[SOLVED] Garbage collection phase1 does not mark (m)any chunks

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pallingerpeter

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Jan 17, 2024
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Currently, garbage collection tries to delete most of my chunks. I have about 4M chunks, at list half of which should be used, and the GC log goes like this:

Code: 
2024-09-27T17:01:19+02:00: starting garbage collection on store backup-cephfs
2024-09-27T17:01:19+02:00: Start GC phase1 (mark used chunks)
2024-09-27T17:09:29+02:00: Start GC phase2 (sweep unused chunks)
2024-09-27T17:10:08+02:00: processed 1% (46618 chunks)
At this point I stopped it manually, then restored the chunks from the synchronised server. 40k*100 ~ 4M, so it was going to delete most of my chunks. It did so twice before, and had to manually rsync chunks back from the replica server.

As far as I understand, GC phase1 should update the access times for referenced chunks so phase2 can delete the chunks with old (>24h+5min) access times.
The problem is, no files access times are updated.
As far as I can tell, the number of chunks with fresh access times does not change during phase1.

The chunks are on a cephfs mount. The *.fidx files are on a different (ext4-on-ceph-rbd) mount. I understand that cephfs has an outstanding feature request https://tracker.ceph.com/issues/43337 for correctly handling atime, and I successfully confirmed that reading a file does not update its atime. However, manually updating chunk times still works:

Code: 
# F=.chunks/0000/0000......... ; ls -lu $F ; cat $F >/dev/null; ls -lu $F ; touch -a $F ; ls -lu $F
-rw-r--r-- 2 backup backup 2605720 Sep 11 09:10 .chunks/0000/0000....
-rw-r--r-- 2 backup backup 2605720 Sep 11 09:10 .chunks/0000/0000....
-rw-r--r-- 2 backup backup 2605720 Sep 28 13:52 .chunks/0000/0000....

I looked into the code and it seems it uses the utimensatlibc call, so the cephfs atime bug should not be a concern (however, I do not really know rust, so maybe I interpreted something wrong).

My main question is: how can I effectively debug the gc procedure to know what the problem is.
  • Maybe it does not scan most images (phase1 time is certainly fast, but it is possible to read the ~2GB of image files in roughly 9 minutes)
    • Can you make it print what image's chunks are being scanned?
  • Maybe it can not/ does not try to update atimes
    • Can you make it print which chunks' atimes are being set?

Thanks in advance!
 
Last edited:
Update: I tried moving the .fidx files back to the cephfs mount, and running a GC.

It runs differently:
Code: 
2024-09-30T09:22:15+02:00: starting garbage collection on store backup-cephfs
2024-09-30T09:22:15+02:00: Start GC phase1 (mark used chunks)
2024-09-30T10:08:09+02:00: WARN: warning: unable to access non-existent chunk 4b38e5765f6615a2c33615ab7a264d4bc12986f358315a3f1bf13085ce7fa88f, required by "/mnt/backup-cephfs/ns/backup/vm/405/2024-02-21T15:40:53Z/drive-virtio0.img.fidx"
...
2024-09-30T10:08:42+02:00: WARN: warning: unable to access non-existent chunk 4d5dbc9c991b1648a6fa3cccf8e24b7c752e97a2f754d6520979ad116fb5436a, required by "/mnt/backup-cephfs/ns/backup/vm/405/2024-02-21T15:40:53Z/drive-virtio0.img.fidx"
2024-09-30T10:11:41+02:00: marked 1% (23 of 2273 index files)
....

It takes a lot more time, and the amount of files with old atime seems to decrease slowly (I measure it with find /mnt/backup-cephfs/.chunks/00* -atime +1 -type f | wc, I obviously do not want to scan all chunks periodically). It is still running, and may take up a whole day.

The preliminary conclusion seems to be: garbage collection does not work properly if the fidx files are not on the same filesystem as the chunks. I will write again when the GC process finishes.
 
Hi there. I just recently experienced a very similar issue, with GC skipping phase 1 and marking everything as unused. The reason I had this happening was that my "ns" directory was behind a symlink. As soon as I stopped using symlinks for files representing snapshots (pretty much everything alongside the ".chunks" direkctory) the GC started to behave.

Check if this helps you: https://forum.proxmox.com/threads/garbage-collection-skipped-first-phase.135931/post-602983
And here the longer version of my experience: https://forum.proxmox.com/threads/im-losing-backup-chunks.155575/

Hope this helps
 
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