VSAN-Troubleshooting-Reference-Manual
VSAN-Troubleshooting-Reference-Manual
VSAN-Troubleshooting-Reference-Manual
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Diagnostics and <strong>Troubleshooting</strong> <strong>Reference</strong> <strong>Manual</strong> – Virtual SAN<br />
In the case of all-flash disk groups, the goal is to keep as many hot blocks (written<br />
often) in the flash cache device. Thus, a flash cache device that has a higher<br />
endurance will be able to satisfy a higher rate of write operations. There is no<br />
attempt made to optimize all-flash configurations for proximal destaging, as flash<br />
devices handle random workloads very well.<br />
Latency consideration with distributed cache<br />
A common question related to the latency the network introduces, if you need to<br />
access data that resides on another host in the cluster.<br />
Typical latencies in 10GbE networks are in the range of 5 – 50 microseconds. When<br />
you see the specs of flash devices you hear about latencies in the range of 50 – 100<br />
microseconds. But these are latencies values when you send one I/O at a time. When<br />
you operate those flash devices at thousands of IOPS, then latency values start to<br />
grow to 1 millisecond or even higher in some cases. Adding something like 10<br />
microseconds on top of 1 millisecond of latency doesn’t make a huge difference to<br />
the overall performance.<br />
How VM Storage Policies impact performance<br />
Where stripe width helps and where it may not<br />
While setting disk striping values can sometimes increase performance, that isn’t<br />
always the case. As an example, if a given test is cache-friendly (e.g. most of the data<br />
is in cache), striping won’t impact performance significantly. As another example, if<br />
a given VMDK is striped across disks that are busy doing other things, not much<br />
performance is gained, and may actually become worse.<br />
A word of caution on flash read cache reservation<br />
The policy setting of Flash Read Cache Reservation (applicable to hybrid only)<br />
should be used only in the event of an identifiable performance improvement. If you<br />
set a read cache reservation in a policy, and you associate it with multiple virtual<br />
machines, you could consume a significant amount of flash on the host or hosts in<br />
question, thus starving the other virtual machines of valuable cache resources.<br />
Handle with care!<br />
Since all virtual machines will share the read cache and write buffer equally, it is<br />
strongly recommended that you do not assign a dedicated amount of read cache to<br />
virtual machines unless there is a pressing need to solve an underlying performance<br />
issue for a specific set of virtual machines.<br />
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