RAID5 for Linux

[mark at buttery.org (Mark J. Dulcey)]

Derek Atkins wrote:

> Uh, actually, there is a penalty.  With S/W RAID1 the kernel has to
> perform two writes across the PCI/IDE bus (one to each Raid-1 mirror
> drive), whereas with hardware RAID1 you only need to write across the
> PCI bus once and then the raid controller will send out the duplicate
> writes across the disk busses.  This extra writing will definitely
> cause a peformance penalty (on writes) for software raid that you wont
> see in a hardware raid.

That assumes that the PCI bus is a system bottleneck. It probably isn't. 
Yes, you have to do the extra writes, but you probably won't notice the 
performance hit.

> Also, historically it had NOT been recommended to use both parts of an
> IDE bus because the master/slave relationship reduces the bus
> throughput.  Has this changed recently?  This is another reason why a
> hardware raid is better.  A good card would use direct busses for each
> drive.  This is also why you see CDrom drives on hdc and not hdb.

If you only have two drives (a single RAID1 pair), the best thing to do 
is put them on separate channels. You can put your CD-ROM as a slave on 
one of them. If it's a typical server, the CD-ROM drive will rarely be 
used (it's mostly there to make it easier to install the OS), so it will 
have little effect on performance.

If you have more than two drives, and don't have a motherboard with 
three or four IDE channels (there were some made shortly before Serial 
ATA became the big new thing), you might want to add an IDE card to 
provide more. Maxtor and Western Digital were bundling them with their 
high-capacity drives for a while (in part because most motherboards 
didn't support them), though I think they stopped doing that.

I have one system here with software RAID1 on a single drive pair (two 
80GB drives, one Seagate and one Western Digital), one connected to each 
channel of one of those cards; the CD-ROM and DVD-ROM are on the 
motherboard IDE. (It's an old motherboard that only supports ATA33; the 
ports on the card are faster, so that's why I use them for the hard 
disks.) It works very well. Our mail server has a software RAID1 pair 
using two SCSI drives; they're somewhat old, and probably no faster than 
current IDE drives, but I already had them, They also work well, and I 
sleep a bit easier with my email stored on redundant drives.