On 11/14/2011 01:54 AM, Tom Horsley wrote:
You might want to change the default virtio disk cache mode.
I was just doing some tests (after finally getting the latest
virtio scsi drivers installed correctly on my Windows XP virtual
machine).
The test was running the little "sdelete" tool which writes
a temp file full of zeroes to the disk in order to zero
out all the free space (very handy for converting the resulting
image to a maximally compressed qcow2 image).
This is a bad test. Most likely the 'sdelete' utility insists on
overwriting the file multiple times (but strangely doesn't flush the
buffers); with cache=writeback these writes and overwrites hit the host
RAM and return immediately. It's not the right workload to test
performance.
The recommendation is cache=none which gives good performance and cpu
utilization. cache=writeback offers better performance, but it also
interferes with other workloads on the same host by filling up page
cache. It's okay to use it, but understand that it isn't free.
--
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