How does disk caching work?

[Jim C. Nasby]

On Sat, Apr 17, 2004 at 09:41:19AM +0200, Uwe Doering wrote:
> The disk i/o buffers you refer to (the 'Buf' column in 'top') are the 
> actual interface between the VM system and the disk device drivers.  For 
> file and directory data, sets of VM pages get referred by and assigned 
> to disk i/o buffers.  There they are dealt with by a kernel daemon 
> process that does the actual synchronization between VM and disks. 
> That's where the soft updates algorithm is implemented, for instance.
> 
> In case of file and directory data, once the data has been written out 
> to disk (if the memory pages were "dirty") the respective disk i/o 
> buffer gets released immediately and can be recycled for other purposes, 
> since it just referred to memory pages that continue to exist within the 
> VM system.
> 
> Meta data (inodes etc.) is a different matter, though.  There is no VM 
> representation for this, so for disk i/o they have to be cached in extra 
> memory allocated for this purpose.  A disk i/o buffer then refers to 
> this memory range and tries to keep it around for as long as possible. 
> A classical cache algorithm like LRU recycles these buffers and memory 
> allocations eventually.
> 
> As usual, the actual implementation is even more complex, but I think 
> you got a picture of how it works.

Yes, much clearer now, thanks!

A few questions if I may...

What's a good way to tune amount of space dedicated to IO buffers?

What impact will vm_min|max_cache have on system performance? Is there
any advantage to setting it fairly high?

The machine I'm tuning is a dual Opteron box with 4G of ram, a mirror
and a 6 disk RAID10. It's running PostgreSQL.
-- 
Jim C. Nasby, Database Consultant                  jim at nasby.net
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