Re: strange performance of raid0

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

On Tue, Feb 17, 2004 at 09:50:21AM -0800, Gregory Leblanc wrote:
> 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
> 
> Well, if you're lucky, these might be useful as comparative numbers 
> between the different drives on the same system.  Just as likely not, 
> though, hdparm rather sucks as a benchmark.

Yes, I only use them as crude benchmark measuers, but they seem
indicative.

> >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.
> 
> Let's assume that the above numbers have a basis in reality.  :)  If 
> you've got disks with widely varying speeds, then the best performance 
> can often be hand from setting up a linear RAID volume, rather than a 
> RAID0.  RAID 0 is really designed to have matching disks, as it 
> distributes data evenly across them.  With Linear, and ext2 (erm, I'm 
> assuming 3 as well, I haven't heard anything different), you can 
> sometimes get better performance with smaller writes, because ext2 
> "scatters" data around the filesystem, in order to avoid fragmentation. 

Yes, that would be an idea. Anyway I replaced the hdc1 with hda1 on
a seagate 40 GB disk, which hdparm said could do about 40 MB/s,
and then - when I was lucky, I could actually get about 120 MB/s
thruput on a "cat file >/dev/null", 681 MB in 5.63 secs. 
This on a mildly loaded production system. Not bad!
Then I am beginning to hit the 1 Gbit/s limit on the PCI bus.

Best regards
Keld
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