Multiple varnish instances per server?

[Barry Abrahamson]

Hi,

Is anyone running multiple varnish instances per server (one per disk  
or similar?)

We are currently running a single varnish instance per server using  
the file backend.  Machines are Dual Opteron 2218, 4GB RAM, and 2  
250GB SATA drives.  We have the cache file on a software RAID 0  
array.  Our cache size is set to 300GB, but once we get to 100GB or  
so, IO starts to get very spiky, causing loads to spike into the 100  
range.  Our expires are rather long (1-2 weeks).  My initial thoughts  
were that this was caused by cache file fragmentation, but we are  
seeing similar issues when using the malloc backend.  We were thinking  
that running 2 instances per server with smaller cache files (one per  
physical disk), may improve our IO problems.  Is there any performance  
benefit/detriment to running multiple varnish instances per server?   
Is there a performance hit for having a large cache?

Request rates aren't that high (50-150/sec), but the cached files are  
all images, some of which can be rather big (3MB).

Also, is anyone else seeing similar issues under similar workloads?
--
Barry Abrahamson | Systems Wrangler | Automattic
Blog: http://barry.wordpress.com