On 01/09/2016 02:34 AM, Wukongming wrote:
Hi, all
I notice this sentence "Running GFS or OCFS on top of RBD will not work with caching enabled." on http://docs.ceph.com/docs/master/rbd/rbd-config-ref/. why? Is there any way to open rbd cache with ocfs2 based on? Because I have a fio test with qemu-kvm config setting cache=none, which give a terrible result of IOPS less than 100 ( fio --numjobs=16 --iodepth=16 --ioengine=libaio --runtime=300 --direct=1 --group_reporting --filename=/dev/sdd --name=mytest --rw=randwrite --bs=8k --size=8G)
, while other non-ceph cluster could give a result of IOPS to 1000+. Would disabling rbd cache cause this problem?
OCFS, GFS, and similar cluster filesystems assume they are using the
same physical disk.
RBD caching is client side, so if you are accessing the same rbd image
from more than one
client, they have independent caches that are not coherent. This means
something like
ocfs2 could cache data in one rbd client, try overwriting it in another
rbd client, and still se
the original data in the first client. With a regular physical disk,
this is not possible, since its
cache is part of the device.
Your diagram shows you using qemu - in that case why not use the rbd
support built into
qemu, and avoid a shared fs entirely?
Josh
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