[GLLUG] RAID array performance?

[Adam McDougall]

On Sun, May 01, 2005 at 01:27:29PM -0400, impact wrote:

  Alright. Since I've finally gotten a job, this summer I plan to set up a
  nice RAID array. I have 4x80gb drives here I'd like to throw in a
  RAID-0. Backup will be done on a separate machine, so don't worry about
  the "its not safe" factor. I've done RAID in the past, however, not with
  4 drives, this puts a little situation before me.
  
  As far as I know, you get better performance when the drives are on
  their own controllers. My motherboard only has 2 IDE channels. Now, I
  also need to have a 5th (CD/DVD-RW) on there. So an IDE PCI card comes
  into play. The question is, how much slower is a PCI IDE card? My
  options (at least the ones I've thought of) follow.. Can anyone give me
  any ideas about which would be fastest and why?
  
  1: 4xRAID0 on OnBoard IDE (2 per channel), DVD on PCI card
  2: 4xRAID0 on 2-channel PCI RAID card, DVD on OnBoard
  3: 4xRAID0 on 4-channel PCI RAID card, DVD on OnBoard

Unless your motherboard has more than one PCI bus, I wouldn't 
worry about pci card vs. onboard, since they are all using the 
same PCI bus.  The only realistic factor there might be how efficient
each IDE controller implementation is (one brand/model vs another).

My understanding of parallel ATA is that a master and slave device on
the same channel both contend for access, so that is something to be 
avoided.  One (active) device per channel is optimal.  

I'm guessing an 80 gig drive probably only gives approximately 
25MB/sec sequential performance, therefore 4 of them would be
a comfortable fit for a single PCI bus in any configuration that
doesn't pessimize pairs of drives, such as two master/slave combos.
Standard PCI is 32 bits (4 bytes) at 33 mhz (33,000,000 per second),
so multiply to get 132MB/sec, and round that down to around 100MB/sec
to make room for overhead and any other PCI devices that don't send much 
data.  This rate is also close to Gigabit ethernet speeds, which is 
why a 32-bit 33mhz gigabit card cannot quite live up to full speeds
on a standard PCI bus. 

$50 sounds awful cheap for a "raid" card, it is probably a controller
that has BIOS-assisted raid setup but the OS likely implements the raid
in software.  If it has 4 seperate channels though, that doesn't sound
bad for a plain controller though.  I'm pretty sure you can get a 
Promise tx100 or 133 dual channel IDE controller for around $20. 

Some expansion cards prefer hard drives only and do not like ATAPI 
devices like CDROMS.  Highpoint is one of them, I'm not sure about
Promise.  Given that, I would prefer to leave your CD/DVD onboard
on one channel, and get 4 channels on 1 or 2 PCI cards
for the disks.  

  
  I would assume (3) would be the fastest, however, I worry about the
  limit on a PCI bus... Would 4xRAID0 hit that limit and cap? How would it
  compare to having 4xRAID using the onboard (only 2 channel)?
  
  Anyone have experience in this area? If it didn't cost 50$ for a
  4-channel PCI RAID card, I'd just try them all =P, and it might come
  down to that =P
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