From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ee0-f51.google.com ([74.125.83.51]:48560 "EHLO mail-ee0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751001AbaDQV3T (ORCPT ); Thu, 17 Apr 2014 17:29:19 -0400 Received: by mail-ee0-f51.google.com with SMTP id c13so1065621eek.10 for ; Thu, 17 Apr 2014 14:29:18 -0700 (PDT) Message-ID: <535047AB.6030606@gmail.com> Date: Thu, 17 Apr 2014 23:29:15 +0200 From: "Oliver O." MIME-Version: 1.0 To: Chris Mason , linux-btrfs@vger.kernel.org Subject: Re: File changing in snapshot References: <534FF5C1.8070404@gmail.com> <534FF9B5.4010207@fb.com> <534FFD22.1060500@gmail.com> In-Reply-To: <534FFD22.1060500@gmail.com> Content-Type: text/plain; charset=ISO-8859-15; format=flowed Sender: linux-btrfs-owner@vger.kernel.org List-ID: Am 17.04.2014 18:11, schrieb Oliver O.: > > Am 17.04.2014 17:56, schrieb Chris Mason: >> On 04/17/2014 11:39 AM, Oliver O. wrote: >>> I seem to have observed a file on a (writable) snapshot changing >>> although there were no writes occuring on the snapshot itself. This is >>> not supposed to happen, right? >> >> Was this a nodatacow file? >> >> -chris > > No. Mount options: > > /dev/sda2 on /home type btrfs (rw,noatime,nodiratime,subvol=@home) > Some additional observations: - Subvolume used for writing: @home - Snapshot used for backup: @home-2014-04-16 - Preceding snapshot: @home-2014-04-10 # lsattr -v @home-2014-04-10/path/to/MyFile.xls @home-2014-04-16/path/to/MyFile.xls @home/path/to/MyFile.xls 36066 ---------------- @home-2014-04-10/path/to/MyFile.xls 36066 ---------------- @home-2014-04-16/path/to/MyFile.xls 36066 ---------------- @home/path/to/MyFile.xls # btrfs subvolume find-new @home-2014-04-10 99999999 transid marker was 180026 # btrfs subvolume find-new @home-2014-04-16 99999999 transid marker was 194551 # btrfs subvolume find-new @home 99999999 transid marker was 197735 # btrfs subvolume find-new @home 36066 | grep "MyFile.xls" inode 24426 file offset 0 len 20480 disk start 9108635648 offset 0 gen 193090 flags NONE path/to/MyFile.xls inode 24426 file offset 20480 len 36864 disk start 25510342656 offset 20480 gen 36067 flags NONE path/to/MyFile.xls inode 24426 file offset 57344 len 4096 disk start 9108582400 offset 0 gen 193090 flags NONE path/to/MyFile.xls Conclusions: The sequence of events seems to be: 1. The file was changed with generation 193090. 2. The snapshot for backup (@home-2014-04-16) was taken (generation 194551). 3. As the backup was reading from the snapshot, it was seeing stale data (MyFile still at generation 36067). 4. As the backup comparison was reading from the snapshot, is was seeing more recent data, causing a discrepancy with the stale data just backed up. It seems that the snapshot needed some time to become stable for reading. From a file system user's point of view, I'd expect a snapshot to be atomic and stable at any time. Maybe some kind of caching problem here? Maybe it's time to file a bug report, but first: suggestions and ideas appreciated.