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From Peter Schuller <peter.schul...@infidyne.com>
Subject Re: mmap
Date Sat, 17 Jul 2010 22:24:40 GMT
> Not 100% relevant but I found this to be interesting if you're nodes are doing heavy disk
I/O:
>
> http://rackerhacker.com/2008/08/07/reduce-disk-io-for-small-reads-using-memory/

There are some pitfalls though, or at least there were the last time I
was tweaking such stuff for a PostgreSQL database. One issue I
remember is that, at least at the time, there was a limit to how many
dirty buffers *per inode* that the background dirty buffer flusher
would write. So for lots of outstanding random I/O to the same large
file, tweaking ratio triggers was not necessarily enough and your
eventual fsync() still ended up pushing more data than you may have
wanted.

I also read a pretty good article (can't find it right now) that made
a good point that some of the more obscure settings one might be
tempted to tweak, should not necessarily *be* tweaked be cause the
kernel had hard-coded assumptions about them (this does not apply to
just the dirty background ratio though; IIRC it was things like the
frequency with which the flusher runs. I never experimented with that
though.

-- 
/ Peter Schuller

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