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 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,

Josef

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