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Re: xfsrestore: incorrect restore if file becomes a dir

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: xfsrestore: incorrect restore if file becomes a dir
From: Bill Kendall <wkendall@xxxxxxx>
Date: Wed, 04 Jan 2012 08:56:32 -0600
Cc: David Brown <davidb@xxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <20120103231604.GT23662@dastard>
References: <20111226201856.GA3909@xxxxxxxxxx> <4F036FF6.2080501@xxxxxxx> <20120103213147.GS23662@dastard> <4F0384A4.6000505@xxxxxxx> <20120103231604.GT23662@dastard>
User-agent: Thunderbird (X11/20080502)
Dave Chinner wrote:
On Tue, Jan 03, 2012 at 04:43:48PM -0600, Bill Kendall wrote:
On 01/03/2012 03:31 PM, Dave Chinner wrote:
On Tue, Jan 03, 2012 at 03:15:34PM -0600, Bill Kendall wrote:
On 12/26/2011 02:18 PM, David Brown wrote:

I've had this happen again. It appears to be the case if between
incremental dumps, a file is deleted and a directory is created that
gets the same inode number. The restore leaves a file in place of the
directory. If the new directory has any contents, xfsrestore prints a
warning, and doesn't restore the subdirectory contents.

Given the sparseness of inodes, this doesn't seem to occur all that
frequently, but I do have a couple of backups that exhibit the
behavior. If no one has any ideas, I'll start digging through
xfsrestore to see if I can figure out what is happening.
I haven't looked at the relevant code, but it sounds like the inode
generation number would also have to be the same in order for this
to happen. Two inodes from separate backups are only considered to
be the same file or directory if the inode number and the lower 12
bits of the inode generation number are the same.
Why does dump only use the lower twelve bits? The on-disk generation
number is 32 bits and we use all of it (by way of random numbers) to
distinguish between different inode generations. That sounds like
something that needs to be fixed....
I don't know the history there, but it dates back to when the generation
number was not randomly initialized. So an inode had to be reused 4,096
times for a collision to occur.

That's kind of what I thought. But even so, with the way XFS reuses
inodes (especially for short term temporary files), those 12 bits
can eaily be burnt through in under a second....

With the current scheme (initially
random, then incremented) there would be cases where a collision
happens more frequently. I agree, it should be changed.

Is that difficult to do?

It requires a change to the dump format, so most of the work is
probably in maintaining backwards compatibility.


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