Re: determining sunit

[Joshua Baker-LePain <jlb17@xxxxxxxx>]

On Tue, 4 Jul 2006 at 5:10pm, pgs@xxxxxxxxxxxx wrote

> Unfortunately, it didn't really answer to my question.
> Maybe it's because I don't know enough about RAID arrays.
> Well, what I want to know is :
> how can I know the value to assign to sunit and swidth?
> All I know at this time for my RAID array is that there are 12 disks of 500 GB, 
> so 11 * 500 GB of usable space (last disk for checksum), and it's not enough to 
> know which value to set to sunit, it seems that sunit must not be equal to the 
> size of one of the disk in my RAID array.
> I heard here and there about chunk size and stripe size. But I don't know what 
> is the cunk size of a RAID array and how can I know it, (it's not me that 
> bought this RAID array), someone of my company told me that its stripe size was 
> 64k, but are stripe size and chunk size the same? And must sunit be equal to 
> the chunk size? And swidth?

Quoting an email sent by Steve Lord *long* ago regarding my 8 disk RAID5 
w/ 32KB stripe size (also referred to as chunksize):

\begin{quote}
Actually this is incorrect, you need

        -d sunit=64,swidth=448

The sunit is the amount of data on each disk in 512 byte chunks, the swidth
is the total amount of data before we go back to the first disk again. You
have 8 disks in raid5 so 7 data disks and a parity disk. You need 7 * 64 
as the stripe width.
\end{quote}

So for your system, it'd be "-d sunit=128,swidth=1408" (assuming you're 
not using a hot spare).

--
Joshua Baker-LePain
Department of Biomedical Engineering
Duke University