Re: [RFC PATCH 5/5] Btrfs: fix broken free space cache after the system crashed

[Josef Bacik <jbacik@fb.com>]

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On 03/05/2014 02:02 AM, Miao Xie wrote:
> On Tue, 4 Mar 2014 10:19:20 -0500, Josef Bacik wrote: On 01/15/2014
> 07:00 AM, Miao Xie wrote:
>>>> When we mounted the filesystem after the crash, we got the 
>>>> following message: BTRFS error (device xxx): block group
>>>> 4315938816 has wrong amount of free space BTRFS error (device
>>>> xxx): failed to load free space cache for block group
>>>> 4315938816
>>>> 
>>>> It is because we didn't update the metadata of the allocated
>>>> space until the file data was written into the disk. During
>>>> this time, there was no information about the allocated
>>>> spaces in either the extent tree nor the free space cache.
>>>> when we wrote out the free space cache at this time, those
>>>> spaces were lost.
>>>> 
>>>> In ordered to fix this problem, I use a state tree for every
>>>> block group to record those allocated spaces. We record the
>>>> information when they are allocated, and clean up the
>>>> information after the metadata update. Besides that, we also
>>>> introduce a read-write semaphore to avoid the race between
>>>> the allocation and the free space cache write out.
>>>> 
>>>> Only data block groups had this problem, so the above change
>>>> is just for data space allocation.
>>>> 
> 
> I didn't like this idea at first but I've come around to it.  The
> only thing is the data_rwsem thing, we don't need it as we are
> protected by the transaction being blocked when we do writeout, so
> nobody can data allocations during this time.  Thanks,
> 
>> But this protection was removed by the patch
> 
>> Commit ID: 00361589d2eebd90fca022148c763e40d3e90871
> 

Excellent point, then I'm good overall, I'll pull it in once I finish
qgroups.  Thanks,

Josef
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