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Old 01-08-2008, 08:30 AM
"Ph. Marek"
 
Default Use device mapper to redirect to cache server?

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.


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.

If I read the multipath documentation correctly (at
http://christophe.varoqui.free.fr/multipath.html):
For completion, here is an example of a pure failover target definition for
the same LU :
...
multipath should do part of what I want - use another device if the first one
fails.
But that doesn't account for pushing writes to both; and how is failing
defined for NBD? I don't see some configurable timeout.


Thank you for all ideas/suggestions! [[ but please CC me ]]


Regards,

Phil

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Old 01-08-2008, 06:30 PM
 
Default Use device mapper to redirect to cache server?

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.

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