Re: strange performance of raid0

[Gregory Leblanc <gleblanc@linuxweasel.com>]

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.

> 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. 
  HTH,
	Greg
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