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[Linux-cluster] [ddraid] extensibility



hi philipp and all,

I am very interested in ddraid for having sios systems over the network.
A few questions though:

I've looked shortly at your implementation (ddraid-0.5.0), but have a
few questions, which I thought to ask you:

* how will you track extensibilty:

afaik you assign a fixed number of logical sectors to the device mapper
 device. but lets take a trivial example:

you have 3 disks (2^k+1), each in a sperate computer.
then you want to plug a 4th and 5th disk (2^k+1) disk (again separate
computer) in the ddraid - how should that look like regarding the dm setup?

what problems could arise given the striping information on the blocks.
do you simply extend a stripe from 2 to 4 data blocks? is it possible to
raise the number of logical sectors with device mapper (sorry for my
ignorance - I haven't looked the details of the dmsetup man page). do
you have to rearrange block sizes or stripe information?

* have you investigated raid-x - would that be possible to implement via
device mapper?

I've looked closer on a the papers by hwang et al regarding Raid-x,
which also provides a single io space for servlerless clusters. The
design is basically a mixture of mirroring and striping, making it
possible for one node to fail completly.
On stripe is made of (n-1) blocks and you have (n-1) mirror blocks. They
have implemented as part of their trojan cluster project, wich I think
isn't alive any more.

I look forward to hearing from you.

-- Jakob


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