Re: [gentoo-user] 1-Terabyte drives - 4K sector sizes? -> bar performance so far

[Stroller <stroller@stellar.eclipse.co.uk>]

On 9 Feb 2010, at 15:43, Neil Bothwick wrote:

> On Tue, 9 Feb 2010 15:11:14 +0000, Stroller wrote:
>
>> You cannot remove one disk from the array and repartition it, because
>> the partition is across the array, not the disk. The single disk,
>> removed from a RAID 5 (specified by Paul Hartman) array does not
>> contain any partitions, just one stripe of them.
>
> A 3 disk RAID 5 array can handle one disk failing. Although  
> information
> is striped across all three disks, any two are enough to retrieve it.
>
> If this were not the case, it would be called AID 5.

Of course you can REMOVE this disk.

However, in hardware RAID you cannot do anything USEFUL to the single  
disk.

In hardware RAID it is the controller card which manages the arrays  
and consolidates them for the o/s. You attach three drives to a  
hardware RAID controller, setup a RAID5 array and then the controller  
exports the array to the operating system as a block device (e.g. /dev/ 
sda). You then run fdisk on this virtual disk and create the  
partitions. You cannot connect just a partition to a hardware RAID  
controller.

Thus in hardware RAID there are no partitions on each single disk,  
only (as I said before) stripes of the partitions. You cannot usefully  
repartition a single hard-drive from a hardware RAID set - anything  
you do to that single drive will be wiped out when you re-add it to  
the array and the current state of the virtual disk is propagated on  
to it.

I hope this explanation makes sense.

I was not aware that Linux software RAID behaved differently. See  
Joost's explanation of 9 February 2010 15:27:32 GMT. I asked if you  
were referring to LVM because I set that up several years ago, and it  
also allows you to add partitions as PVs. I can see how it would be  
useful to add just a partition to a RAID array, and it's great that  
you can do this in software RAID.

So this:

On 9 Feb 2010, at 00:27, Neil Bothwick wrote:
> With the RAID, you could fail one disk, repartition, re-add it,  
> rinse and
> repeat. But that doesn't take care of the time issue

only applies in the specific case that Paul Hartman is using Linux  
software RAID, not the general case of RAID in general.

Stroller.