Mixing Magnetic Drive Types in single VSAN Drive Group

[tehkuhnz]

Hey All,

I have some questions about some possible VSAN configurations etc.

So lets say - I have numerous amounts of Dell R720's servers configured in the 8* 3.5 inch drive bay configuration that I want to retrofit for VSAN.

I need to have as much capacity as possible in each unit, but I also need the storage to perform well. ( Trying to get the best of both worlds, i know )

So my proposed solution would be to provide:

1) 3.2TB ioScale card for the SSD layer

2) Three - 1.2TB 10K SAS Drives

3) Four - 6TB NL-SAS Drives

4) 1 low capacity SSD  for boot in a 2.5 inch converter( these host have greater than 512 gb of ram )

All of the above would make up 1 drive group per host ( minus the boot drive of course). I plan to have at minimum of three copies of each file ( i believe that's FTT=2)

This should net aprox 30tbish raw and roughly 10tb useable per host assuming FTT=2.

These host's would be backing a rather large VCD deployment that has demonstrated the I/O blender effect on lesser arrays. ( We were using a tier 3/d array, and it maxed out and died on us when hit around the 5000 IOPS mark )

It's hard to predict, and the constant need for more storage is staggering. ( Hopefully we can mitigate by scaling vertically with VSAN)

I suppose my main concern/question is mixing the 10K and large capacity drives. My hope is that it would be beneficial from an I/O perspective and vSAN would help balance I/O once data starts to de-stage from the flash.

A vendor suggested this approach, But I haven't really found anything out on the web supporting this theory.

I am not sure if its even recommended/supported. ( i.e. mixing magnetic drive types in a single VSAN Drive group and even if you do, that there will be notable performance increases.)

[depping · Accepted solution]

I would not recommend mixing large capacity drives with SAS drives. it will make your performance very unpredictable as it will very much depend on from which drive the IO is coming. Some may have microsecond latency when from SSD. Some may have milisecond latency when from 10K RPM and others may be tens of miliseconds when from NL-SAS and that will depend on overall load on the system. It is just not recommended as VSAN will not differentiate between the 10K and 7200 RPM drives. On top of that, with the 7200 RPM drives you also have less IOPS for more TBs.

[tehkuhnz]

Thanks for the response Duncan. I suspected mixing could be problematic as from my understanding the only kind of I/O balancing is between the SSD and magnetic layer. But confirmation of this is very helpful and very much appreciated!