Hi Nathan,
could be a regression. The write append bug was a known issue for older kernel clients. I can try to find the link. We have one of the affected kernel versions and asked our users to use a single node for all writes to a file.
In general, for distributed/parallel file systems, this is expected behaviour. It is explicitly the duty of the developer to coordinate write processes to prevent simultaneous writes to the same file area. In principle, ceph fs should do this for you using client CAPs, however, I personally think at a way too high performance cost.
Depending on what you write to, try to use a collector process on a single node that collects all input from the remote nodes and writes a single consistent stream. Basically what rsyslogd does (maybe you can abuse rsyslogd?). This will also greatly outperform any such application running on a non-broken ceph-fs client, because the coordination of meta data updates and write locks between clients is unreasonably expensive.
Best regards,
=================
Frank Schilder
AIT Risø Campus
Bygning 109, rum S14
________________________________________
From: Nathan Fish <lordcirth@xxxxxxxxx>
Sent: 07 September 2021 21:17:05
To: ceph-users
Subject: Data loss on appends, prod outage
As of this morning, when two CephFS clients append to the same file in
quick succession, one append sometimes overwrites the other. This
happens on some clients but not others; we're still trying to track
down the pattern, if any. We've failed all production filesystems to
prevent further data loss. We added 3 new OSD servers last week, they
finished backfilling a few days ago. Servers are Ubuntu 18.04, clients
mostly 18.04 and 20.04, with HWE kernels (5.4 and 5.11 respectively).
Ceph was upgraded from nautilus to octopus months ago. There were no
relevant errors or even warnings in "ceph health" before we stopped
the filesystems:
HEALTH_ERR mons are allowing insecure global_id reclaim; 20 OSD(s)
experiencing BlueFS spillover; 6 filesystems are degraded; 6
filesystems are offline
ceph versions
{
"mon": {
"ceph version 15.2.14
(cd3bb7e87a2f62c1b862ff3fd8b1eec13391a5be) octopus (stable)": 3
},
"mgr": {
"ceph version 15.2.14
(cd3bb7e87a2f62c1b862ff3fd8b1eec13391a5be) octopus (stable)": 3
},
"osd": {
"ceph version 15.2.14
(cd3bb7e87a2f62c1b862ff3fd8b1eec13391a5be) octopus (stable)": 200
},
"mds": {
"ceph version 15.2.14
(cd3bb7e87a2f62c1b862ff3fd8b1eec13391a5be) octopus (stable)": 48
},
"rgw": {
"ceph version 15.2.13
(c44bc49e7a57a87d84dfff2a077a2058aa2172e2) octopus (stable)": 1
},
"overall": {
"ceph version 15.2.13
(c44bc49e7a57a87d84dfff2a077a2058aa2172e2) octopus (stable)": 1,
"ceph version 15.2.14
(cd3bb7e87a2f62c1b862ff3fd8b1eec13391a5be) octopus (stable)": 254
}
}
I looked for bugs on the tracker but didn't see anything that seemed
like our issue. Any advice would be appreciated.
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