rsync - Jun 2010 - ...failed: too many links (31)

We do backups using rsync --link-dest.  On one of our volumes, we just 
hit a limit in ext3 which generated the error:

rsync: link "..." => ... failed: Too many links (31)

This appears to be related to a limit in the number of directory entries 
to which an inode may be connected.  In other words, it's a limit on the 
number of hard links that can exist to a given file.  This limit is 
apparently 32000.

This isn't specifically an rsync problem, of course.  I can recreate it 
with judicious use of "cp -Rl", for example.  But any site using
--link-
dest as heavily as we are - and ext3 - is vulnerable to this.  So I 
thought I'd share our experience.

This is admittedly an extreme case: We've a lot of snapshots preserved 
for this volume.  And the files failing are under /usr/lib/locale; there 
is a lot of hardlinking already occurring in there.

I've thought of two solutions: (1) deliberating breaking linking (and 
therefore wasting disk space) or (2) using a different file system.

This is running on CentOS 5, so xfs was there to be tried.  I've had 
positive experiences with xfs in the past, and from what I have read this 
limit does not exist in that file system.  I've tried it out, and - so 
far - the problem has been avoided.  There are inodes with up to 32868 
links at the moment on the xfs copy of this volume.

I'm curious, though, what thoughts others might have.

I did wonder, for example, whether rsync should, when faced with this 
error, fall back on creating a copy.  But should rsync include behavior 
that exists only to work around a file system limit?  Perhaps only as a 
command line option (ie. definitely not the default behavior)?

Thanks...

	- Andrew

On Wed, 30 Jun 2010 01:43:02 +0000, Andrew Gideon wrote:
> I've thought of two solutions: (1) deliberating breaking linking (and
> therefore wasting disk space) or (2) using a different file system.
> 
> This is running on CentOS 5, so xfs was there to be tried.  I've had
> positive experiences with xfs in the past, and from what I have read
> this limit does not exist in that file system.  I've tried it out, and
-
> so far - the problem has been avoided.  There are inodes with up to
> 32868 links at the moment on the xfs copy of this volume.
> 
> I'm curious, though, what thoughts others might have.
> 
> I did wonder, for example, whether rsync should, when faced with this
> error, fall back on creating a copy.  But should rsync include behavior
> that exists only to work around a file system limit?  Perhaps only as a
> command line option (ie. definitely not the default behavior)?
I know it's been a while, but I thought I'd follow up on this.

First: The problem is occurring with yum databases.  A change was 
introduced a while back that saves space under /var/lib/yum by hard-
linking at least some files (eg. the changed_by files).  This isn't the 
only situation where our backups are failing due to "too many links",
but
it is the most reliable failure.

The yum change included a fallback: if the linking failed, a new file is 
created.  I mention this because I'm wondering (see above) if this is an 
appropriate solution for rsync.  Apparently, it is so for yum.

Second: xfs does seem to completely eliminate this issue.  I don't quite 
trust xfs as much as I do ext3, so we're only using it where the "too 
many links" problem occurred.  But as our systems are upgraded to the new 
yum, this will be more and more of our backup volumes.

So I'm still wondering if an rsync-centric solution, perhaps similar to 
yum's fallback, is appropriate.

	- Andrew