Re: [PERFORM] Arguments Pro/Contra Software Raid

On Tue, May 09, 2006 at 12:10:32PM +0200, Jean-Yves F. Barbier wrote:
> > I myself can't see much reason to spend $500 on high end controller
> > cards for a simple Raid 1.
>
> Naa, you can find ATA &| SATA ctrlrs for about EUR30 !

And you're likely getting what you paid for: crap. Such a controller is
less likely to do things like turn of write caching so that fsync works
properly.

> > + Hardware Raids might be a bit easier to manage, if you never spend a
> > few hours to learn Software Raid Tools.
>
> I'd the same (mostly as you still have to punch a command line for
> most of the controlers)

Controllers I've seen have some kind of easy to understand GUI, at least
during bootup. When it comes to OS-level tools that's going to vary
widely.

> > + There are situations in which Software Raids are faster, as CPU power
> > has advanced dramatically in the last years and even high end controller
> > cards cannot keep up with that.
>
> Definitely NOT, however if your server doen't have a heavy load, the
> software overload can't be noticed (essentially cache managing and
> syncing)
>
> For bi-core CPUs, it might be true

Depends. RAID performance depends on a heck of a lot more than just CPU.
Software RAID allows you to do things like spread load across multiple
controllers, so you can scale a lot higher for less money. Though in
this case I doubt that's a consideration, so what's more important is
that making sure the controller bus isn't in the way. One thing that
means is ensuring that every SATA drive has it's own dedicated
controller, since a lot of SATA hardware can't handle multiple commands
on the bus at once.

> > + Using SATA drives is always a bit of risk, as some drives are lying
> > about whether they are caching or not.
>
> ?? Do you intend to use your server without a UPS ??

Have you never heard of someone tripping over a plug? Or a power supply
failing? Or the OS crashing? If fsync is properly obeyed, PostgreSQL
will gracefully recover from all of those situations. If it's not,
you're at risk of losing the whole database.

> > + Using hardware controllers, the array becomes locked to a particular
> > vendor. You can't switch controller vendors as the array meta
> > information is stored proprietary. In case the Raid is broken to a level
> > the controller can't recover automatically this might complicate manual
> > recovery by specialists.
>
> ?? Do you intend not to make backups ??

Even with backups this is still a valid concern, since the backup will
be nowhere near as up-to-date as the database was unless you have a
pretty low DML rate.

> BUT a hardware controler is about EUR2000 and a (ATA/SATA) 500GB HD
> is ~ EUR350.

Huh? You can get 3ware controllers for about $500, and they're pretty
decent. While I'm sure there are controllers for $2k that doesn't mean
there's nothing inbetween that and nothing.
--
Jim C. Nasby, Sr. Engineering Consultant jnasby(at)pervasive(dot)com
Pervasive Software http://pervasive.com work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461