On 09/14/2012 10:06 AM, Tommi Virtanen wrote:
On Fri, Sep 14, 2012 at 8:51 AM, Travis Rhoden <trhoden@xxxxxxxxx> wrote:
On a running Ceph cluster using XFS for the OSD's, is it safe to
defrag the OSD devices while the system is live?
I did a quick check of one device:
xfs_db -c frag -r /dev/sdd
actual 637596, ideal 144935, fragmentation factor 77.27%
If it's safe to defrag xfs while it's mounted in general, it's safe to
do it when an OSD is running. Xfs either keeps its promises as a
filesystem, or doesn't.
How that affects performance is another question..
While I'm talking about XFS... I know that RBD's use a default object
size of 4MB. I've stuck with that so far.. Would it be beneficial to
mount XFS with -o allocsize=4M ? What is the object size that gets
used for non-RBD cases -- i.e. just dumping objects into data pool?
Don't know about -o allocsize -- benchmark it!
Objects are the size they are; Ceph does not dictate any size. RBD and
CephFS both stripe a thing (image/file) over multiple objects, at a
constant size; you already know that, RBD defaults to 4MB. Other users
of RADOS create objects of any size they please, and an OSD stores
those as files in the underlying filesystem.
Also keep in mind that objects can be sparse - the 4MB stripe size
doesn't mean the full 4MB are used by an object in CephFS or RBD.
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