Re: Promise RAID-1 vs software RAID-1

[Christopher Chan <cchan outblaze com>]

Rodolfo J. Paiz wrote:
> At 10:39 6/16/2004, Alan Horn wrote:
>
> > Raid 1 is mirroring only. For data integrity reasons I can't see how 
> > they could read from any but the master mirror.
>
>
> WAG alert:
>
> Once the two disks are synchronized, it sounds perfectly reasonable to 
> writes to take the same amount of time (since all data has to be written 
> to both drives), but for reads to be significantly faster by reading 
> from both spindles. There really is no "master" in an equal set of two, 
> I think. Both contain identical data, so you should be able to read from 
> both. I think.

The Linux kernel does give you the ability to read simultaneously from 
both drives in a Linux mirror array.
> By the way, the Promise cards *are* software RAID... all the real RAID 
> computations are done by the driver using the computer's resources. The 
> trick is that the binary driver for Linux is probably less mature and 
> less flexible than Linux native software RAID, so I've always used 
> Promise and HighPoint controllers simply as additional EIDE/ATA 
> controllers and used Linux software RAID. Excellent results. Note that 
> Linux software RAID is also capable of RAID-5, IIRC, which the Promise 
> drivers are not.

Heh, exactly the same reason here. Any Promise installation we had was 
for ATA100 support.
> If you have the budget, I'd strongly recommend the 3Ware cards. Real 
> hardware RAID, native Linux kernel support since 2.2.x, fast, very 
> reliable, and not expensive.

Some gotchas:

3ware RAID 5 sucks and especially when in degraded mode (and this goes 
for a lot of other hardware RAID cards) because the onboard processor is 
not fast enough to handle the calculations. Hint: If you see an Intel 
960 processor on your RAID board, forget RAID 5.

Linux RAID 5 will be much faster so forget the 3ware RAID card if you 
want to do RAID 5.

3ware RAID 1+0 also sucks. You are better off configuring two mirrors 
and using the Linux kernel to achieve the RAID 0 part. This means that, 
depending on what you got (4 port/8 port/12 port), you will need other 
disks on the IDE channels for the system if there is nothing left for 
the system.