KVM scalability and consolidation ratio: cache none vs cache writeback

Written by Gionatan Danti on . Posted in Virtualization

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Host scalability: disks load

We previous stated that enabling the write-back cache led to increased disk performance. The following chart proves our affirmation:

The wb-enabled tiles have similar disk utilization than the no-cache ones, but they provide superior speed: if we normalize for performance, write-back cache provide much higher efficiency.

Lets spends some word on the increased average access time (await). What happens here? The answer is simple: as the write-back cache enable more I/O threads to be concurrency active, total throughput is higher but at the same time the average single-request access time grows.

Don't let await fear you: when the cache is disabled, the running threads enjoy lower access time, but you have a lower total number of I/O active threads. If your application depends on multiple, concurrent I/O write operations, it will be condemned to serially executing many of them, leading to the perception of a slower system. Enabling the wb-cache give your application a real chance to execute multiple concurrently I/O writes, and the host system can even coalesce some of them.

So, while in some specific, controlled, low-latency workload the nocache configuration can be the better choice, generally the write-back cache is the preferred one.

Detailed disk perf data:

The wb-enabled cases show much higher read and write speeds, indeed.


#1 Troels Arvin 2014-01-20 20:24
Where can one read more about the potential issues with write barriers and live migration?
#2 Gionatan Danti 2014-01-21 10:20
Quoting Troels Arvin:
Where can one read more about the potential issues with write barriers and live migration?

here you can find some informations on barriers:

On live migration and why it is better to not use it with caching, you can read libvirt and qemu man page.


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