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

[Saisha Kamat]

Thank you for the clarification. I am proceeding to check the
CFS_FAIL_CHECK().

On Thu, Feb 1, 2024 at 3:59 PM Andreas Dilger <adilger at whamcloud.com> wrote:

> 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> 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> 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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>
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>
>
> Cheers, Andreas
> --
> Andreas Dilger
> Lustre Principal Architect
> Whamcloud
>
>
>
>
>
>
>
>
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