[lustre-discuss] Question about lctl changelog_deregister

[Mohr Jr, Richard Frank (Rick Mohr)]

I have started playing around with Lustre changelogs, and I have noticed a behavior with the “lctl changelog_deregister” command that I don’t understand.  I tried running a little test by enabling changelogs on my MDS server:

[root at server ~]# lctl --device orhydra-MDT0000 changelog_register
orhydra-MDT0000: Registered changelog userid ‘cl5'

On a client system, I ran “lfs changelog” to look at some of the entries.  In particular, I was looking at the last entry index number to see how quickly entries were being added:

[root at client ~]# lfs changelog orhydra-MDT0000 | tail -1
432339 11CLOSE 22:18:01.882533366 2018.01.26 0x3 t=[0x20000b4a7:0xe8de:0x0]

…waited a bit...

[root at client ~]# lfs changelog orhydra-MDT0000 | tail -1
694305 08RENME 22:24:39.345273014 2018.01.26 0x1 t=[0x20000b4d2:0x16f3d:0x0] p=[0x20000b528:0xe56a:0x0] restart.xml s=[0x20000b4d2:0x16f45:0x0] sp=[0x20000b528:0xe56a:0x0] restart.xml.tmp

At this point, I didn’t have any changelog consumer in place.  I was just simply looking at the entries.  When I was done, I deregistered the changelog userid:

[root at server ~]# lctl --device orhydra-MDT0000 changelog_deregister cl5
orhydra-MDT0000: Deregistered changelog user 'cl5'

Based on what I read in the manual, this should remove all the changelog entries.  However, when I checked, I found that there were still several thousand entries:

[root at client ~]# lfs changelog orhydra-MDT0000 | head -1
705647 11CLOSE 22:24:57.281171001 2018.01.26 0x3 t=[0x20000ac25:0x1e308:0x0]
[root at client ~]# lfs changelog orhydra-MDT0000 | tail -1
742926 11CLOSE 22:26:04.108790913 2018.01.26 0x3 t=[0x20000ac25:0x1e313:0x0]

I tried to clear them using the lfs changelog_clear command, but of course that failed because the userid was no longer registered.  I also waited a while in case it just took some time to clear the entries, but after several hours, they were still there.

Am I misunderstanding what is supposed to happen when a userid is deregistered?  Or did I mess up a command somewhere?  Or is this a bug?

--
Rick Mohr
Senior HPC System Administrator
National Institute for Computational Sciences
http://www.nics.tennessee.edu