[rescue] RAID (was Re: Insane drives)

[Bran Tregare]

I've been installing Compaq, IBM and Clariion(iirc) arrays for several 
years, and the data is striped across the logical units(each logical unit 
is 1 drive usually), you CANNOT implement a raid 5 stripe on a single disk, 
isn't gonna happen, I know of NOT raid software that would allow 
this.   you do NOT lose 1/3 of your space.  the stripe is written across 
all logical units with 1 used for parity.  in december-ish I installed a 
Raid chassis using 8 80 GB drives for a total of 560Gigabytes of disk 
space. the stripe was across 7 drives ant the parity was written on the 8th 
drive.

your assumption is correct -IF- you are only using 3 drives, RAID5 uses 1 
drive for the parity data and the rest of the data is striped across all 
the units.

At 10:40 PM 7/10/01 -0400, you wrote:
>At 03:48 PM 7/10/2001 -0400, you wrote:
>>Joshua D. Boyd writes ---
>> >On Tue, 10 Jul 2001, Robert Novak wrote:
>> >> If you have 10 9.1GB disks in your set, you get 75.6GB or so (assuming no
>> >> spares). Might want to use a hotspare if you have the capacity (Sun's
>> >> Disksuite will let you create a hotspare pool, so put one disk in it and
>> >> attach it to your RAID 5 metadevice).
>> >
>> >So, you are saying that you always only use one disk for parity?  I
>> >thought that was what raid 3 did.
>>
>>Raid 3-5 all use 1 disk's worth of space for parity.
>
><snip>
>
>
>>Raid 5: 1 disk worth of space for parity. Parity for each stripe is rotated
>>         through the disks.  Parity calculated by the block/stripe.
>
>I'm not convinced this is right.
>
>(Everybody shout if I'm wrong and I'll learn something...)
>
>My understanding is it has nothing at all to do with "disks". You could 
>implement an entire RAID-5 "array" on a single disk if you wanted, though 
>it would have somewhat limited usefulness.
>
>A simplified description of the data storage model, as I understand it, is 
>that any given block is paired with another block and the result of XORing 
>those two blocks is then stored in a third block. You can lose any one of 
>these three blocks and it can be recreated from the remaining two. Ergo, 
>put each block on a different physical device and you can lose any one 
>physical device while being guaranteed that all the blocks on it can be 
>recreated.
>
>This also explains why write performance blows on a RAID-5. A single block 
>write is actually one read (the "partner" of the block we're trying to 
>write) and two writes (one for the block we're trying to write and one to 
>update the XOR).
>
>So you lose 1/3 of your total storage capacity plus any overhead 
>associated with maintaining the array and stripe metadata.
>
>--Adam
>
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