[dm-devel] Sending reads/writes to a userspace program?

["Nick Johnson" <arachnid notdot net>]

Does dm provide any way for a custom target type to redirect a read or
write to a userspace program? Or even for it to notify the program of
the read/write and wait until it gives the goahead, somewhat like the
way page faulting works? I'm wondering if it's possible to create a
target type that implements a 'remote' block device with blocks stored
on something like Amazon S3, keeping only a local cache of mapped
blocks, and fetching / writing blocks as needed.

-Nick Johnson