Re: [dm-devel] Use device mapper to redirect to cache server?

[malahal us ibm com]

Ph. Marek [philipp marek bmlv gv at] wrote:
> Hello everybody!
> 
> 
> I'd like to mirror write accesses to a block-device to two devices, get all 
> all *read* accesses from one of these, and if it fails, to fallback to the 
> other device.

This is exactly what 'mirror' target does (dm-raid1.c) without
read-balancing. But read on...

> The usage would be to use some kind of NAS box as persistent network block 
> device, with some other machine with a lot of RAM as cache server; the cache 
> server would get asked for data, and if it's not available, the request 
> should go to the NAS directly.

I think, you are better of using the cache server as some kind of NFS
server as 'mirror' target's failure handling is heavy-weight.