To: linux-btrfs@vger.kernel.org
From: Duncan <1i5t5.duncan@cox.net>
Subject: Re: btrfs problems on new file system
Date: Sat, 26 Dec 2015 10:47:17 +0000 (UTC)
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covici posted on Sat, 26 Dec 2015 02:29:11 -0500 as excerpted:

> Chris Murphy <lists@colorremedies.com> wrote:
> 
>> If you can post the entire dmesg somewhere that'd be useful. MUAs tend
>> to wrap that text and make it unreadable on list. I think the problems
>> with your volume happened before the messages, but it's hard to say.
>> Also, a generation of nearly 5000 is not that new?
> 
> The file system was only a few days old.  It was on an lvm volume group
> which consisted of two ssd drives, so I am not sure what you are saying
> about lvm cache -- how could I do anything different?
> 
>> On another thread someone said you probably need to specify the device
>> to mount when using Btrfs and lvmcache? And the device to specify is
>> the combined HDD+SSD logical device, for lvmcache that's the "cache
>> LV", which is the OriginLV + CachePoolLV. If Btrfs decides to mount the
>> origin, it can result in corruption.
> 
> See above.

I think he mixed up two threads and thought you were running lvm-cache, 
not just regular lvm, which should be good unless you're exposing lvm 
snapshots and thus letting btrfs see multiple supposed UUIDs that aren't 
actually universal.  Since btrfs is multi-device and uses the UUID to 
track which devices belong to it (because they're _supposed_ to be 
universally unique, it's even in the _name_!), if it sees the same UUID 
it'll consider it part of the same filesystem, thus potentially causing 
corruption if it's a snapshot or something that's not actually supposed 
to be part of the (current) filesystem.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman