Finally Dell Support budged. This is the latest update from support: -- Write Back Cache Enablement We are actively driving a plan to deliver a Write-Back option (via firmware) for Dual PERC configurations. We are working this plan with great urgency and will have it implemented as soon as we are able complete our development and Enterprise validation of the firmware. --
Hi, I have been struggling with a storage performance issue for a few months. I found out that the culprit in this performance hit, is the Fault Tolerant Shared PERC 8 in the VRTX. In this configuration you are bound to the write-through caching policy, which is currently performing very bad. Because of this big performance hit on the storage (caused by the caching policy) I had the following problems: The release notes of the latest firmware (23.8.12-0061) for the SPERC 8 state: Firmware supports VRTX running with single SPERC8 controller;- Write-back cache is enabled for VDs. - Does not have high availability capability  Firmware supports VRTX running with dual SPERC8 controller; - High availability capability present (Both SPERC8 needs to be connected to expanders) - One of the controllers is active and the other is passive - Write-back cache is disabled for VDs; only option for write cache is Write Through. To downgrade from a dual controller to a single controller, I removed the second SPERC8 controller, removed the second expander, re-cabled the SAS connections and downgraded the chassis to 1.25. Just removing the controller and expander wasn't enough, because then I still was unable to set "write-back" as caching policy. As soon as I downgraded to 1.25 I was able to set write-back as caching policy. With the single controller configuration, the storage performance is as it should be. All the earlier mentioned problems are gone: From this experience I currently strongly advise to not go for the dual SPERC8 configuration and stick with the single configuration. Hopefully a firmware update will solve this performance issue, because at the moment having the dual / fault tolerant SPERC 8 configuration with this very poor storage performance makes no sense at all. Yeah, you're fault tolerant... but no backup and bad performing VM's aren't really a good option in a production environment I think... Or might there be an essential setting I am missing for the write through setup? (I have tried different settings already...) Below are the benchmarks made using Atto. The difference is tremendous! Funny fact: while doing the benchmark with the write back policy enabled, the virtual disk was still in its background initialization  process, and with the write through it was not.  [IMAGE]  NOTE: Both VD's were configured as RAID6
Performance issue / poor performance: VRTX with Fault Tolerant / Dual Shared 
PERC 8