[German]Another small piece of information for administrators of a Hyper-V cluster under Windows Server 2019. It looks like Intel has disabled RSC support for all NICs on the driver side as of release 1903. In addition, storage stacks write operations tagged as write-through, bypassing caching. A user has informed me about that.
I got that information from blog reader Alexander F. a couple of weeks ago in beginning of January 2021. Alex notified me via email about the issues he recently stumbled across (thanks for that).
Intel RSC support for NICs disabled in Hyper-V
In his email he wrote: I stumbled across the following yesterday while updating from a 2019 Hyper-V cluster.
It looks like Intel has disabled RSC support for all NICs, driver-side for Server 2019 as of release 1903!
The acronym RSC stands for Receive Segment Coalescing. Microsoft introduced the process in Windows Server 2012 to reduce the network load on the server CPU. The background is that incoming network packets on the network adapter (NIC) trigger an interrupt of the CPU for processing. For virtualized guests, the load from these incoming network packets and thus the CPU can become very high. Receive Segment Coalescing (RSC) is designed to reduce this load by offloading packet handling to the network interface card (NIC). The NIC can combine the network traffic into larger packets and pass them to the processor for processing. An explanation of this topic can be found in this Microsoft article. Alexander wrote:
But I haven’t found any more detailed info anywhere, why this is now all of a sudden. Well, I never really understood the sense of offloading in a virtualized environment.
I have deactivated all offloading on the NIC’s of several Hyper-V clusters in the last months and have received only positive feedback from the respective customers.
Alexander has explained the whole thing in the Spiceworks community in this article. He noticed the whole thing when he had to update a Hyper-V 2019 cluster. In doing so, he also installed the latest Intel network driver with version 25.6 on the two nodes of the cluster. When checking the NIC settings in the advanced settings, he noticed after the update that the NIC could no longer be configured with regard to RSC. This exists on both the Intel X722 NICs and the Intel I350 NICs. Before the update, he could still retrieve and adjust the RSC configuration in the NIC settings.
Hyper-V storage stacks write operations as write-through
Alexander pointed out another oddity related to Windows Server 2019 in his email and wrote:
I uncovered yet another mess from Microsoft at the end of the year starting with Server 2019. Starting with [Windows] Server 2019, all write operations of the Hyper-V storage stack, towards the end storage (RAID/SAN) are hard tagged as “write-through”.
Regardless of what the software causing the IO has previously specified. This means that the write-back cache of any RAID controller behind it, or even of a SAN, is more or less forcibly leveraged.
Alexander described the whole thing in more detail in this Spiceworks community post. He also pointed out in his mail that the only reference from Microsoft to the change can be found in the document Hyper-V storage Caching layers and implications for data consistency – Windows Server | Microsoft Docs.
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