Hi,
On 1/18/19 3:11 PM, [email protected] wrote:
Hi.
We have the intention of using CephFS for some of our shares, which we'd
like to spool to tape as a part normal backup schedule. CephFS works nice
for large files but for "small" .. < 0.1MB .. there seem to be a
"overhead" on 20-40ms per file. I tested like this:
root@abe:/nfs/home/jk# time cat /ceph/cluster/rsyncbackups/13kbfile >
/dev/null
real 0m0.034s
user 0m0.001s
sys 0m0.000s
And from local page-cache right after.
root@abe:/nfs/home/jk# time cat /ceph/cluster/rsyncbackups/13kbfile >
/dev/null
real 0m0.002s
user 0m0.002s
sys 0m0.000s
Giving a ~20ms overhead in a single file.
This is about x3 higher than on our local filesystems (xfs) based on
same spindles.
CephFS metadata is on SSD - everything else on big-slow HDD's (in both
cases).
Is this what everyone else see?
Each file access on client side requires the acquisition of a
corresponding locking entity ('file capability') from the MDS. This adds
an extra network round trip to the MDS. In the worst case the MDS needs
to request a capability release from another client which still holds
the cap (e.g. file is still in page cache), adding another extra network
round trip.
CephFS is not NFS, and has a strong consistency model. This comes at a
price.
Regards,
Burkhard
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