Re: [gentoo-user] RAID

[Dan Farrell]

On Tue, 13 Mar 2007 08:14:28 -0000
"Nelson, David \(ED, PAR&D\)" <David.Nelson2@×××××××××××.com> wrote:

> > -----Original Message-----
> > From: mwq [mailto:mwq@××.pl]
> > Sent: 10 March 2007 21:00
> > To: gentoo-user@l.g.o
> > Subject: [gentoo-user] RAID
> >
> >
> > I have one laic question which may not be directly connected
> > to Gentoo but I think you'll forgive me that.
> > Imagine such a situation: I have two hard drives but drive A
> > is twice faster when reading and writing then drive B. I want
> > to make RAID 0 using A and B. Why are the stripes sizes on
> > both drives excacly the same? (I've googled and didn't find
> > any information about different spripes sizes) I think that
> > using twice greater stripe on A gives more speed then using
> > equal stripes.
> > And my question is: where am I doing a mistake?
> > PS
> > Sorry for my poor English
> >
> > --
> > gentoo-user@g.o mailing list
> >
> >
>
> Forgive me the maths approach here, but if drive A reads/writes at
> speed a, and drive B at speed b.
>
> a = 2*b from what you have said.
> In raid 0 drive speed is limited by the slower drive (if I recall
> correctly) so the speed your raid array would be limited to is .... b
> + b = 2b = a (from above).
>
> Hence you would be as well not bothering with RAID if drive A is 2x
> as fast as drive B.
>
> Naturally it's not as clear cut but should be pretty close. RAID 0ing
> these drives would gain you little in terms of speed while any 2
> drive RAID 0 setup increases the chance of failure by 2.
>
> If you just want them to appear as one drive look at something like
> LVM which can create one volume from both drives, although parts of
> the volume on drive A would be faster than those parts on drive B.
>
> Just my £0.02, feel free to poke holes in my reasoning ;)
well, I'll try a bit perhaps.  Given my scheme, we should really be
talking about 3 drives -- well, actualy you generally raid
_partitions_.  
A = fast disk, or speed thereof
B = slow disk. or speed thereof
A1 = first partition, size R/3, fast disk
A2 = second partition, size R/3, fast disk
B1 = first partition, size R/3, slow disk
raid-0 is A1 + A2 + B1
now let's say you have a file N to pull from the disk.  Naturally, one
third of it will come from each partition.  that is, two-thirds of it
comes from A, and A being twice as fast as drive B, the one-third
that's on B gets done in about the same amount of time.  so N mb is
copied in max( (2/3*N)/A , (1/3*N)/B ) seconds which should be roughly
the same. 

if N was on A alone obviously the speed would be N/A seconds.  (N/A) >
(2/3*N)/A.  

in reality, though, I think the best performance would probaby involve
just using the fast drive.  RAID introduces too much overhead to make
up for itself in this situation I think.  
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