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In VMWare ESXi / vSphere, you can mark a disk or LUN as "Flash", indicating that the disk is an SSD (or the LUN is SSD/flash storage backed)

When you do, you get the following warning:

Marking HDD disks as flash disks could deteriorate the performance of datastores and services that use them. Mark disks as flash disks only if you are certain that those disks are flash disks.

Marking HDD disks as flash disks could deteriorate the performance of datastores and services that use them. Mark disks as flash disks only if you are certain that those disks are flash disks.

This warning makes me curious: what does marking a LUN as flash actually do? How does it change VMware's behavior / performance? How could it deteriorate performance, I.E. what technically changes at a low-level?

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    Great question. I've casually wondered about this for quite a while now, but have never endeavored to find the answer. – EEAA Jun 16 '17 at 2:24
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    Flash drives have no seek time unlike mechanical drives, so perhaps it removes efforts to keep data defragmented. – Jason Martin Jun 16 '17 at 2:24
  • I suspected something similar @JasonMartin but didn't want to just guess, wanted to know for sure! – Josh Jun 16 '17 at 2:26
  • kb.vmware.com/selfservice/microsites/… It would seem that VMware tries to avoid writing to it or putting it in a citical path – quadruplebucky Jun 16 '17 at 3:31
  • That's very helpful @quadruplebucky, reading now. Thanks! – Josh Jun 16 '17 at 3:36
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When you mark a disk as flash disk, VMware primarily tries to use it as a cache for the datastore. Since SSDs are much much faster than hard disks it improves the performance of the datastore. If you mark a hard disk as flash disks VMware tries to use it as a cache, expecting very low seek times and high throughput. Since a hard disk can't deliver this performance, you'll notice degraded performance on the datastore.

A word of caution: It's been a while I heard this on a VMware training, I might remember it wrongly or in the wrong context, so take it with a grain of salt.

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    I'm pretty sure ESXi won't use SSDs automatically for caching, as far as I know you need to configure caching devices explicitly. Nevertheless, you're right about this: If you mark slow HDDs as SSDs and use them for caching, you will see less performance instead of more. As far as I know, marking devices as SSD is a feature for situations where you (RAID-) controller hides the true fetaures of the device. Don't mark HDDs as SSDs if you need SDD performance! – Mario Lenz Jun 21 '17 at 20:55

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