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Re: IDE controller card and Fedora Core 3



Robert L Cochran wrote:
> You want to RAID5 640 Gb  worth of drives on a Pentium II, 400 MHz 
> machine? I'm not a RAID expert. I question whether doing it on an old, 
> slow machine with large drives is practical. It seems to me you would 
> simply saturate the PCI bus.

Depends. (And I know the Original Poster later said performance wasn't
critical...)

Firstly, unlike certain OSes, Linux should find that a Pentium II is
fine for shoving data around. It's not like it needs to do complex
calculations on each byte: it shouldn't even need to examine each byte
while reading.

Of course, while it's writing, something will have to calculate
checksums: I don't think the Original Poster's suggested card has any
RAID acceleration features.

Secondly, it depends exactly what you're going to put on that RAID
array. If you're putting a mail server or a database on it, you'll
almost certainly find that disk usage is dominated by lots and lots of
small (~8K) reads. And you'll be very lucky to get more than about 150
of those per second on ATA drives. 

It's all about physics: on a hard disk that spins about 100 times per
second, a particular byte only goes near the read heads once every 10
ms. And no matter how fast the rest of the computer is, 10 ms is an age
in computing. Database administrators swear by the "number of spindles",
for this reason: I've got much chunkier disk arrays on not much faster
systems. (They also swear *at* RAID 5 for a lot of usages...)

And 8K times 150 transfers per second = 1200 K/s per disk: that's not
going to set anything on fire.

(The Original Poster mentioned SAMBA, but it still depends on what sort
of files are being stored).

Thirdly, on a slow PCI implementation, you're going to get 90 - 100 MB/s
of usable bandwidth. Hard drives are only just going past the 70 MB/s at
peak performance. At the best of times, you are going to be limited by
the PCI bus, but you're still going to get better than a single drive on
the latest systems.

Of course, on a Samba server, you're going to be sharing that bandwidth
with the network card (on a PII 400). If that's gigabit, then yes,
that's going to halve the available bandwidth and slow things down. If
it's only 100 Mbit/s, then that's 12.5 MB/s max, and that's your
bottleneck: that's the physical maximum you'll get out of the system.

(When did 100 Mbit/s become "only"?)

But this is why PCI Express is a Good Thing. It's designed for this sort
of job: a separate network card and a separate fast hard drive adapter.
The bandwidth isn't shared: they each get their own 2.5 Gbit/s per lane
in each direction.  Although you might *still* need more than 1x PCIe
for the hard drive adapter...

James.
-- 
E-mail address: james | Anonymous:          What do you think of Stainer's
@westexe.demon.co.uk  |                     "Crucifixion"?
                      | Sir Thomas Beecham: Good idea! 


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