On Tue Oct 13, 2009 at 09:13:13AM -0400, Antonio Perez wrote:

1.- Should the "rest of the disk" be only one partition?
I have read that making several partitions and setting several md disks:
	sd[a..d]2 --> md1
	sd[a..d]3 --> md2
	sd[a..d]4 --> md3
would help with the rebuild time of each md, which sounds correct. It is 
also proposed that the md on the outer area of the disk would be faster 
allowing for better control of performance, assigning faster mds to the more 
used filesystems.

However, and this I don't know, those sda[2..4] are not really different 
devices (spindles) and reads to one md would conflict (or not?) with reads 
to the other mds.

Any reads to the same disk will conflict - whether they're on separate
arrays or within a single array shouldn't make any difference (the IO
elevator is set on the disk rather than the array, so I assume it will
gather requests across all arrays).  md itself knows that the arrays
share disks, so will ensure any maintenance tasks (resyncs, etc) only
run on a single array at a time.

2.- On the Raid 1: How many sectors to copy? 63?
On an update of grub code, core.img could change, which means that the first 
63 sectors (to be on the safe side) of the disk which gets the update should 
be copied to the other 3 disks.
Or is it that the md code would mirror sectors 1-62 and only the MBR needs 
to be manually mirroed?

Not sure what you mean here.  If you have /boot on a RAID1 array then
it's mirrored, so the stage1.5 and stage2 bootloaders get mirrored via
that.  The stage1 bootloader is usually installed into the MBR, so only
that needs replicating separately.

2.- Is there a recomended way to trigger the said copy of question 2?
Where should a call to copy the MBR should be placed? On update-grub?

I always just rerun the grub install for each disk manually (see
http://lists.us.dell.com/pipermail/linux-poweredge/2003-July/008898.html
for example).  Automating it will depend on your distribution - if
update-grub is what's called after a grub update (and it isn't
overwritten by the update) then this would seem the logical place to do
it.

HTH,
    Robin

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