Re: strange performance of raid0

["Keld Jørn Simonsen" <keld@dkuug.dk>]

On Sun, Feb 15, 2004 at 01:54:11AM -0800, Matt Thrailkill wrote:
> The you don't get as good performance because hdc1 is slow enough to
> really drag it down.  Think about it, if reads are all going about
> sequentially, it has to spend alot of time waiting on that 16mb/s drive
> compared to the others.

I would have thought the disks would have worked towards gaining
performance enhancements in some cumulative way, is that not so, in theory?

So why is 3 disks slower than 2?

> You probably can't get 100mb/s with the two because that would be pretty
> efficient, and there's probably more overhead than that.
> 
> Shouldn't the 80gb drive be going faster than 16mb/s though?  Have you
> checked hdparm to make sure dma and all the goodies are turned on for
> it?

I have a 40 GB segate disk also, on the same motherboard IDE
controller, it runs about 40 MB/s. So yes, the hdc1
should go faster, and I have measured something like 40 MB/s
on it in idler times. I think it is because it is running other 
processes. The machine is a ftp mirror and the disk has RedHat
and Fedora ISO images, so it is quite popular.

> On Sun, 2004-02-15 at 02:04, Keld Jørn Simonsen wrote:
> > Hi
> > 
> > I have some strange performance results on a raid0
> > I have 4 IDE disks on two controllers, the one on the 
> > motherboard of the duron 1 GHz mackine, the other a promise TX2 plus 
> > SATA + PATA controller. I run kernel 2.4.22
> > 
> > The disks and hdparm -t on each of them
> > 
> > /dev/hdc1 seagate 80 GB                   16 MB/s
> > /dev/sda1 maxtor sata 200 GB              50 MB/s
> > /dev/sdb7 maxtor 160 GB                   54 MB/s
> > 
> > The partitions are al about 5 GB each.
> > 
> > If I make a raid0 device of all of them I get a thruput of 45 MB/s
> > IIf I exclude the hdc1 partition, I get around 75 MB/s.
> > The system is a little loaded - but that would be normal operating
> > conditions. CPU is 90 % idle. I have about 100 MB free RAM.
> > 
> > Why is it slower with 3 disks in the raid0 than
> > 2 disks? Why don't I get approx 100 MB/s out of just the two?
> > 
> > Best regards
> > Keld
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