VMFS datastore vs RDM in unity 300

Question

Hello

I wanted to compare performance of VMFS Datastore vs RDM .I used IOMETER IOPS and latency measuring in EMC Unity 300 , I created one pool that include 8 SSD with 100 GB LUN

i test the performance of lun with 2 situation ,in 1st test I created VMFS Datastore that present to a VM and test that with IOMETER after that in 2nd test i present LUN to vm with RDM the result was very different.

the specification of IOMETER include :

block size : 64kbyte

write/read : 30/70

random / sequential : 100/0

I extracted these result for VMFS:

total IO : 500

latency: 3 ms

I extracted these result for RDM:

total IO: 2000 l

atency: 0.3 ms

why the values very different?

Rainer_EMC · Answer

assuming that everything else in your test is the same and you arent seeing other side effects
keep in mind you are testing two very different configs

when you use RDM your client I/O gets sent unchanged to the Unity

when you use a vmdk in a ESX datastore you are going through the vmfs file system on the ESX server.
Not only does that have some overhead it will also change the I/O type and block size

Take a look at the performance stats on Unity - you should see differences there

Its not really "fair" to compare these - vmfs has quite some benefits and functionality that arent available with RDM

Unity

VMFS datastore vs RDM in unity 300

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