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Re: [Bug-tar] tar does not recognize nfs device

From: Jordan Desroches
Subject: Re: [Bug-tar] tar does not recognize nfs device
Date: Sun, 27 Jan 2008 10:48:55 -0500

I'll try to be more specific this time. Our institution has a Netapp NAS that exports files via NFS. We are mounting the exports on a heterogenous group of systems, running different OS's. On some of the systems, we have a need to use the incremental option that tar provides on the NFS mounts (what I previously referred to as an NFS device). Each of the NFS mounts has a device number associated with it that is not necessarily static across reboots. According to the tar documentation (http://www.gnu.org/software/automake/manual/tar/Incremental-Dumps.html), tar has a way of handling this situation:

"Metadata stored in snapshot files include device numbers, which, obviously is supposed to be a non-volatile value. However, it turns out that NFS devices have undependable values when an automounter gets in the picture. This can lead to a great deal of spurious redumping in incremental dumps, so it is somewhat useless to compare two NFS devices numbers over time. The solution implemented currently is to considers all NFS devices as being equal when it comes to comparing directories; this is fairly gross, but there does not seem to be a better way to go."

The indication that tar has recognized a particular file as part of an NFS mount is a leading 1 in the entry of a Version 2 formatted tar snaphots file, as described in http://www.gnu.org/software/tar/manual/html_node/tar_170.html . As I stated in the initial post, tar does not recognize that it is working on a NFS mounted area of the file system as indicated by the leading 0 in the snapshots file. 

Which brings me back to my original question, why is this happening, and what can I do to fix it? If I run stat on the files that I am attempting to create a tar archive out of, it reports that they are NFS mounted, but tar does not recognize them as such.



On Jan 27, 2008, at 5:57 AM, Vitaly V. Ch wrote:

>"nfs device"

as far as I understand it's sensless _expression_.

Please detail Your situation


On Jan 24, 2008 2:40 PM, Jordan Desroches <address@hidden> wrote:
Hello all,

I'm running into a problem where tar does not seem to recognize that
it is running on a nfs device. The reason I think that is the case are
the leading zeroes in the snapshots file:

address@hidden@address@hidden@address@hidden/sp....  etc, etc

this is causing some trouble, as when the nfs device number changes,
all of a sudden there is a full backup instead of an incremental. tar
is being called through AMANDA.

I have seen the same results under two environments:

Ubuntu Edgy
GNU tar-1.15.91-2
kernel: 2.6.17-12-generic

Ubuntu Gutsy
GNU tar-1.18-2
kernel 2.6.22-14-generic

any insights that might help me track down why tar does not recognize

Thanks for you help!

Jordan Desroches

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