Re: How does disk caching work?

From: Jim C. Nasby (jim_at_nasby.net)
Date: 04/19/04

  • Next message: Jim C. Nasby: "Re: command piped into bzip not using all available CPU" 
    Date: Sun, 18 Apr 2004 21:20:43 -0500
To: Uwe Doering <gemini@geminix.org>

    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@nasby.net
Member: Triangle Fraternity, Sports Car Club of America
Give your computer some brain candy! www.distributed.net Team #1828
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