On Wed, Jun 29, 2005 at 05:32:23PM +0100, Conor Wynne wrote:
> I do not fully agree with this. IDE/SCSI?SATA/Whatever controllers on
> which the disks reside can fail just as easily as a raid controller.
> Although I fully take you points about chipsets/firmware.
There is a universal standard for how HBA's are supposed to behave.
Linux cannot perform the job of a HBA.
> With linux raid, you just move the disks in the correct order to the new
> system. However, LSI (megaraid controllers) are backward compatible with
> older versions.
If you have it set up correctly, you won't need to preserve order with
linux RAID1. It will be detected automatically (mdadm helps lots to
get this right).
> The good old days, chipsets changed between vendors, and I cite the
> adaptec/LSI change as a perfect example.
>> But now everything is pure LSI based, and they work very well.
No use when we have systems bought accross 4 years and want to minimise
on-site spares though :/
> > Number 3 is that linux is just better at RAID1 than most raid cards.
>> Yes, except under load. True hardware raid controllers will be faster as
> they offload the work to the controllers cpu's, not to the host cpu.
>> In these situations, I would prefer to donate cpu time to the db's etc.
We're using software RAID1 for the system disks in ftp.heanet.ie. It's
certainly under load, and we're logging (syncronously!) to that RAID1 volume.
It's never proved a problem. We have some pretty active mail spools on
software RAID1 volumes, no problem there either.
What do you think takes CPU time? There's no striping, there's no parity
to calculate. It's all IO-bound, and if you've got two identical disks
they're going to give you near-identical IO times. The overhead is truly
tiny. If you're really concerned about maximising IO performance, then
you're using striping of some kind anyway, RAID1 isn't for you.
So, you won't get blindingly fast IO, but it will be only slightly slower
than just using one disk, but you're not going to do much better with
a hardware card. CPU is not the bottleneck.
> > Although some less OS's can't boot from software raid 1 of course.
>> Wot? Linux can for sure, you cannot have /boot on a LVM though.
> I assume you are talking about doze? I though it could boot from RAID1 non?
No, Linux can, definitely. Lilo and grub both support booting from RAID1
(lilo is much better at it though). I don't think Windows can though.
Colm MacCárthaigh Public Key: colm+pgp at stdlib.net
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