[LU-8521] ZFS OST is unwritable

• Type: Bug
• Resolution: Fixed
• Priority: Major
• Affects Version/s: Lustre 2.5.3
• Environment: TOSS 2.4-9
• Severity: 2
• Assignee: nasf (Inactive)
• Reporter: Joe Mervini

Description

After a power outage we encountered a hardware error on one of our storage devices that essentially corrupted ~30 files on one of the OSTs. Since then the OST has been read-only and is throwing the following log messages:

[ 351.029519] LustreError: 8974:0:(ofd_obd.c:1376:ofd_create()) fscratch-OST0001: unable to precreate: rc = -5
[ 360.762505] LustreError: 8963:0:(ofd_obd.c:1376:ofd_create()) fscratch-OST0001: unable to precreate: rc = -5
[ 370.784372] LustreError: 8974:0:(ofd_obd.c:1376:ofd_create()) fscratch-OST0001: unable to precreate: rc = -5

I have scrubbed the device in question and rebooted the system bring up the server normally but I am still unable to create a file on that OST.

zpool status -v reports the damaged files and recommended restoring from backup and I'm inclined to simply removing the files. I know how to do this with ldiskfs but I don't know how to with ZFS. At this point I don't know how to proceed.

Issue Links

• is related to: LU-7585 Implement OI Scrub for ZFS

Comments

[Joe Mervini]

So just to be clear, if I do a stat on a particular file in ROOT/O/<username> that could be wildly different than the associated object on the OST? For example I have a file similar to this:

File: `jamervi/scripts/check-key-image-files'
Size: 0 Blocks: 0 IO Block: 4096 regular empty file
Device: fd00h/64768d Inode: 844107533 Links: 1
Access: (0700/rwx-----) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2015-04-14 13:54:09.000000000 -0600
Modify: 2015-04-14 11:31:58.000000000 -0600
Change: 2015-04-14 11:31:58.000000000 -0600

Is it possible that what is stored on the OST could be perhaps a year or more different?

I am not particularly interested in the file sizes. What I need is a WAG at the age of the files on the file system.

[nasf (Inactive)]

For a given Lustre striped regular file, its attributes are distributed to several targets (MDT and OSTs). The filename, owner, mode and permission are stored on the MDT. Each stripe (or OST-object) stores its own size/blocks attribute, client needs to calculate the file size/blocks attribute by sum all related OST-objects' size/blocks attributes. As for the xtime (atime, mtime, ctime) attribute, both the MDT-object and OST-objects store the timestamp. Which one will be used as the file xtime, depends on which ctime is newer.

So directly mount the target as backend filesystem, in spite of "ldiskfs" or "ZFS", you can NOT directly get the file size and xtime attributes. But you can calculate those attributes. For a given OST-object, you can get its MDT-object's FID from the OST-object's PFID EA, the OST-objects with the same MDT-object's FID belong to the same file. Then you can sum related OST-objects' size, and also compare their ctime. So you can get the file's size/time attributes. But consider such process, I am not sure whether it is really more efficient than directly collecting the information from Lustre client.

[Andreas Dilger]

With ldiskfs it is relatively safe to mount the MDT as type ldiskfs and collect read-only data from the filesystem. The local MDT data will not contain file sizes, only filenames, owners, timestamps, etc. We don't test this mode officially, but I use it to poke around in my home filesystem, though that filesystem has very low activity. It is not possible to do this with ZFS.

How many OSTs do you have? I would think if there is currently 71% free space in the filesystem, and you have more than 10-12 OSTs (each 70% full), then removing one of them would only increase space usage by 6-7% or less on the remaining OSTs, which should still be OK? I'd suspect that the OST in question itself already has less than 70% usage because it can delete files but not create new ones, so it may be less data that needs to be migrated.

[Joe Mervini]

I've circled back to this issue and since we are unable to repair the target our only option is to either migrate the files on the read only target to other OSTs or punt. One of the problems we have at the moment is that the capacity of the file system is at ~71% and as a result migration is problematic.

We concluded that the best course of action would be to purge older data from the file system to free up space and tried using lfs find to gather file stats. Unfortunately, this method has to be abandoned because of the ~1400 top level directories we have only been able to collect data on roughly 300 of those directories in the course of a month.

