baber
Expert
Expert

Why we choose low disk capacity for cache tier when we are using high endurance disks

As I find out there is a direct relation between the size of cache tier and disk's endurance for cache tier according to attach pic when we are using mixed use with DWPD =3 , we have to select large disk capacity 2.4T but when we are using optane ssd as it has DWPD 30 
we can use low capacity 300G . Would you please tell me why we need low disk capacity when we are using high endurance disks ?

 

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TheBobkin
VMware Employee
VMware Employee

@baber , "I find out there is a direct relation between the size of cache tier and disk's endurance for cache tier" - not necessarily, though for any model of SSD/NVMe (e.g. using same flash type) the endurance *should* scale roughly linearly e.g. a 2TB P4600 TLC NVMe may have ~10500 TBW where a 4TB of the same model would be ~21000 TBW. However, this is is not cross-comparable with other models of SSD/NVMe as there is a lot of possible differences including type of NAND memory used, %/TB of over-provisioned cells etc. .

 

"Would you please tell me why we need low disk capacity when we are using high endurance disks ?"
For example:
375GB P4800X - 20500 TBW:
https://www.vmware.com/resources/compatibility/detail.php?deviceCategory=ssd&productid=42342
2TB P4600 - 10490 TBW:
https://www.vmware.com/resources/compatibility/detail.php?deviceCategory=ssd&productid=41826

 

As you can clearly see, both of these devices are rated for vSAN AF Cache-tier and are also Endurance Class D (>=7300 TBW), though you can also see that the above P4800X is less than 1/5th of the size and has 2x the endurance of the above P4600.

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