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

[Wang Shilong <wangshilong1991@gmail.com>]

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.

#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.^_^

Thanks,
Wang
> 
> Josef