Hi all, we've been seeing persistent problems when trying to create hardlinks
on cephfs; it's returning EXDEV in a way that makes no sense given typical
POSIX behaviour and ceph documentation. Here's a typical strace of the problem:
78 13:47:26.572435
link("/data/db/hdb/data/2023.08.06/table1.0/column1",
"/data/db/hdb/data/2023.08.06/table1.1/column1") = -1 EXDEV (Invalid
cross-device link)
78 13:47:26.577661 write(1,
"{\"time\":\"2025-03-03T13:47:26.577z\",\"component\":\"MSVC\",\"level\":\"INFO\",\"message\":\"[eoi-78]
Retrying in 500 milliseconds\",\"service\":\"eoi\"}\n", 136) = 136
78 13:47:26.577762 clock_nanosleep(CLOCK_REALTIME, 0, {tv_sec=0,
tv_nsec=500000000}, NULL) = 0
78 13:47:27.078037
link("/data/db/hdb/data/2023.08.06/table1.0/column1",
"/data/db/hdb/data/2023.08.06/table1.1/column1") = 0
We try creating a link, get EXDEV, wait 500 milliseconds, then try the same
operation again and it succeeds. The link and its target are both on the same
cephfs mount (/data/db/hdb in this case), so the normal POSIX 'linking between
filesystems' explanation doesn't apply. I've looked through the ceph client
and server code and from what I've seen EXDEV is only returned in a couple of
other situations: linking between snapshots, and linking across quotas. Neither
snapshots nor quotas were in use here, and if they were the culprit it seems
unlikely the automatic retry would have worked. Web searches on EXDEV errors in
ceph have also proven to be a dead end. My best guess, although it's not a very
good one, is that stale MDS cache data is somehow involved -- in one case the
issue reportedly got much worse after increasing (!) the MDS memory limit.
This error has been occurring for a particular client for upwards of 9 months
and has proven stubbornly resistant to reproduction elsewhere (we are working
on migrating them to a more recent ceph version to see if the error remains),
so our technical investigations haven't got particularly far. I was hoping
someone here on ceph-users would have seen similar EXDEV errors in the wild or
in development and have some insight into what could be causing them.
Regards, Domhnall
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