[rt-users] Migrating from Postgres to MySQL

[Robert Nesius]

On Tue, Jul 28, 2009 at 3:56 PM, Kenneth Marshall <ktm at rice.edu> wrote:

> Kage,
>
> The main advantage is gained by avoiding I/O through the virtual
> disk. The layout of the virtual disk tends to turn most I/O into
> random I/O, even I/O that starts as sequential. The factor of
> 10 performance difference between random/sequential I/O causes
> the majority of the performance problem. I have not had personal
> experience with using an NFS mount point to run a database so I
> cannot really comment on that. Good luck with your evaluation.
>

You're trading head-seeking latencies for network latencies, and those are
almost certainly higher.  Hosting your database server binaries and such
forth in NFS is possible, though again, not optimal both from a performance
and risk standpoint (NFS server drops, your DB binaries vanish, your DB
server drops even though the machine hosting it was fine).

I think hosting databases in NFS can cause serious problems - I seem to
remember older versions of mysql wouldn't support that.  I don't know if
newer ones do...but I do know in the *very large* IT environment I worked
in, all database servers hosted the DBs on their local disks or in
filesystems hosted on disks (SANS?) attached via fibre-channel.

Could solid-state drives side-step the random-access issue with
virtualization, or at least make it suck less? Based on how many people I
know who have said "Wow, my SSD died.  I thought those were supposed to be
more reliable?" ... I wouldn't bet my service uptime on it. ;)

-Rob
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