xRAID disks....

Erik Trulsson ertr1013 at student.uu.se
Tue Jun 10 19:37:46 UTC 2008

On Tue, Jun 10, 2008 at 05:13:31PM +0200, Wojciech Puchar wrote:
> x> hardware support.
> >
> >> actually there is not much need to have it for RAID-0/1/10 where there is
> >> almostnothing to process.
> >
> > For mirrors it can actually be a big win with hardware support.
> > If you use software RAID then you will have to perform each write twice
> > (once to each disk),
> in parallel
> > while with hardware support for RAID you only need
> > to transfer the data once.
> which saves at most 100MB/s bandwidth - compare this to 5-10GB/s in modern 
> machines.

You do not normally have that much bandwidth even in a modern machine.
Typical bandwidth for the northbridge/southbridge connection is 1-2 GB/s
for most machines sold today. (For example just about all machines with
a recent Intel desktop chipset. The connection between north- and south-bridge
on those is equivalent to a PCI-E x4 connection (which provides 1GB/s in each
And that is for modern machines. Older ones have even less bandwidth

> If the controller resides on a PCI-bus together
> > with several other devices (which is not uncommon) then the reduced
> > bandwidth usage can be very useful.
> true. but not if it's builtin in chipset or on PCI express.

PCI-E controller cards are still fairly uncommon, and many of them 
require a x4 or x8 slot, while most motherboards only have x1 slots
(apart from the x16 slot intended for a graphics card.)
(And PCI-express is still fairly new, so there are lots of computers
in use that do not have any PCI-E slots at all.)

If you go back just a few years you will find that chipset itself
provides only two IDE-channels and nothing more.
Any other devices reside on a single PCI-bus (which provides a total
bandwidth of 133MB/s.)

> there are really not worth price. unless you need RAID-5.
> but with todays disk prices it's better to just use RAID-1+0 and bigger 
> drives.

That depends on what your goals are, and what constraints you operate under.
RAID 10 is nice, but it requires more disks then RAID5. Extra disks create
extra noise and require more power and generate more heat and (most
importantly) require extra space.  There are a limited amount of space
available in most computer cases, which might not be able to accomodate
the extra disks needed for RAID10.

> with software RAID you are not forced to operate on whole disks. usually 
> not everything has to be mirrored.

If you have reason to use mirroring at all, then I would say that just about
everything should be mirrored.
(RAID is in no way a substitute for backups.  The main reason for using RAID
is either performance (which is often better served by several independent
disks anyway) or to minimise downtime.  If some parts of your disks are not
mirrored then you be able to avoid that downtime anyway.)

<Insert your favourite quote here.>
Erik Trulsson
ertr1013 at student.uu.se

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