Re: Load balancing across multiple network paths

Nikos Gkorogiannis <[email protected]> wrote:
> Interesting scheme. However, from my point of view, there are some
> disadvantages to this: 
> * three logical volumes instead of one, therefore three file servers
> (maybe one can re-export them to a single host but it starts getting
> nasty in that the advantages of the separation are lost).

Use nfs to reassemble the file systems on them on every node you care
about.

> * if one host dies, one of the three arrays is taken out (of course, one
> can setup a failover system that would serve the (degraded) array from a
> different machine - again quite complex).

That's the idea. It's precisely why the arrangement keeps enough data
to reconstruct the array off of the local node and on remote nodes. It
makes the assembly robust aganst the loss of any node. If you lost the
whole net, you would lose all three nodes (along with contact with them
:-).

> * is not the network still the main basket (compared to the above
> scheme)? :)

Well, if you wanted to instead keep two disks local and get a third
from the net, then you would need three disks on each node, and donate
one of them to another node, and import one from a different other
node.

So you would have a total of 9 physical disks, and you would produce
3 raid disks, each the size of 2 physical disks.  This arrangement
allows each node to survive the loss of the net, individually, but
doesn't allow a missing node to be reconstructed from the data kept on
its remote nodes. I don't see the point of that.


> > If you are running raid5, you are interested in achieving stability and 
> > rapid recovery, not speed!
> 
> Um, yes, obviously. I wasn't referring to speed necessarily in my
> question.

The network is not a stable thing. Timeouts occur. People trip over
cables. Routers go down or need resetting. You will go crazy if you are
sensitive to any of those things, and yu have made yourself sensitive
to n of them. The only sensible thing to do is not to have an n-1 fan
out!

(try 3-3*3-3*3*3)


Peter