[lustre-devel] [EXTERNAL] Re: Direct Modification of Lustre Metadata on Disk

[Andreas Dilger]

You cannot use debugfs to modify a mounted filesystem, no differently than you cannot replace the tire on your car while you are driving it on the highway...

If you want to inject corruption into the filesystem while it is mounted (without corrupting the whole filesystem), you could use "CFS_FAIL_CHECK()" to add fault injection points directly into the code, as is done by LFSCK to create specific corruptions and then repair them (see lustre/tests/sanity-lfsck.sh).

Cheers, Andreas

On Jan 31, 2024, at 18:41, Saisha Kamat via lustre-devel <lustre-devel at lists.lustre.org<mailto:lustre-devel at lists.lustre.org>> wrote:

Hello Artem,

I apologize for the delay in my response. I also want to thank you for
the suggestion.
However, our objective is to inject modifications while Lustre is
mounted, specifically targeting xattr faults such as LMA, Linkea, and
Lovea. Could you please advise on which debugfs utility I can use for
this purpose?

We would greatly appreciate your guidance and clarity on these matters.

Thanks and Regards,
Saisha

On Fri, Jan 26, 2024 at 5:24 PM Blagodarenko Artem
<artem.blagodarenko at gmail.com<mailto:artem.blagodarenko at gmail.com>> wrote:

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Hi Saisha,



Thanks for your interest in Lustre FS and research efforts.

Regarding your question. Had MDS been mounted when there was the xattre direct modification attempt?

If yes, then xattr data consistency is not guaranteed.

If it is possible, unmount MDS, make modifications, and mount it back.



If MDS backend is LDISKFS, after unmounting MDS, you can mount it as



Mount -t ldiskfs <path_to_dev> <mount_point> and modify any file.

There is also debugfs utility which allows perform many useful modifications without mounting.



Best regards,

Artem Blagodarenko



Best rega

From: lustre-devel lustre-devel-bounces at lists.lustre.org<mailto:lustre-devel-bounces at lists.lustre.org> on behalf of Saisha Kamat via lustre-devel lustre-devel at lists.lustre.org<mailto:lustre-devel at lists.lustre.org>
Date: Friday, 26 January 2024 at 19:58
To: lustre-devel at lists.lustre.org<mailto:lustre-devel at lists.lustre.org> lustre-devel at lists.lustre.org<mailto:lustre-devel at lists.lustre.org>
Subject: [lustre-devel] Direct Modification of Lustre Metadata on Disk

Hello,

I am a Ph.D. student at UNC-Charlotte, focusing on research related to
the Lustre File System. As part of my project, I am investigating
scenarios involving the direct modification of xattr metadata on the
Lustre disk, without unmounting the Lustre servers.

To achieve this, I have attempted to open the MDS (Metadata Server)
disk partition as a file descriptor, locate the target file and its
xattr, and write a faulty value. However, I have encountered an
unexpected issue where my changes appear to be saved to memory and are
not being synchronized with the disk.
After completing the write operation, when I read the same xattr
again, it reflects the corrupted value. Strangely, when using the
"getfattr" command, the original, correct value is displayed. This
discrepancy has raised doubts about whether Lustre permits direct
modifications to its metadata on the disk.
Furthermore, I observed that even after unmounting and remounting the
Lustre file system, the xattr continues to display the corrupted value
upon reading, whereas "getfattr" still returns the original, correct
value.

Please help me understand whether Lustre allows direct modifications
to its metadata on the disk and if there are any inherent limitations
or considerations that I should be aware of.
Additionally, any recommendations or alternative approaches for
simulating faulty conditions for testing purposes would be highly
valuable to my research.

Thanks and Regards,
Saisha
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Cheers, Andreas
--
Andreas Dilger
Lustre Principal Architect
Whamcloud







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