From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from userp1040.oracle.com ([156.151.31.81]:48323 "EHLO userp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751168AbaEVDGR (ORCPT ); Wed, 21 May 2014 23:06:17 -0400 Message-ID: <537D6A58.5040907@oracle.com> Date: Thu, 22 May 2014 11:09:12 +0800 From: Anand Jain MIME-Version: 1.0 To: Qu Wenruo , linux-btrfs Subject: Re: Should btrfs reuse the src_dev's dev UUID when doing dev replacing? References: <537D5468.3040808@cn.fujitsu.com> In-Reply-To: <537D5468.3040808@cn.fujitsu.com> Content-Type: text/plain; charset=UTF-8; format=flowed Sender: linux-btrfs-owner@vger.kernel.org List-ID: Thanks Qu for bringing up this topic. We definitely need some focus on the btrfs volume management related bugs/features/enhancements. more inline.. On 22/05/14 09:35, Qu Wenruo wrote: > Hi, > > [Current dev replace] > As kernel codes show, 'btrfs dev replace' will swap tgt_dev's uuid with > src_dev's uuid. > This method works fine most of the time, since it doesn't need to change > the chunk tree. > > [Problem with re-appear missing device] > (Anand Jain reported the problem in Jan 2014) > Take the following suitiuation as example: > /dev/sda, /dev/sdb, /dev/sdc as btrfs RAID1. > 1, 2, 3 as their dev id. > > 1)/dev/sdb is missing, > Mount them in degraded mode. > > 2) 'btrfs dev replace start 2 /dev/sdd' will replace missing /dev/sdb. > > 3) /dev/sdb is online again. > > 4) umount /BTRFS/MOUNT/POINT; mount /dev/sda > After mount, btrfs will still use /dev/sdb but not /dev/sdd Yeah its weird that grouping depends on the mercy of chronological oder of device probing. The _last_ device probed stays in the list. But the most weird is if FS is mounted and is followed with the dev scan it would just overwrite the btrfs_device struct. I have sent out interim fix to both of these bugs a long time back. > [Cause of the bug] > When this comes to missing device, since the src_dev is missing, neither > UUID swap nor superblock wipe will > work. So if the device reappears, next mount will scan the the fsid and > dev uuid, and if btrfs scan the re-appeared > device first, it will use the re-appeared device. > > [Method to fix] > IMO there are 2 possible method to fix the bug. > 1) Don't reuse the src_dev's dev UUID. > I don't think any of the UUID in btrfs should be reused, so if every > device in btrfs has its own UUID, > it is quite easy to distinguish different devices, and even don't need > to wipe the superblock of src_dev. > (But superblock wipe is still needed for other reasons) Yep that the right way IMO too. UUID must be unique to disk, even in the case of replace. > 2) Do generation check in device_list_add. > When multiple devices with same dev UUID is found, only add the one > whose generation is the same with > other deivces. > IMO this is just a workaround. yes an interim fix, patch was sent out a long time back. > I think it is better to be decided before any related patch sent. > > Any suggestions? > > Thanks, > Qu > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html