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

[Saisha Kamat]

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> wrote:
>
> [Caution: Email from External Sender. Do not click or open links or attachments unless you know this sender.]
>
>
> 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 on behalf of Saisha Kamat via lustre-devel lustre-devel at lists.lustre.org
> Date: Friday, 26 January 2024 at 19:58
> To: lustre-devel at lists.lustre.org 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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