[Gllug] A SATA Raid card worth having?

[Ben Fitzgerald]

On Thu, Jan 26, 2006 at 10:24:07PM +0000, Russell Howe wrote:
> On Thu, Jan 26, 2006 at 03:23:09PM +0000, Ben Fitzgerald wrote:
> > on attending a redhat course not long ago an instructor stated that in
> > his opinion s/w raid was now as good as h/w raid as:
> > 
> > a) most cards use crappy processors to perform the parity calculation
> > b) modern cpus are very powerful and can easily cope with parity calculation
> 
> The CPU speeds these days probably mean the RAID calculations aren't
> going to kill them. I guess the main problem is the shift of I/O
> bottlenecks..
> 
> With a hardware RAID card in a PCI slot, you do your I/O to the PCI
> card, and then it farms out the relevent I/O to whatever disks it has
> attached.
> 
> With software RAID, the OS needs to do all the I/O itself, and in all
> the boxes I've used, that would mean sharing a single PCI bus to access
> either the same card multiple times, or multiple cards. I can't remember
> what the bandwidth of PCI is, but it's not hugely fast.. If you're doing
> RAID1, you effectively cut the PCI bandwidth available for disk
> operations in half (since you'll be writing everything twice).

see what you are saying. on modern 64-bit pci:

"Later versions of PCI enable true 64-bit data transfers using up to a
133MHz clock to enable transfer speeds of up to 1066 Mbytes/sec.)" [1]

ata drives reach 133mb/s max transfer but scsi goes much higher so I
guess this could be a bottleneck if you had a scsi controller with
several fast units hanging off it.

> It does mean you have to think about this when speccing a system, and
> probably means software RAID on a box which wasn't built with it in mind
> isn't going to offer stunning performance.
> 
> Also, what if your OS and bootloader are installed on a RAID1 set and one
> of the drives dies? Can the box boot up successfully with a dead boot
> disk, bypassing it and booting off of the remaining good one? I suspect
> most if not all PC BIOSes would have problems here, depending on just
> how the drive had died. Maybe the software RAID cards have more
> intelligent BIOSes than your typical SCSI/IDE/SATA-supporting
> motherboard?

you can put grub onto both mirror components and use root=LABEL=/ [2]

cheers,

ben.

[1] http://www.quatech.com/support/comm-over-pci.php
[2] http://www.dur.ac.uk/a.d.stribblehill/mirrored_grub.html

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