Re: Can't install GRUB

[Chris Murphy]

On Nov 5, 2012, at 10:42 PM, Paolo <address@hidden> wrote:
> 
> How can I calculate the right values?

The first partition should start on sector 2048, which is what all new 
partitioning tools do, including the fdisk and parted in CentOS 6 as far as I 
know - unless you are explicitly telling them to do something differently. But 
the tool you're using seems to be reporting CHS values instead of LBA which is 
what originally confused me: that start value of 1 is actually a cylinder 
start, but a sector value of 63 which is fine for GRUB to fit into the MBR gap. 
You need an LBA start value of 2048, not 63.


> and .. is this a real problem for a working system (performance or other 
> problems)?

Yes. It can massively and negatively affect the performance of the array with 
excessive RMW. Your disk is a disk with 4K sectors, but masks them behind a 
512e layer. To avoid excessive RMW, you need to make sure that raid chunks are 
on 4K boundaries instead of in between. And they aren't on 4K boundaries if you 
use a start sector of 63 like you have now. It's widely considered best 
practices to partition in 1MB boundaries to make this easier to compute, and 
2048 512 byte sectors is 1MB, hence that suggestion.

I suggest you look at the linux-raid archives and post there if you have 
specific questions on what the layout should be.


Chris Murphy