mark

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Patch
diff mbox

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index dabd721..016d2ec 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -2421,7 +2421,7 @@  static int run_one_delayed_ref(struct btrfs_trans_handle *trans,
 				 * a new extent is revered, then deleted
 				 * in one tran, and inc/dec get merged to 0.
 				 *
-				 * In this case, we need to remove its dedup
+				 * In this case, we need to remove its dedupe
 				 * hash.
 				 */
 				btrfs_dedupe_del(trans, fs_info, node->bytenr);
@@ -5675,6 +5675,7 @@  int btrfs_delalloc_reserve_metadata(struct inode *inode, u64 num_bytes)
 	bool delalloc_lock = true;
 	u64 to_free = 0;
 	unsigned dropped;
+	int loops = 0;
 
 	/* If we are a free space inode we need to not flush since we will be in
 	 * the middle of a transaction commit.  We also don't need the delalloc
@@ -5690,11 +5691,12 @@  int btrfs_delalloc_reserve_metadata(struct inode *inode, u64 num_bytes)
 	    btrfs_transaction_in_commit(root->fs_info))
 		schedule_timeout(1);
 
+	num_bytes = ALIGN(num_bytes, root->sectorsize);
+
+again:
 	if (delalloc_lock)
 		mutex_lock(&BTRFS_I(inode)->delalloc_mutex);
 
-	num_bytes = ALIGN(num_bytes, root->sectorsize);
-
 	spin_lock(&BTRFS_I(inode)->lock);
 	nr_extents = (unsigned)div64_u64(num_bytes +
 					 BTRFS_MAX_EXTENT_SIZE - 1,
@@ -5815,6 +5817,23 @@  out_fail:
 	}
 	if (delalloc_lock)
 		mutex_unlock(&BTRFS_I(inode)->delalloc_mutex);
+	/*
+	 * The number of metadata bytes is calculated by the difference
+	 * between outstanding_extents and reserved_extents. Sometimes though
+	 * reserve_metadata_bytes() fails to reserve the wanted metadata bytes,
+	 * indeed it has already done some work to reclaim metadata space, hence
+	 * both outstanding_extents and reserved_extents would have changed and
+	 * the bytes we try to reserve would also has changed(may be smaller).
+	 * So here we try to reserve again. This is much useful for online
+	 * dedupe, which will easily eat almost all meta space.
+	 *
+	 * XXX: Indeed here 3 is arbitrarily choosed, it's a good workaround for
+	 * online dedupe, later we should find a better method to avoid dedupe
+	 * enospc issue.
+	 */
+	if (unlikely(ret == -ENOSPC && loops++ < 3))
+		goto again;
+
 	return ret;
 }