[ale] "Cost" of raid?

[Robert L. Harris]

Thus spake Danny Cox (danscox at mindspring.com):
> 
> > Could anyone say that writes to Raid 5 take 1.25x as long, 4x as long,
> > etc?  When the array looses one disk, reads take 5x as long as 0+1,
> > writes take 7x, etc?
> 
> 	Someone may say, but they'd be wrong!  This, as in most things, depends
> on many factors.  
> 
> 	For example, the SGI XFS fs (which we use here) was quite a bit slower
> on RAID5, due to the way it writes it's log file in smaller chunks. 
> When a log write comes along, the software RAID5 driver flushes all of
> it's stripe headers for the "old" size, and reallocates them for the
> "new" size.  Then, it switches back.  Recently, though, SGI has made
> some changes in the way they write the log which helps greatly.
> 
> 	Regardless, RAID5 will typically write slower than RAID1 (even though
> you're writing the data twice in R1).  The RAID5 driver uses every trick
> I've ever heard of (and one new one that I'd not heard of ;-) to speed
> itself up.  It's very good.
> 
> 	Another point of view is: can/will you sacrifice some speed for the
> capacity?  The size of a RAID5 device is (size_of_drive * (# disks -
> 1)).

 Yeah, thats why I tried to give the hardware specs and what I know of
the software requirements orriginally and asked for a guestimate :>  I'm
meeting with them on monday and it'll be one of those "ok here are our
details, give us answers now" type meetings.  Maybe not but that's what
I'm trying to plan for.



:wq!
---------------------------------------------------------------------------
Robert L. Harris                
                               
DISCLAIMER:
      These are MY OPINIONS ALONE.  I speak for no-one else.
FYI:
 perl -e 'print $i=pack(c5,(41*2),sqrt(7056),(unpack(c,H)-2),oct(115),10);'


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