Re: remote mysql is too slow
On Wed, Apr 11, 2012 at 09:23:09PM +0100, Bob Proulx wrote:
> Karl E. Jorgensen wrote:
> > Bob Proulx wrote:
> > > But at some point the daemon is going to need to write a file to disk.
> > > That data will get cached at that time. Or are you saying that mysql
> > > is using or should using O_DIRECT and avoid the cache explicitly?
> >
> > No - although that might help (marginally) too. The point is that
> > caching of data in MySQL will be more efficient than caching in the
> > kernel disk cache; less layers and whatnot.
>
> The point I was trying to make (and apparently failing miserably) is
> that unless the mysql daemon is really trying hard to avoid it then as
> soon as it writes a file it will end up in the filesystem buffer
> cache.
ah. The miserable failing was at the receiving end - sorry :-)
> In any case, from the original 'free' output it isn't possible to know
> if the filesystem buffer cache is due to mysql or due to other
> activity across the filesystem. Such as backup for example. It could
> have come from any other activity. There could be files being copied
> to or from the system. Could be anything.
Agreed
> Normally filesystem buffer cache is a good healthy indication.
> Because if the operating system becomes starved for memory it will
> free filesystem buffer cache in order to obtain it. If a system has
> no files cached and lots of truly free ram it may indicate that some
> process is frequently consuming a lot of memory, forcing all buffers
> to be flushed, and then exiting causing that memory to be free memory.
> Free memory as opposed to working-for-you memory in the buffer cache.
> Normally I would say memory used for filesystem buffer cache is good
> healthy use of the memory. And when I say normally here I am not
> talking about a database daemon which will of course have special
> needs.
Definitely special needs.
--
Karl E. Jorgensen
IT Operations
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