Default use of caching on raw volumes

[Nikolai K. Bochev]

Looks like i'm in a writing mood today, so just bare with me :) 


A few weeks ago i had problems on one of my kvm hosts with cpu loads. It wasn't very obvious problem, since none of the vm's were having a high cpu usage. Things started to get weird when i upgraded all of the vm's to lucid ( some reinstalled clean, some upgraded ). At times loadavg did peak to 32 on a 16 core server. I started disabling services on the vm's until i found the reason - the zabbix monitoring i use ( for total of 10 or so hosts ) was writing ( apparently ) too much data to the disk in it's pgsql database. The zabbix server and the pgsql server are on separate vm's. I went on and disabled zabbix for a while. I am using raw, allocated images for performance and it was still hanging up the system during extensive read/writes. Then i stumbled upon this : 


https://www.redhat.com/archives/virt-tools-list/2010-August/msg00027.html 


I went ahead and put the cache='none' on the drives of the pgsql vm and there you have it - loadavg went down to 1-3 from 30+ with zabbix running. 
I did some testing with force destroying the vm around 20 times and i didn't encounter any data loss whatsoever. 
Now the question is - did the ubuntu team do some testing with this ? If yes, did you find anything that breaks up so bad, that it's not ready for an LTS or any other sort of release. 
And if testing shows things are ok, would there be any merit in changing either libvirt or virt-manager to do this by default just for ubuntu ( not really sure what upstream will do about it ) ? 

-- 


Nikolai K. Bochev 
System Administrator 



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