[lustre-discuss] [EXTERNAL] Re: Data recovery with lost MDT data

Thu Sep 21 15:06:21 PDT 2023

I knew an lfsck would identify the orphaned objects.  That’s great that it will move those objects to an area we can triage.  With ownership still intact (and I assume time stamps too), I think this will be helpful for at least some of the users to recover some of their data.  Thanks Andreas.

I do have another question.  Even with the MDT loss, the top level user directories on the file system are still showing current modification times.  I was a little surprised to see this – my expectation was that the most current time would be from the snapshot that we accidentally reverted to, 6/20/2023 in this case.  Does this make sense?

[dvicker at dvicker ~]$ ls -lrt /ephemeral/ | tail
  4 drwx------     2 abjuarez               abjuarez             4096 Sep 12 13:24 abjuarez/
  4 drwxr-x---     2 ksmith29               ksmith29             4096 Sep 13 15:37 ksmith29/
  4 drwxr-xr-x    55 bjjohn10               bjjohn10             4096 Sep 13 16:36 bjjohn10/
  4 drwxrwx---     3 cbrownsc               ccp_fast             4096 Sep 14 12:27 cbrownsc/
  4 drwx------     3 fgholiza               fgholiza             4096 Sep 18 06:41 fgholiza/
  4 drwx------     5 mtfoste2               mtfoste2             4096 Sep 19 11:35 mtfoste2/
  4 drwx------     4 abenini                abenini              4096 Sep 19 15:33 abenini/
  4 drwx------     9 pdetremp               pdetremp             4096 Sep 19 16:49 pdetremp/
256 drwxrwx---   199 ccp_fast_boeing_runner ccp_white          258048 Sep 19 22:03 ccp_fast_boeing_runner/
160 drwxr-x---  2070 ccp_fast_spacex_runner ccp_white          159744 Sep 19 22:04 ccp_fast_spacex_runner/
[dvicker at dvicker ~]$

From: Andreas Dilger <adilger at whamcloud.com>
Date: Thursday, September 21, 2023 at 2:33 PM
To: "Vicker, Darby J. (JSC-EG111)[Jacobs Technology, Inc.]" <darby.vicker-1 at nasa.gov>
Cc: "lustre-discuss at lists.lustre.org" <lustre-discuss at lists.lustre.org>
Subject: [EXTERNAL] Re: [lustre-discuss] Data recovery with lost MDT data

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In the absence of backups, you could try LFSCK to link all of the orphan OST objects into .lustre/lost+found (see lctl-lfsck_start.8 man page for details).

The data is still in the objects, and they should have UID/GID/PRJID assigned (if used) but they have no filenames.  It would be up to you to make e.g. per-user lost+found directories in their home directories and move the files where they could access them and see if they want to keep or delete the files.

How easy/hard this is to do depends on whether the files have any content that can help identify them.

There was a Lustre hackathon project to save the Lustre JobID in a "user.job" xattr on every object, exactly to help identify the provenance of files after the fact (regardless of whether there is corruption), but it only just landed to master and will be in 2.16. That is cold comfort, but would help in the future.
Cheers, Andreas

On Sep 20, 2023, at 15:34, Vicker, Darby J. (JSC-EG111)[Jacobs Technology, Inc.] via lustre-discuss <lustre-discuss at lists.lustre.org> wrote:
Hello,

We have recently accidentally deleted some of our MDT data.  I think its gone for good but looking for advice to see if there is any way to recover.  Thoughts appreciated.

We run two LFS’s on the same set of hardware.  We didn’t set out to do this, but it kind of evolved.  The original setup was only a single filesystem and was all ZFS – MDT and OST’s.  Eventually, we had some small file workflows that we wanted to get better performance on.  To address this, we stood up another filesystem on the same hardware and used a an ldiskfs MDT.  However, since were already using ZFS, under the hood the storage device we build the ldisk MDT on comes from ZFS.  That gets presented to the OS as /dev/zd0.  We do a nightly backup of the MDT by cloning the ZFS dataset (this creates /dev/zd16, for whatever reason), snapshot the clone, mount that as ldiskfs, tar up the data and then destroy the snapshot and clone.  Well, occasionally this process gets interrupted, leaving the ZFS snapshot and clone hanging around.  This is where things go south.  Something happens that swaps the clone with the primary dataset.  ZFS says you’re working with the primary but its really the clone, and via versa.  This happened about a year ago and we caught it, were able to “zfs promote” to swap them back and move on.  More details on the ZFS and this mailing list here.

https://zfsonlinux.topicbox.com/groups/zfs-discuss/Tcb8a3ef663db0031-M5a79e71768b20b2389efc4a4

http://lists.lustre.org/pipermail/lustre-discuss-lustre.org/2022-June/018154.html

It happened again earlier this week but we didn’t remember to check this and, in an effort to get the backups going again, destroyed what we thought were the snapshot and clone.  In reality, we destroyed the primary dataset.  Even more unfortunately, the stale “snapshot” was about 3 months old.  This stale snapshot was also preventing our MDT backups from running so we don’t have those to restore from either.  (I know, we need better monitoring and alerting on this, we learned that lesson the hard way.  We had it in place after the June 2022 incident, it just wasn’t working properly.)  So at the end of the day, the data lives on the OST’s we just can’t access it due to the lost metadata.  Is there any chance at data recovery.  I don’t think so but want to explore any options.

Darby

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