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The following is an archive of a post made to our 'vox-tech mailing list' by one of its subscribers.

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Re: [vox-tech] Software RAID question
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Re: [vox-tech] Software RAID question



> Let's say, for hypothetical purposes, that a RAID
> array is set up on a SCSI card (and disks), on a
> high-end AMD or PIII, and (for sake of argument) Red
> Hat 7.0 for the rest of the os with kernel 2.4.  Oh,
> and let's say that we were using RAID5.
> 
> Can this kind of setup handle writing rates of <32KB

> expect?  Does it take lots of tuning/tweaking to get
> this kind of performance?  Would other raid levels
> improve (writing) performance?

Okay first, most new SCSI or EIDE drives can handle somewhere
around 40 MB/sec (outside tracks) to 20 MB/sec inside tracks.
>        -------Sequential Output-------- ---Sequential Input-- --Random--
>        -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
>     MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU  /sec %CPU
>    864  8465 89.8 28931 30.9 11671 15.5  8345 87.9 35108 25.6 147.4  1.7

Here's a 30 GB ibm EIDE disk:
1.00a       ------Sequential Output------ --Sequential Input- --Random-
            -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
            MB K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
            640          33250  13 14079  22           32966  19  

Pretty much bandwidth scales with the number of disk busses you have, typical
rates are 1/2 of your bus bandwidth.  I.e. 2 U160 Channels = 320 MB/sec
= 160 MB/sec peak.

So if you have 8 20 MB/sec drives on 2 U160 channels you could hit in
the neighborhood of 160 MB/sec.

Unfortunately the PCI bus normally handles 132 MB/sec peak, so it's
tough to get more then 64 MB/sec real world.  Unless you spring for
a machine/motherboard that has 64 bit pci.

As far as raid levels go:
        Raid-5 requires TWO writes for each write.  This is spread across
the number of heads/disks you have in the array.  So performance 1/2
of an optimally balanced collection of disks.  But the performance
is similar to single disk write speeds, since the second write happens
in parallel on a second disk.

        Raid-0/striping with 2 disks basically doubles the read/write 
bandwidth for long consecutive read/writes.  Of course it also doubles
your chance of losing all your data because of a hardware failure.

All the above goes for scsi or eide (as long as you use 1 disk
per bus on eide).  I've seen raid result for over 100 MB/sec read/write
using only eide disks (limited by pci).

--
Bill


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