I was playing around on one of my test file systems and discovered that I can get all the data I want from the MDT (i.e., create, modify and access timestamps) by mounting the MDT ldiskfs and running stat on the files in /ROOT/O. My question is, would it be prudent to take a system outage to collect this data or am I safe having the device mounted both lustre and ldiskfs (read only) at the same time?

Thanks in advance.

[Joe Mervini]

I've circled back to this issue and since we are unable to repair the target our only option is to either migrate the files on the read only target to other OSTs or punt. One of the problems we have at the moment is that the capacity of the file system is at ~71% and as a result migration is problematic.

We concluded that the best course of action would be to purge older data from the file system to free up space and tried using lfs find to gather file stats. Unfortunately, this method has to be abandoned because of the ~1400 top level directories we have only been able to collect data on roughly 300 of those directories in the course of a month.

I was playing around on one of my test file systems and discovered that I can get all the data I want from the MDT (i.e., create, modify and access timestamps) by mounting the MDT ldiskfs and running stat on the files in /ROOT/O. My question is, would it be prudent to take a system outage to collect this data or am I safe having the device mounted both lustre and ldiskfs (read only) at the same time?

Thanks in advance.

[Andreas Dilger]

Besides the OI scrub to repair the corruption in the ZFS filesystem, I think the only other option is to migrate the files off this OST onto other OSTs and then reformat it. How much space this will consume on the other OSTs depends on how many OSTs there are.

That isn't a great solution, but the repair tools for ZFS are somewhat less robust than for ldiskfs since ZFS itself doesn't get corrupted very easily. Once the OI Scrub functionality is available for ZFS we will be able to repair a fair amount of corruption in the backing filesystem, though it still wouldn't be possible to recover the user data in any OST objects that were corrupted.

[nasf (Inactive)]

What others can we do for this ticket in addition to the on-going ZFS OI scrub?

[nasf (Inactive)]

I will work on LU-7585 to implement ZFS OI scrub, that will allow us to rebuild "/O" directory on the OST. On some degree, such functionality may be helpful for this issue. But the OI scrub can rebuild the OI mappings, it cannot repair the corrupted data. If related objects themselves corrupted (quite possible), not (only) related entries missed from the "/O" tree. Then ZFS OI scrub cannot help much. Anyway, such work will not be ready in short time, so your current solution is quite necessary.

[Andreas Dilger]

OK, so it is good news at least that the OST is currently mounted and readable, and not totally offline. I assume you do not have a snapshot of the OST that would allow recovery, and it has likely been online long enough that the old uberblocks which might reference an uncorrupted version of the filesystem have been overwritten (there are only 256, and they are updated at 1s intervals).

Note that deactivating the OST on the MDS will prevent the OST objects from being deleted due to LU-4295, but that may a benefit if the OST is corrupt and deleting files may cause more problems, and also not an issue if you would be draining the whole OST and reformatting it. Since the OST can't precreate objects it may be that marking it deactivated on the MDS is redundant, but not otherwise harmful.

[Joe Mervini]

Andreas - Yes, you are correct with regard to both configurations and we were pretty much discussing the same things here. We have kicked around the idea of perhaps trying to migrate the data on the bad OST but then the file system is at >76% full and we start running into problems when it approaches 80% so that is problematic.

As it turns out, even if I try force a file to be created on that OST via lfs setstripe -i 1 it will bounce it to a different OST. I have been able to access files on that OST identifying them with lfs find and can at least read/delete and actually modify existing files that I have examined in my own directory space (the write operation took some time but it surprisingly completed.) I have deactivated the OST in the meantime.

I look forward to any additional feedback or suggestions. Thanks Andreas

Fan Yong - Thanks for the info.

[nasf (Inactive)]

I did some examination of ZFS OSTs on a working test file system and I could not find a LAST_ID anywhere. Is it supposed to exist under ZFS?

The "LAST_ID" for ldiskfs backend is named as "LAST_ID" under "/O/<seq>" directory. For ZFS backend, it is named as "0" under "/O/<seq>/d0" directory.