NOCOW + compress-force = bug

From: Roman Mamedov <rm@romanrm.ru>
To: dave@jikos.cz
Cc: dima <dolenin@parallels.com>, linux-btrfs@vger.kernel.org
Subject: NOCOW + compress-force = bug
Date: Thu, 16 Feb 2012 19:55:15 +0600	[thread overview]
Message-ID: <20120216195515.643f4aa7@natsu> (raw)
In-Reply-To: <20120213141040.GG15350@twin.jikos.cz>

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Hello,

Please be aware that there seems to be a possible problem with using NOCOW
flag on files situated on a filesystem mounted with compress-force(=lzo, in my
case).

Since experimenting with NOCOW, I started regularly hitting this BUG at
extent-tree.c:5813 

  5813                 BUG_ON(!(flags & BTRFS_BLOCK_FLAG_FULL_BACKREF));

I was unable to make netconsole work over a bridged interface, so can only
post screenshots of this OOPS:
  http://romanrm.ru/pics/2012/2012-02-16-btrfs-bug-1.jpg
  http://romanrm.ru/pics/2012/2012-02-16-btrfs-bug-2.jpg

This happened four times already, and always on snapshot creation (but not
every case). I have hourly snapshots in crontab, and only one case out of about
ten fails with this problem. Did not try to deliberately reproduce it yet by
manually making snapshots very often, etc.

On Mon, 13 Feb 2012 15:10:40 +0100
David Sterba <dave@jikos.cz> wrote:

> Hi,
> 
> On Mon, Feb 13, 2012 at 04:40:03PM +0900, dima wrote:
> > Actually it is possible. Check out David's response to my question from 
> > some time ago:
> > http://permalink.gmane.org/gmane.comp.file-systems.btrfs/14227
> 
> this was a quick aid, please see attached file for an updated tool to set
> the file flags, now added 'z' for NOCOMPRESS flag, and supports chattr
> syntax plus all of the standard file flags.
> 
> Setting and unsetting nocow is done like 'fileflags +C file' or -C for
> unseting. Without any + or - options it prints current state.
> 
> 
> david

-- 
With respect,
Roman

~~~~~~~~~~~~~~~~~~~~~~~~~~~
"Stallman had a printer,
with code he could not see.
So he began to tinker,
and set the software free."

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