PVE/KVM and a lot of RAM

[achekalin]

Hello,

here is the config problem I face:

if I need to run as fast as it possible two windows guest machines (win 2003 r2 x86) with 4 Gb of RAM each, if I have say 16 or 32 Gigs of RAM in the host server will there be any significant difference in VMs speed due to in-RAM caching on host machine vs situation when host machine be equipped with say 8 or 10 Gigs of RAM? The host server is HP server with 2 x Xeons (4 cores) and 2 x SAS disks mirrored with BBU-equipped RAID card (410i), so the storage won't be too slow itself.

These Windows machines are terminal servers, so users on them may produce noticeable disk activity so disk caching may help, the only question is will PVE will use all of free RAM on host server to cache VMs disk operations?

 

[spirit]

Hello,

the only question is will PVE will use all of free RAM on host server to cache VMs disk operations?

yes, if you choose cache=writethrough or cache=writeback for you vm disk config, it will use all the ram available for buffer cache.

(Note: that you can achieve the same thing by adding more memory in your windows guest, but maybe linux can handle this better ..?..)

 

[achekalin]

yes, if you choose cache=writethrough or cache=writeback for you vm disk config, it will use all the ram available for buffer cache.

I really wonder if 'default' setting may be slow or not?

 

[JimBeam]

I really wonder if 'default' setting may be slow or not?

Only 1 way to find out. What other answer would you expect...

 

[e100]

I have done extensive benchmarks with all of the cache modes.
cache=writethrough and cache=writeback do result in faster reads but writes are slower.

cache=none has best write speed but no host buffer cache for reads, if you want buffer cache add more RAM to the guest.

When I have heavy write VMs I use cache=none, heavy read VMs I use cache=writethrough.