Todd Lipcon has submitted this change and it was merged. ( 
http://gerrit.cloudera.org:8080/8995 )

Change subject: fs: update default data dir group size
......................................................................

fs: update default data dir group size

This patch makes fs_target_data_dirs_per_tablet non-experimental and
sets the new default to 3. Upon experimenting with the flag via YCSB
workloads, only read workload tail latency seemed to be affected at very
small group sizes (e.g. 1). 3 seems like a reasonable choice, and we can
always update it in the future.

Additionally, this change was tested on an 8-node cluster with an
analytical workload (TPC-H, scale factor: 300), each node with 7
data dirs and a separate WAL dir. I updated the disk group size to 3
and ran a TPC-H workload, results below averaged across 10 runs.

Size     | q1      q2      q3      q4      q5      q6     q7      q8      q9    
 q10     q12     q13     q14    q15    q16     q17     q18      q19    q20     
q21
All      | 13.931  10.628  30.917  47.363  37.468  1.914  54.252  23.196  
52.618 19.074  19.171  34.751  4.077  2.738  14.617  64.92   95.475   22.659 
25.13   155.384
Three    | 14.343  11.174  30.932  48.172  38.072  1.906  54.489  22.494  53.88 
 18.478  18.408  33.6    3.991  2.933  15.567  79.578  100.285  25.182 26.979  
153.592
Slowdown | 0.412   0.546   0.015   0.809   0.604   -0.008 0.237   -0.702  1.262 
 -0.596  -0.763  -1.151  -0.086 0.195  0.95    14.658  4.81     2.523  1.849   
-1.792

Note that existing data will not be updated with the new flag. Only new
tablets will honor the sizing.

Change-Id: I2dd0d3f8cf140f684318c0dffb45a1091302ecdc
Reviewed-on: http://gerrit.cloudera.org:8080/8995
Tested-by: Kudu Jenkins
Reviewed-by: Todd Lipcon <[email protected]>
---
M src/kudu/fs/data_dirs.cc
M src/kudu/tserver/tablet_server-test.cc
2 files changed, 13 insertions(+), 10 deletions(-)

Approvals:
  Kudu Jenkins: Verified
  Todd Lipcon: Looks good to me, approved

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Gerrit-Project: kudu
Gerrit-Branch: master
Gerrit-MessageType: merged
Gerrit-Change-Id: I2dd0d3f8cf140f684318c0dffb45a1091302ecdc
Gerrit-Change-Number: 8995
Gerrit-PatchSet: 7
Gerrit-Owner: Andrew Wong <[email protected]>
Gerrit-Reviewer: Adar Dembo <[email protected]>
Gerrit-Reviewer: Andrew Wong <[email protected]>
Gerrit-Reviewer: Kudu Jenkins
Gerrit-Reviewer: Todd Lipcon <[email protected]>

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