[mythtv-users] question about RAID

Johnathon Meichtry johnathon-dev at meichtry.org
Mon Jan 9 20:36:14 UTC 2006


Thanks, I was putting it in layman's terms but your right I should have been 
more precise.

If one was going to be technically correct then it isn't even that straight 
forward as with most systems you will probably have three random access 
reads for every sequential write therefore with RAID0 you may get an 
alternating disk write (some would say simultaneous but really there is no 
such thing as there is only one bus) but between them you will probably have 
three reads with two probably being on the same disk therefore RAID0 only 
giving you a marginal performance improvement without even taking into 
account disk 1 and disk 2 being on the same controller (i.e. IDE PATA master 
and slave) and the resultant IO wait (not to mention the bottleneck on the 
PCI bus).

To me striping is one of the things where it looks great in theory but not 
until you have three or more disks across multiple controllers do you get a 
real-world noticeable performance improvement.  Not that I am knocking RAID0 
but I believe that all it gives you is aggregation of partitions and not 
much else so you might as well have at least three or more disks/slices and 
go for RAID5 so that if you lose one you don't lose the lot and you have a 
better chance of getting a performance improvement.


----- Original Message ----- 
From: "James C. Dastrup" <jc at dastrup.com>
To: "Discussion about mythtv" <mythtv-users at mythtv.org>
Sent: Monday, January 09, 2006 7:53 PM
Subject: Re: [mythtv-users] question about RAID

>All RAID0 does is join (or grow) partitions together.  It is very useful if
>you have, for example, three 100GB disks about like them joined into a 

Not exactly. A true RAID 0 (or a striped set) alternates writing each block
from one disk to the other. This increases performance because 2 chunks
of data can be written at the same time - one chunk to disk 1, one chunk to
disk2, etc.

Joining partitions together, or extending a volume, is not RAID 0. This does
not necessarily increase performance, because data typically won't be 
to the second disk until the first is full.

In both scenarios, your useable space equals all your disks combined, but
only the striped set improves performance.

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