Re: [ukha_d] Re: [OT] UltraATA133PCI RAID Controller - £19.99

[Bruno Prior <bruno@xxxxxxx>]

Gerard,

I think we are at cross-purposes. I didn't say you couldn't get drivers for these sorts of cards (in fact, I specifically mentioned that you
could for the Promise controller). I was replying to the poster who
suggested the card might be hardware RAID, which I very much doubt.

The difference between hardware and software RAID is where the
processing is carried out. With hardware RAID, the processing is carried out by a processor on the card. This removes all load from the m/board
CPU, and is transparent to the OS. Software RAID uses the m/board CPU to carry out parity etc. calculations, which requires drivers and adds an
extra load to the CPU. I don't know the Adaptec ATA-RAID cards very
well, but if it cost less than =A3100, it is almost certainly really a
software RAID device. In practice, these are little more than souped-up ATA controllers, which is why you can get them at such reasonable
prices. True hardware RAID solutions are _a_lot_ more expensive and
usually SCSI-based.

None of which is to suggest that the ATA-RAID solutions are not useful. It doesn't really matter whether you use drivers to make the card appear like a RAID card, or use it simply as another ATA controller and use
generic software-RAID. The net effect is the same - you get cheap RAID
control that can utilise cheap (i.e. IDE not SCSI) disks. And as I
mentioned in my previous post, most CPUs are under-utilised, so you are probably getting better performance than hardware RAID for a fraction of the price. Hardware RAID really comes into its own for other features,
such as hot-swapping (although this should also become possible with
ATA-RAID, once the forthcoming Serial-ATA becomes common), arrays with
more than (say) four disks, or where the computer is used for
computationally intensive purposes.

It is up to the user whether to use a driver specific to an ATA-RAID
card or to use linux's generic software-RAID. However, the latter has
many advantages, not least of which is the ability to create arrays not just from whole devices, but from partitions on the devices. This is a
very powerful feature, that allows you to mix and match your RAID-levels and filesystem types across the same disks. For instance, my server's
root filesystem is ext2 on RAID-1, but my much larger (320 Gb) /home
filesystem is ext3 on RAID-5. Yet both filesystems contain partitions
>from across various machines, using network block devices. I'm not sure if
any of this is feasible with the Adaptec ATA-RAID drivers, but I'm
pretty sure it's not possible with true hardware RAID.

Incidentally, SuSE _is_ linux, not another OS.

Cheers,

Bruno


Gerard McGovern wrote:
>>Anything this cheap is unlikely to be genuine hardware RAID.
>>The Promise
>>and Adaptec ATA-RAID cards, for example, are not real hardware RAID= -
>>most of the work is actually done by the driver which comes
>>with them.
>>The fact that the manual says the card requires Windows
>>indicates that
>>this is also the case with this card.
>
>
> Well that is not the case in the Adaptec ATA-RAID case. They use the > HighPoint Technologies chipset and you can get drivers to work under > Linux, FreeBSD, SuSE and I think that even Solaris is supported.
> Software only RAID it is not.

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