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Is it possible to turn off caching for the whole partition which stores the virtual machines?
I know it can be done individually for each VM but considering that all virtualized operating systems will always be doing their own caching it seems more correct to turn it off for the whole partition which stores all of my VMs.
You could use sync on the file system mount options but there may be significant performance penalties (would have to test). If the data the VMs are writing is critical it might be worth investigating whether they can use raw partitions instead of files on the host.
You could use sync on the file system mount options but there may be significant performance penalties (would have to test). If the data the VMs are writing is critical it might be worth investigating whether they can use raw partitions instead of files on the host.
Thanks catkin, I will try the 'sync' mount option, see how that performs. It is not so much that the data is critical but that I want to prevent double caching, all of the host VMs will be doing their own caching anyway just don't want to double up with caching on top of caching and the problems that entails.
Speed; double caching is very slow turning off caching on the VMs allows them to run much faster.
Thanks Nick
I am surprised. Normally caching is faster because the data is written less often (and in larger chunks, but that has a small effect on the time taken), hence caching vs. not caching is usually a choice between performance and data security. Do you have any test results to demonstrate the scale of the problem?
Why I don't know. Just that I have found Win 7 VMs install much quicker with individual machine caching set to none rather than default. Now instead of having to do this for each VM I thought that because I have all VMs in the same partition perhaps I could turn off caching for all of them by using this mount option.
Why I don't know. Just that I have found Win 7 VMs install much quicker with individual machine caching set to none rather than default. Now instead of having to do this for each VM I thought that because I have all VMs in the same partition perhaps I could turn off caching for all of them by using this mount option.
which caching option is this? where do you install the VM from? you cannot post such claims without investigating the underlying root cause, that's all I'm saying, and your conclusions are pretty far fetched
---------- Post added 03-28-12 at 01:20 PM ----------
Quote:
Originally Posted by Nick_C
How can I see what mount options are currently in use
Code:
mount
Quote:
and then how can I add that sync mount option to the mounting of my /vm partition?
edit your fstab, and add the option after "defaults"
would be great to find out, at the lowest possible level, what is causing this. I don't think double caching as such is the culprit, but maybe in the way qemu implements it's caching, some extra overhead gets introduced - this needs to be proven of course.
In addition, you might be running buggy builds that do weird things, seen plenty of that before
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