Re: H/W RAID 5 on slower disks versus no raid on faster HDDs

>>>>> "nikolaus" == nikolaus  <nikolaus@dilger.cc> writes:

nikolaus> Of course can software RAID be faster than hardware RAID.
nikolaus> But then you are not comparing the best offerings of each
nikolaus> category.  Software RAID is usually cheaper than hardware.
nikolaus> But again you may be able to construct a product pairing
nikolaus> where it is reverse.

No... I admit... if you're willing to spend huge $$'s, there exist
hardware solutions that outpace software.  It usually means that you
have hardware that is more powerful than your main system, though.
Often more costly, too.

nikolaus> raw versus file system is an old battle.  raw is faster
nikolaus> because it does not have the filesystem overhead.  However,
nikolaus> raw is a little harder to administer.  Question is how much
nikolaus> faster raw is and if it is worth the trouble.  Numbers
nikolaus> usually quoted are 10-20%. And here things may be changing
nikolaus> as operating systems and chaching algorithems get better to
nikolaus> mask the file system overhead.  As I mentioned the question
nikolaus> is mute for PostgeSQL because raw is currently not
nikolaus> supported.

Well... I don't think your raw write performance (in, say, FreeBSD)
would be any more than 5% better than filesystem writing.  Old
filesystem overhead consisted of the in-order writes that were
required to update filesystem metadata... which (consequently) was
about 20% in-order (vs. un-ordered) writes.

Softupdates (FreeBSD) only requires 1%-ish (or less) in-order writes
(or to put it another way: 1% of writes chosen before they would
otherwise happen).

DAve.

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|David Gilbert, Velocet Communications.       | Two things can only be     |
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