So what you want to do is have 3 partitions where you have /, /boot & the rest?

For desktop usage it's OK to use that setup since you won't be writing
to / and the other segments a lot at the same time.

If you're running an application which writes a lot of data to / and
you require to read/write a lot of data of the rest of the disk, it
will conflict and slow things down a lot.

Basically, you're partitioning each disk and making each partition
belong to an array. So if the collective partitions of Array1 are busy
with something and the partitions of Array2 are also busy, you'll slow
down because you're reading/writing to/from the same disk from two
different partitions.

On Tue, Oct 13, 2009 at 4:13 PM, Antonio Perez [off-list ref] wrote:

If I'm posting to the wrong group, sorry. just point to the RTFM link.

This post is about setting up a Debian box with four disks (size should not
be important, me thinks), let's assume that a Raid 5 is the correct type for
the intended use.

Keeping aside LVM and/or layering of md (just for simplicity), and taking
into account that /boot, / and maybe other areas should go in a Raid 1
configuration, for booting reliability. I have three questions that perhaps
you could help to clarify:

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.

Setting the whole disk as one partition would prevent any conflict but would
take longer to rebuild and files would be spread over the whole area of the
disk.

I really don't know the internals of md well enough to tell what advantages
and problems one setup has over the other.

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?

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?

TIA

--
Antonio Perez

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