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Old 10-05-2002, 07:13 PM   #1
Vlad_M
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Software RAID question


Hi all,

I've been playing with RAID on my linux box, and I have some questions that
maybe someone can help me with.
This is what I have:

Adaptec 29160N PCI SCSI controller
4 IBM Ultrastar fast wide disks

and I am using software RAID provided in the Linux 2.4.x kernel.
When the machine boots up, the speed of the SCSI bus is set to 40 MB/s. Now my
first question is is that 40MB/s *per disk* or is he whole bus set to 40 MB/s
since the disks are fast wide?

I am asking this cos when I am running the disks in RAID 0, the speed goes up in
increments of 10MB/s as I am adding the disks (10 MB/s per disk, which is
exactly as it should be as I get 10MB/s per single disk when benching it on its
own). But, the speed tops off at 30 MB/s, so I am getting exactly the same results (speed wise) with 3 disks as I am with 4 disks.
I am benching with files large enough to avoid any caching and I have also experimented with block and chunk sizes.

What are your opinions, am I getting the figures I should be getting, or is something wrong?
If you feel that something is wrong can you please point it out to me and also give me your view on how to go about fixing it.

Thanks!
 
Old 10-07-2002, 08:37 PM   #2
mcleodnine
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It's the maximum speed that the SCSI bus can support.

When you're using software RAID you are depending on your CPU (rather than a deicated RAID controller) to do the gymnastics involved in reading/writing to each disk. Faster CPU should garner faster read/writes (up to the max SCSI bus speed).

What's the maximum throughput for each drive in the array? If you've got fast I/O on your disks then you may want to look at getting a better SCSI card.

Also - how are you testing the array - hdparm, bonnie, or soemthing else?
 
Old 10-08-2002, 05:43 PM   #3
Vlad_M
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Original Poster
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Hi,

I am testing the array with bonnie, IOzone and Tiobench.
The question I have now is this - Linux software raid supports dual level raid - for the case where two raid 0 arrays are used to make up 1 raid 1 array.

I did the following - two raid 0 arrays (made up of 2 disks each) make up 1 raid1 array. All the disks are on the same SCSI controller. The thing worked, but the rates I am getting are shocking. It is resyncing right now at 1 MB/s. Horrible. Any idea why this is happening?

P.S. I know that the resync is being done using idle bandwidth only. The point is that I am not on the machine at the moment, I am writing this from a different box. There are no users on the RAID machine.
 
Old 10-08-2002, 09:28 PM   #4
mcleodnine
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First - you're not benchmarking it _while_ it's rebuilding the array, right?

If it's the first rebuild you're watching then it could be same thing that happened to me. I started by installing to just one disk and then migrating to a RAID-5 setup. My initial setup was done on standard linux partitions of type '83' (linux) instead of type 'fd' (linux raid autodetect). When I went to build the array to include the other two drives it was extremely slow. (hours). I've made that boneheaded move twice! Subsequent rebuilds of the array using the proper partition type work normally and I get about 850-900 MB/minute rebuilds on a system with no user load (rebuilds were fast and the drives get nice and toasty - my first disk cooling lesson) . BTW my setup used reiserfs for both the initial build and the SW RAID.
 
  


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