Re: CephFS file to rados object mapping

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Hi,

On 08/10/2015 22:25, Gregory Farnum wrote:

> So that means there's no automated way to guarantee the right copy of
> an object when scrubbing. If you have 3+ copies I'd recommend checking
> each of them and picking the one that's duplicated...

It's curious because I have already tried with cephfs to "corrupt" a
file in the OSD backend. I had a little text file in cephfs mapped to
the object "$inode.$num" and this object was in the PG $pg_id, in the
primary OSD $primary and in the secondary OSD $secondary (I had indeed
size == 2). I thought that the primary OSD was always taken as reference
by the "ceph pg repair" command, so I have tried this:

    # Test A
    echo "foo blabla..." >/var/lib/ceph/osd/ceph-$primary/current/$pg_id_head/$inode.$num
    ceph pg repair $pg_id

and the "repair" command worked correctly and my file was repaired
correctly. I have tried to change the file in the secondary OSD too with:

    # Test B
    echo "foo blabla..." >/var/lib/ceph/osd/ceph-$secondary/current/$pg_id_head/$inode.$num
    ceph pg repair $pg_id

and it was the same, the file was repaired correctly too. In these 2
cases, the good OSD was taken as reference (the secondary for the test
A and the primary for the test B).

So, in this case, how did ceph know which copy was the correct object?

-- 
François Lafont
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