Re: Hardware: HP StorageWorks MSA 1500 - Mailing list pgsql-performance

On Tue, 25 Apr 2006, Mark Kirkwood wrote:

> Mikael Carneholm wrote:
>
> >
> >> There are two SCSI U320 buses, with seven bays on each. I don't know
> > what the overhead of SCSI is, but you're obviously not going to get >
> > 490MB/s for each set of seven even if the FC could do it.
> >
>
> You should be able to get close to 300Mb/s on each SCSI bus - provided
> the PCI bus on the motherboard is 64-bit and runs at 133Mhz or better
> (64-bit and 66Mhz give you a 524Mb/s limit).

I've no idea if the MSA1500's controllers use PCI internally. Obviously
this argument applies to the PCI bus you plug your FC adapters in to,
though.

AIUI it's difficult to get PCI to actually give you it's theoretical
maximum bandwidth. Those speeds are still a lot more than 200MB/s, though.

> >> Of course your database may not spend all day doing sequential scans
> > one at a time over 14 disks, so it doesn't necessarily matter...
> >
>
> Yeah, it depends on the intended workload, but at some point most
> databases end up IO bound... so you really want to ensure the IO system
> is as capable as possible IMHO.

IO bound doesn't imply IO bandwidth bound. 14 disks doing a 1ms seek
followed by an 8k read over and over again is a bit over 100MB/s. Adding
in write activity would make a difference, too, since it'd have to go to
at least two disks. There are presumably hot spares, too.

I still wouldn't really want to be limited to 200MB/s if I expected to use
a full set of 14 disks for active database data where utmost performance
really matters and where there may be some sequential scans going on,
though.

> > That's probably true, but *knowing* that the max seq scan speed is that
> > high gives you some confidence (true or fake) that the hardware will be
> > sufficient the next 2 years or so. So, if dual 2GBit FC:s still don't
> > deliver more than 200Mb/s, what does?
> >
>
> Most modern PCI-X or PCIe RAID cards will do better than 200Mb/s (e.g.
> 3Ware 9550SX will do ~800Mb/s).
>
> By way of comparison my old PIII with a Promise TX4000 plus 4 IDE drives
> will do 215Mb/s...so being throttled to 200Mb/s on modern hardware seems
> unwise to me.

Though, of course, these won't do many of the things you can do with a SAN
- like connect several computers, or split a single array in to two pieces
and have two computers access them as if they were separate drives, or
remotely shut down one database machine and then start up another using
the same disks and data. The number of IO operations per second they can
do is likely to be important, too...possibly more important.

There's 4GB FC, and so presumably 4GB SANs, but that's still not vast
bandwidth. Using multiple FC ports is the other obvious way to do it with
a SAN. I haven't looked, but I suspect you'll need quite a budget to get
that...