On 02/06/2014 01:41 PM, Christian Kauhaus wrote:
Am 06.02.2014 16:24, schrieb Mark Nelson:
Hi Christian, can you tell me a little bit about how you are using Ceph and
what kind of IO you are doing?
Sure. We're using it almost exclusively for serving VM images that are
accessed from Qemu's built-in RBD client. The VMs themselves perform a very
wide range of I/O types, from servers that write mainly log files to ZEO
database servers with nearly completely random I/O. Many VMs have slowly
increasing storage utilization.
A reason could be that the OSDs issue syncfs() calls and ext4 cuts FS extents
from just what has been written so far. But I'm not sure about the exact
pattern of OSD/filesystem interaction.
Ok, so the reason I was wondering about the use case is if you were
doing RBD specifically. Fragmentation has been something we've
periodically kind of battled with but still see in some cases. BTRFS
especially can get pretty spectacularly fragmented due to COW and
overwrites. There's a thread from a couple of weeks ago called "rados
io hints" that you may want to look at/contribute to.
Thanks!
Mark
HTH
Christian
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