We create one million empty files through filebench, here is the test env:
MDS: one MDS
MON: one MON
OSD: two OSD, each with one Inter P3700; data on OSD with 2x replica
Network: all nodes are connected through 10 gigabit network

We use more than one client to create files, to test the scalability of
MDS. Here are the results:
IOPS under one client: 850
IOPS under two client: 1150
IOPS under four client: 1180

As we can see, the IOPS almost maintains unchanged when the number of
client increase from 2 to 4.

Cephfs may have a low scalability under one MDS, and we think its the big
lock in
MDSDamon::ms_dispatch()::Mutex::locker(every request acquires this lock),
who limits the
scalability of MDS.

We think this big lock could be removed through the following steps:
1. separate the process of ClientRequest with other requests, so we can
parallel the process
of ClientRequest
2. use some small granularity locks instead of big lock to ensure

Wondering this idea is reasonable?

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