Hi Josef, > > 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, Yeah, very reasonable, if you don't mind, i would give a patch for this issue. Thanks, Wang > > Josef -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
