[ceph-users] 2x replication: A BIG warning

Wido den Hollander wido at 42on.com
Wed Dec 7 00:08:51 PST 2016


As a Ceph consultant I get numerous calls throughout the year to help people with getting their broken Ceph clusters back online.

The causes of downtime vary vastly, but one of the biggest causes is that people use replication 2x. size = 2, min_size = 1.

In 2016 the amount of cases I have where data was lost due to these settings grew exponentially.

Usually a disk failed, recovery kicks in and while recovery is happening a second disk fails. Causing PGs to become incomplete.

There have been to many times where I had to use xfs_repair on broken disks and use ceph-objectstore-tool to export/import PGs.

I really don't like these cases, mainly because they can be prevented easily by using size = 3 and min_size = 2 for all pools.

With size = 2 you go into the danger zone as soon as a single disk/daemon fails. With size = 3 you always have two additional copies left thus keeping your data safe(r).

If you are running CephFS, at least consider running the 'metadata' pool with size = 3 to keep the MDS happy.

Please, let this be a big warning to everybody who is running with size = 2. The downtime and problems caused by missing objects/replicas are usually big and it takes days to recover from those. But very often data is lost and/or corrupted which causes even more problems.

I can't stress this enough. Running with size = 2 in production is a SERIOUS hazard and should not be done imho.

To anyone out there running with size = 2, please reconsider this!



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