300-610 · Question #295
A Cisco engineer is analyzing a customer environment where dozens of VMs are running on a physical server and UCS chassis are cascaded. The setup uses two ISLs in a port channel to the SAN switch (per
The correct answer is C. Configure 802 1Qbb priority flow control. The described scenario of high VM oversubscription on SAN ISLs indicates a network congestion issue for storage traffic, leading to an IOPS bottleneck.
Question
A Cisco engineer is analyzing a customer environment where dozens of VMs are running on a physical server and UCS chassis are cascaded. The setup uses two ISLs in a port channel to the SAN switch (per fabric). The oversubscription from VM to physical SAN port is often 20 (VMs/host) x 72 (9 UCS chassis cascaded = 72 hosts):
2 ISLs from fabric interconnect to SAN switch port on the NPV core (NPIV enabled switch) This results to 20 x 72 2 per single ISL = 720 VMs per ISL 20 (VMs per host) x 8 (hosts per chassis) x 9 (chassis per port channel to SAN switch) / 2 (ISLs per port channel) = 20 x 8 x 9 / 2 = 720 VMs per SAN F-port What is the best way to resolve possible IOPS bottleneck?
Options
- ADeploy a content delivery network
- BEnable Windows 1Gb server full duplex
- CConfigure 802 1Qbb priority flow control
- DRemove data encryption on the SSD
How the community answered
(20 responses)- A15% (3)
- B5% (1)
- C75% (15)
- D5% (1)
Why each option
The described scenario of high VM oversubscription on SAN ISLs indicates a network congestion issue for storage traffic, leading to an IOPS bottleneck.
Deploying a content delivery network (CDN) is for optimizing content delivery to end-users over the internet, not for resolving internal SAN IOPS bottlenecks in a data center.
Enabling Windows 1Gb server full duplex is a basic network configuration for a server's Ethernet interface and does not address SAN-level IOPS bottlenecks related to inter-switch links or high VM oversubscription.
802.1Qbb Priority Flow Control (PFC) is a Data Center Bridging (DCB) standard designed to provide lossless Ethernet for Fibre Channel traffic (FCoE). By selectively pausing traffic on specific priority queues during congestion, PFC prevents frame drops and ensures storage traffic, which is highly sensitive to packet loss, can maintain its expected performance even in oversubscribed environments, thereby resolving IOPS bottlenecks.
Removing data encryption on the SSD might improve the performance of individual local SSDs, but it does not address a SAN-level IOPS bottleneck caused by network oversubscription on the Fibre Channel ISLs.
Concept tested: 802.1Qbb Priority Flow Control for SAN IOPS optimization
Source: https://www.cisco.com/c/en/us/td/docs/unified_computing/ucs/sw/qos/2-0/b_Cisco_UCS_QoS_Management_Guide/Cisco_UCS_QoS_Management_Guide_chapter_010.html
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