Xfs_repair and journalling

[Martin Steigerwald]

Am Sonntag, 17. März 2013 schrieb Stan Hoeppner:
> On 3/16/2013 10:56 AM, Subranshu Patel wrote:
> > This is not observed in EXT4, fsck successfully recovers without
> > mounting the filesystem.
> 
> And this is the real problem.  You're *assuming* XFS should behave in
> the same manner as EXT4.  Why would you assume a Ferrari should behave
> like a Tata Nano?
> 
> XFS is far more sophisticated than EXT4 in many, many ways, including
> recovery after unclean shutdown.  XFS kernel code performs journal
> playback/recovery automatically when the filesystem is mounted.
> xfs_repair is a tool for fixing filesystems that are broken, not simply
> in need of journal playback.  Thus xfs_repair has no code to perform
> journal recovery.
> 
> EXT4 (and EXT3) lacks this sophistication and must call a user space
> tool, e2fsck, to perform journal playback/recovery.

Stan, to all what I know this is just not true.

Ext4 and Ext3 are perfectly capable to replay their journals as well. I 
would have notices with various Ext4 installations on laptops and 
workstations that had a unclean shutdown way more often than the two unclean 
shutdowns you mentioned. My previous main laptop a ThinkPad T42 would have 
been broken by every month or so, considering me playing around and managing 
to crash it every once in a while.

Thats the whole point of being a journaling filesystem.

> XFS is the Ferrari of Linux filesystems and EXT is the Tata.  Keep that
> in mind as you discover many of the other differences in the future.

XFS may be more advanced in many areas - for example the capacity it can 
hold and the scalability to really huge setups -, but the basic ability to 
replay a journal is not one of this areas.

Maybe both filesystem differ under what circumstances they are still able to 
replay their journal, but from what I gathered the fully block based journal 
of Ext filesystem is quite robust.

Additionally also BTRFS meanwhile recovers just fine from an unclean 
shutdown. At least it did on the last dozen hard crashes due to BTRFS not 
being able to deal nicely with out of memory situations in combination with 
some form of a memleak with Planeshift and/or Intel gfx driver unless I 
tweak the VM subsystem settings a bit.

Also ReiserFS 3 is able to replay a journal. It was the first Linux 
filesystem that had journaling. Not that I would use it but thats for other 
reasons. (Yes, I did not look it up, but I think XFS did journaling by that 
time or even earlier on IRIX already.)

And with all the mentioned filesystem, I would always first mount after a 
unclean shutdown and expect that all is fine. Just if it isn´t I would 
consider other options like repair tools. The XFS tools may the tools suite 
that guides to this behaviour more than tools for other filesystems however.

Thanks,
-- 
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
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