FreeBSD MySQL still WAY slower than Linux

[Oliver Fromme]

Michael Schuh <michael.schuh at gmail.com> wrote:
 > As i sayed i have the installations always made in the same way, so that i mean.
 > I mean i have alwas made the swap on the first gig of the disk, and
 > the installation
 > on the rest of the disk. and i have no multiple os'es on these disk.

The problem is that the file (from "dd of=foo") can still
end up at completely different physical places on the disk.

It depends on the filesystem (ext2, ext3, UFS, whatever)
and on the allocation strategies of the filesystem code.
UFS might start filling cylinder groups from the beginning
of the disk, while ext3 might start at the end (does ext3
even _have_ cylinder groups?).  This was just an example,
but you get the idea.

Of course, it also depends on how much data there already
is on the filesystem, and how it is distributed over the
disk.

For accurate measurements and comparisons, you have to make
sure to use _exactly_ the same physical location on the
disk.  From userland you don't have a way to control the
physical allocation of files.  Therefore, the only reliable
way is to leave an unused partition on the disk, do _not_
put a filesystem on it, and use the raw device in the »dd«
command.  If you do this, you will always hit the same
physical location on the disk.

But then again -- as others have already mentioned, serial
write speed is not the most important factor for database
performance (although the WAL journal files of advanced
transactional databases like PostgreSQL are written in a
sequential way), so the usefulness of this "benchmark" is
very debatable.

Best regards
   Oliver

-- 
Oliver Fromme, secnetix GmbH & Co KG, Oettingenstr. 2, 80538 München
Any opinions expressed in this message may be personal to the author
and may not necessarily reflect the opinions of secnetix in any way.

"I started using PostgreSQL around a month ago, and the feeling is
similar to the switch from Linux to FreeBSD in '96 -- 'wow!'."
        -- Oddbjorn Steffensen