On 2018-02-14 20:14, Steven Vacaroaia wrote:

> Hi, 
> 
> It is very useful to "set up expectations" from a performance perspective  
> 
> I have a  cluster using 3 DELL R620 with 64 GB RAM and  10 GB cluster network 
> 
> I've seen numerous posts and articles about the topic mentioning the 
> following formula  
> ( for disks WAL/DB on it ) 
> 
> OSD / replication / 2 
> 
> Example  
> My HDD are capable of 150 MB/s 
> If I have 6 OSDs, expected throughput should be aroung 250 MB/s  for a pool 
> withe replication =2  
> ( 150 x 6 / 2 / 2 ) 
> 
> How would one asses the impact of using SSD for WAL/DB i.e what performance 
> gains should I expect ? 
> Example: 
> adding an 500MB/s SDD for every 2 HDD 
> 
> Should I expect that kind of throuput on the client ( e.g Windows VM running 
> on datastore create on RBD image shared via iSCSI ) ? 
> 
> The reason I am asking is that despite rados bench meeting the expectaion, 
> local performance test are 4 times worse 
> 
> rados bench -p rbd 120 write --no-cleanup && rados bench -p rbd  120 seq 
> 
> Total time run:         120.813979 
> Total writes made:      6182 
> Write size:             4194304 
> Object size:            4194304 
> Bandwidth (MB/sec):     204.678 
> Stddev Bandwidth:       36.2292 
> Max bandwidth (MB/sec): 280 
> Min bandwidth (MB/sec): 44 
> Average IOPS:           51 
> Stddev IOPS:            9 
> Max IOPS:               70 
> Min IOPS:               11 
> Average Latency(s):     0.312613 
> Stddev Latency(s):      0.524001 
> Max latency(s):         2.61579 
> Min latency(s):         0.0113714 
> 
> Total time run:       113.850422 
> Total reads made:     6182 
> Read size:            4194304 
> Object size:          4194304 
> Bandwidth (MB/sec):   217.197 
> Average IOPS:         54 
> Stddev IOPS:          7 
> Max IOPS:             80 
> Min IOPS:             31 
> Average Latency(s):   0.293956 
> Max latency(s):       1.99958 
> Min latency(s):       0.0192862 
> 
> Local test using CrystalDiskMark  
> 
> 57 MB/s seq read 
> 43 MB/s seq write 
> 
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Hi Steven, 

I do not believe there is formula as there are many factors involved.
The "OSD / replication / 2" is probably related to the theoretical peak
of a filestore based OSD with collocated journal, practically this does
not mean you will reach this due to other factors. If you have a link
with performance formulas, it would be interesting to know. 

For your test, i would check: 

The rados benchmark default values is using 4M objects and 16 threads.
You need to set your CrystalDiskMark with similar parameters. 

The iSCSI target gateway should easily pull the rados throughput you are
seeing without too much drop, double check how your client initiator and
target are configured. You can also run atop or other performance tool
on your iSCSI gateway and see if you have any resource issues. 

Maged
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