On 10/09/14 14:24, [email protected] wrote:
...
Now, if there are going to be multiple partitions for the install (e.g.
/home, /var, etc.), my questions is, which is better:
A: Is it better to make "larger" initial partitions for raid assembly,
and then use disklable to create multiple partitions within that one
softraid volume (i.e.: one big softraid volume sd0, broken up into
sd0a, sd0b, sd0c...)?
YES
B: Is it better to make several smaller install partitions, and then
assemble multiple softraid volumes, and then use disklable to place only
one (or two?) system partitions in each softraid volume (i.e.: multiple
softraids like sd0, sd1, sd2..., each with only one partition like sd0a,
sd1a, sd2a...)?
NO! NO! NO! (generally :)
C: Or, does it not matter?
My limited (ok, non-existent) knowledge and/or understanding of disk I/O
makes it impossible for me to being to even guess what may be best.
it matters. :)
The point of RAID isn't just to build the array, but to maintain it,
including replacing failed elements.
So, you replace a failed disk and restart the mirroring process. If you
have one softraid volume, you just start it and let it go. If you have
multiple softraid volumes, you will have to rebuild each. So, you have
to either do them sequentially or at the same time. Sequentially
requires watching for one remirror to finish before starting the next,
so you have to be hovering over the server. So why not just start them
all at the same time? If on different physical disks, sure, go for it.
But on one disk? you will end up with some horrific thrashing of the
heads as it mirrors a block here and another block over there. Your
rebuild time may be 20x to 100x as slow as doing one volume at a time,
your disks will make unpleasant noises, and you may just break your
remaining disk before the rebuild is complete. Remirroring a 2T disk
may take more than a day...so twenty times as long is bad, one hundred
times as long is a complete disaster. Should you need to be FSCK'ing a
disk while a rebuild is happening (you want to avoid this, really)
things can get really really slow.
Nick.
