Re: rados benchmark question

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Having your journals on the same disk causes all data to be written twice, i.e. once to the journal and once to the
osd store.  Notice that your tested throughput is slightly more than half your expected maximum...


On Wed, Jan 1, 2014 at 11:32 PM, Dietmar Maurer <dietmar@xxxxxxxxxxx> wrote:

Hi all,

 

I run 3 nodes connected with a 10Gbit network, each running 2 OSDs.

 

Disks are 4TB Seagate Constellation ST4000NM0033-9ZM (xfs, journal on same disk).

 

# ceph tell osd.0 bench

{ "bytes_written": 1073741824,

  "blocksize": 4194304,

  "bytes_per_sec": "56494242.000000"}

 

So a single OSD can write up to OSD_BW=55MB/s

 

I have 6 OSDs, so I would expect an overall write bandwidth of:

 

6*OSD_BW = 6*55 = 330MB/s

 

To verify that, I created a pool with size=1, and run:

 

# rados -p testpool bench 300 write --no-cleanup

Total time run:         300.905982

Total writes made:      13487

Write size:             4194304

Bandwidth (MB/sec):     179.285

 

Stddev Bandwidth:       106.954

Max bandwidth (MB/sec): 400

Min bandwidth (MB/sec): 0

Average Latency:        0.356925

Stddev Latency:         0.696282

Max latency:            8.5891

Min latency:            0.025648

 

This is much lower than the expected bandwidth (179 < 330).

 

Is this normal? If so, what is the reason for that?

 

 


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