Re: [PATCH] Btrfs: do not set log for full commit when creating non-data block groups

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2018/11/8 下午10:48, Filipe Manana wrote:
> On Thu, Nov 8, 2018 at 2:37 PM Filipe Manana <fdmanana@kernel.org> wrote:
>>
>> On Thu, Nov 8, 2018 at 2:35 PM Qu Wenruo <quwenruo.btrfs@gmx.com> wrote:
>>>
>>>
>>>
>>> On 2018/11/8 下午9:17, fdmanana@kernel.org wrote:
>>>> From: Filipe Manana <fdmanana@suse.com>
>>>>
>>>> When creating a block group we don't need to set the log for full commit
>>>> if the new block group is not used for data. Logged items can only point
>>>> to logical addresses of data block groups (through file extent items) so
>>>> there is no need to for the next fsync to fallback to a transaction commit
>>>> if the new block group is for metadata.
>>>
>>> Is it possible for the log tree blocks to be allocated in that new block
>>> group?
>>
>> Yes.
> 
> Now I realize what might be your concern, and this would cause trouble.
> Surprised this didn't trigger any problem and I had this (together
> with other changes) running tests for some weeks already.

Maybe it's related metadata chunk pre-allocation so it will be super
hard to hit in normal case, or some extent allocation policy preventing
us from allocating tree block of newly created bg.

Thanks,
Qu

> 
>>
>>>
>>> Thanks,
>>> Qu
>>>
>>>>
>>>> Signed-off-by: Filipe Manana <fdmanana@suse.com>
>>>> ---
>>>>  fs/btrfs/extent-tree.c | 3 ++-
>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>>>> index 577878324799..588fbd1606fb 100644
>>>> --- a/fs/btrfs/extent-tree.c
>>>> +++ b/fs/btrfs/extent-tree.c
>>>> @@ -10112,7 +10112,8 @@ int btrfs_make_block_group(struct btrfs_trans_handle *trans, u64 bytes_used,
>>>>       struct btrfs_block_group_cache *cache;
>>>>       int ret;
>>>>
>>>> -     btrfs_set_log_full_commit(fs_info, trans);
>>>> +     if (type & BTRFS_BLOCK_GROUP_DATA)
>>>> +             btrfs_set_log_full_commit(fs_info, trans);
>>>>
>>>>       cache = btrfs_create_block_group_cache(fs_info, chunk_offset, size);
>>>>       if (!cache)
>>>>
>>>


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