From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0b-00082601.pphosted.com ([67.231.153.30]:36947 "EHLO mx0b-00082601.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754307AbaBEUrw (ORCPT ); Wed, 5 Feb 2014 15:47:52 -0500 Message-ID: <52F2A370.2080509@fb.com> Date: Wed, 5 Feb 2014 15:47:44 -0500 From: Josef Bacik MIME-Version: 1.0 To: Wang Shilong CC: , Wang Shilong Subject: Re: [PATCH] Btrfs: convert to add transaction protection for btrfs send References: <1391009539-2326-1-git-send-email-wangshilong1991@gmail.com> <52E94FEA.5060404@fb.com> <52EA78E3.9060009@fb.com> <98BBA3BC-1CB6-4840-A5AF-6CE9C76B72AA@gmail.com> <52EA7C78.2030509@fb.com> <2DED49DB-B86D-46FB-B05C-3FB5E655749C@gmail.com> <15132D45-7C4B-41FD-A240-43BCFE314726@gmail.com> <52F00AAE.6070509@fb.com> <15BD028C-7568-4ACF-84D1-CA39092AD285@gmail.com> <52F244DE.9030108@fb.com> <2ECC0789-2FE8-4644-B439-35B682502867@gmail.com> In-Reply-To: <2ECC0789-2FE8-4644-B439-35B682502867@gmail.com> Content-Type: text/plain; charset="GB2312" Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 02/05/2014 12:23 PM, Wang Shilong wrote: > 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. Go for it, you'll be faster than I will be, all I do is run xfstests and try to reproduce things that will never reproduce for me. Josef