Use of TLC drives in all flash vSAN

[carlo_COT]

Deciding between using Dell (Samsung) PM1725 TLC based PCIe SSD drives and Intel P3700 MLC based PCIe drives for use as the cache drives in an all flash vSAN.  Both drives appear on the vSAN HCL.

Is there a possible performance issue with using the TLC drives?  The performance according to the datasheets on each drive look to be very similar.  I've read that TLC drives use an SLC based write cache to mitigate the performance issues of using TLC, and that when you reach a certain sustained IO the performance hits a wall.  I'm worried about this happening when putting a server in maintenance mode and evacuating all data.  Of course the datasheets are giving you numbers that best case scenarios.

Maybe there are other factors to consider.  Can anyone chime in please?

Thanks!

[admin]

I would say go with MLC drives. There are significant number of articles on this and most of these articles suggest to use MLC drives over TLC drives for better performance, endurance and many more reasons.

You may want to have a look at these articles:

Everything You Need to Know About SLC, MLC, and TLC NAND Flash | My Digital Discount - MyDigitalDiscount.com‌

SLC, MLC or TLC NAND for Solid State Drives ? :: SG FAQ‌

Hope this helps you to decide!

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[Doug Arcidino]

For the cache layer, absolutely do not use the TLC drive. This will cause inconsistency in the performance and definitely would have noticeable performance issues during heavy use. You can use VCG supported TLC drives for your capacity layer if you want but I would not do it for cache.

[carlo_COT]

Thanks for the feedback.  I thought this was the case, and I can't understand why the PM1725 drives are on the vSAN HCL.

VMware Compatibility Guide - ssd

[Doug Arcidino]

Vendors supply hardware to us(VMware) and then we run them against a series of tests. We test SSD drives for use as cache in AF and hybrid and as capacity in AF configurations. As long as they pass all of our tests, we publish them. That's my understanding of the process. It's worth noting that SSDs of this model are only support as cache for AF configurations and not hybrids. This is telling me that the drive can't keep up on read and writes enough to front end HDDs. It should work fine but if you have the ability and money to use MLC, go for it.

[carlo_COT]

Thank you.  That is precisely the response I was looking for!

[Doug Arcidino]

It's worth noting that we retest hardware with every new version of VSAN. So all of these drives will be retested for compatibility with 6.5 and the VCG may change.... Not all devices will be approved before 6.5 goes GA.

Happy to help!