Re: [PATCH] Btrfs: convert to add transaction protection for btrfs send

[Josef Bacik <jbacik@fb.com>]

On 02/05/2014 03:59 AM, Wang Shilong wrote:
> Hi Josef,
>
> [..SNIP..]
>> On 01/31/2014 11:37 AM, Wang Shilong wrote:
>>> Hello Josef,
>>>
>>>>  
>> 2) Remove the per-root rwsem for the commit root and just make one big
>> rwsem that covers all commit root switching. This way everybody who
>> wants to search with the commit root can just use this semaphore and all
>> be safe. It will mean that the inode cache stuff may block longer than
>> normal but I don't think that's too big of a deal.
>>
> I am ok with this fix,  I wanted to talk something about protecting searching commit file root, this is really a
> problem especially for full send.
>
> I have some ideas about this issue:
>
> #1.don't use commit file root to search.
> This will become a nightmare when we are doing full send which will iterate the whole file tree,
> at the same time, we snapshot send root, snapshots will be blocked until send finished.
>
> #2. don't allow snapshot if we are sending root.
> This may be a little confusing, snapshots are readonly, but users can not snapshot it.
I think this is the best bet. The fact is we don't want to hold this
commit_root_sem for the entire duration of the send, it would block
people trying to commit the transaction. We could check for contention
and drop the sem and re-search down to where we were but I think that
would be prone to errors. If we just check to see if the snapshot is
being sent and just return -EBUSY when we try to create a snapshot I
think that's perfectly reasonable.
> #3. after one iteration, we do check send_root's generation, and make sure it doesn't
> change, if it changed, then we restart send again.
>
> I don't know which approach is better,and also snapshot-aware defragment will change
> read-only snapshot?
>
> Did you have any better ideas about this issue? Share it with me here.^_^
>
Snapshot-aware defrag will definitely screw us here. I think we need to
do the same thing above as we do here, which is to simply skip the
snapshot aware defrag if we are currently using that root for send. This
sound reasonable to you? Thanks,

Josef