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Subject: Re: large postfix installations?
From: Matthias Andree (madt.e-technik.uni-dortmund.de)
Date: Tue Apr 04 2000 - 04:43:51 CDT

Ian C.Sison <ian.sqsr.com.ph> writes:

> > Better yet, switch to FreeBSD and use softupdates, where you
> > should see a factor of five to ten performance improvement.
>
> Question on this. Isn't the issue on async updates vs sync updates,
> on any OS moot if your disk controller or HD contains a large enough
> cache that buffers writes to the disk anyway?

Not really. With classical systems, sync update means that the whole lot
you're writing has been actually written to the physical disk until your
write or close or fsync calls return (depending which way you enforce
data synchronization).

This means large write caches don't really help with writing (well,
reordering caches may reduce the intermediate seeks while writing), but
you still will have to wait until all blocks have actually been written
to disk before you can continue. Large caches will probably still help,
since write-through-buffered data is available for reading with low
latency.

I'm not exactly sure where softupdates help out of this, but from what I
heared, these give up the "physical sync" requirement while still
guaranteeing data integrity upon crash. And this data integrity is what
sync eventually is all about. 

-- 
Matthias Andree

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