Bug 1156637

Description of problem:

Gluster small-file creates have negative scalability with brick count.  This prevents Gluster from getting reasonable small-file create performance with 

a) JBOD (just a bunch of disks) configurations and
b) high server counts

Version-Release number of selected component (if applicable):

RHSS 3.0 = glusterfs*-3.6.0.28-1.el6rhs

client does not matter.

How reproducible:

Every time.

Steps to Reproduce:
1.  Create Gluster volume with 2, 4, 8, 16, 32 .. bricks (easy to do with JBOD)
2.  run smallfile or similar workload on all clients (glusterfs for example)
3.  measure throughput per brick

Note: Dan Lambright and I were able to take this testing out to 84 bricks using virtual machines with a single disk drive as a brick for each VM, and 3 GB + 2 CPU cores/VM.  We made sure replicas were on different physical machines.  See article below for details.  

Actual results:

At some point throughput levels off and starts to decline as brick count is increased.  However, with Gluster volume parameter cluster.lookup-unhashed off instead of default value of on, throughput continues to increase, though perhaps not linearly.

A dangerous workaround is "gluster v set your-volume cluster.lookup-unhashed off", but if you do this you may lose data.

Expected results:

Throughput should scale linearly with brick count, assuming number of bricks/server is small.

Additional info:

https://mojo.redhat.com/people/bengland/blog/2014/04/30/gluster-scalability-test-results-using-virtual-machine-servers

for Red-Hat-external folks, available upon request.

Gluster volume profile output shows that without this tuning, LOOKUP FOP starts to dominate calls and eventually %latency as well.  For example, with just 2 servers and 6 RAID6 bricks/server in a 1-replica volume, we get something like this:

Interval 2 Stats:
   Block Size:              65536b+ 
 No. of Reads:                    0 
No. of Writes:                 4876 
 %-latency   Avg-latency   Min-Latency   Max-Latency   No. of calls         Fop
 ---------   -----------   -----------   -----------   ------------        ----
      0.00       0.00 us       0.00 us       0.00 us           4881      FORGET
      0.00       0.00 us       0.00 us       0.00 us           4876     RELEASE
      0.08      46.11 us      18.00 us     208.00 us            160      STATFS
      0.44      37.75 us      14.00 us     536.00 us           1081        STAT
      1.54      29.12 us       6.00 us    1070.00 us           4876       FLUSH
      8.44     160.01 us      80.00 us     935.00 us           4876       WRITE
     14.74     279.62 us     126.00 us    2729.00 us           4877      CREATE
     74.76     100.29 us      33.00 us    2708.00 us          68948      LOOKUP
 
    Duration: 10 seconds
   Data Read: 0 bytes
Data Written: 319553536 bytes

The number of LOOKUP FOPs is approximately 14 times the number of CREATE FOPs, which makes sense because there are 12 DHT subvolumes and it checks all of them for existence of a file with that name before it does a CREATE.   However, this shouldn't be necessary if DHT layout hasn't changed since volume creation or last rebalance.  

Jeff Darcy has written a patch at https://review.gluster.org/#/c/7702/ to try to make "cluster.lookup-unhashed=auto" be a safe default where we don't have to do exhaustive per-file LOOKUPs on every brick, unless the layout changes, and in that circumstance we can get back to a good state by doing a rebalance (did I capture behavior?)