Re: [dm-devel] [PATCH 1/1] dm-mpath: Extend path selector interface for supporting Dell EqualLogic path selector

[Alasdair G Kergon <agk redhat com>]

On Mon, Jun 28, 2010 at 09:53:20AM -0400, Narendran Ganapathy wrote:
> This patch extends the dm-path-selector interface to allow path selectors to
> use extra information from the IO request when selecting a path.

> To do location based routing, we need the address information of the request.

What other inputs - in addition to offset - will the path selector need to take
into account to make its decision and how will it get those inputs?
Presumably you envisage some sort of semi-static or cached information,
and not asking the hardware before every piece of I/O?
 
How many ranges are there likely to be in this offset-based routing table?
How frequently is the offset-based routing table likely to change?

As Hannes points out, the dm table layer is already designed to handle
offset-based routing, so I'll need some convincing there's a need to duplicate
part of this inside path selectors.
 
If this information is rapidly changing - many reconfigurations per minute,
then we may need to consider some in-kernel solution.  Otherwise I'll be
seeking solutions performing the reconfiguration from userspace first.

> We also propose extending the dm_mpath_io structure used to hold information

I'll consider proposals for any new fields alongside patches that make good use
of them, but I won't add fields in advance.

Alasdair