VSAN All Flash Writes first to Cache Tier Buffer !!!

[VSANNewbie]

HI There, I have a quick question around that according to the document:

Writes are performed at the cache layer and then de-staged to the capacity layer, as needed.

Above excerpt is from "VMware VSAN 6.6 Technical Overview".

In a hybrid environment, mostly, if we have FTT = 1, the LSOM needs to confirm it its written both the components to Capacity Tier.

Now, my question is if we have All Flash, and most of the writes performed with write buffer, so with FTT =1, whether it will write to Cache Tier of both the hosts where the components lies or just one buffer and then later it will re-stage.

Need the confirmation for the above.

Many thanks.

[jameseydoyle · Accepted solution]

Writes will be performed to both cache devices for the 2 disk groups. The cache device is used only for a single diskgroup. Therefore, for a write to be a success, acknowledgements need to be received back from the cache device for both diskgroups.

Actually, until the data has been destaged, it will also be used as read cache for that particular block of data. In other words, if the VM subsequently performs a read on the newly written block, it will will be serviced by the cache tier. Therefore, it must be written to both cache devices/hosts, because if we had a failure on one of the hosts, we would want to be sure that we could read the block from the other host.

[VSANNewbie]

Many thanks James for a quick response.