shell wrapper to set ioctls before & after a command execution?

[Giacomo Mulas <gmulas@oa-cagliari.inaf.it>]

Hello everybody.

I am looking for hints to create a wrapper for an arbitrary command, so that
I can set a ioctl using its PID and reset the ioctl after the command is
finished.

The problem I need to solve is the following: I have an hybrid spinning+ssd
drive on my laptop, and I set up flashcache to use the small (~20G) ssd to
cache the 500G spinning disk. It works well, but is defeated every time a
backup runs, or any cron job which goes through the whole disk, thereby very
effectively wiping the cache (i.e. throwing away the most frequently used
data from the cache and making it useless until it is repopulated).
Flashcache does offer the possibility to "blacklist" PIDs so that they will
not go through the cache. It cannot remove the PIDs automatically, so they
must be removed from the blacklist after the process finishes.
So my idea would be to use wrappers which call flashcache_setioctl before
the command to be blacklisted, and immediately after it is finished.
Setting is easy: I can just do, e.g.

flashcache_setioctl -a -b $$ /dev/mapper/mycache ; this adds the current PID
                                                 ; to the black list
exec mycommand $@

however, in this way the shell exits after mycommand finishes, and I cannot
add at the end the final command

flashcache_setioctl -r -b $$ /dev/mapper/mycache

which would remove the PID from the black list of the cache.

Can someone suggest a way to wrap a command so that I can know its PID
(before it is launched or immediately after) and get back control when it is
finished? It sounds relatively easy, and more general than my specific
problem, I would be surprised if noone else has yet faced a similar case
(and solved it).

Thanks, bye
Giacomo

--
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Giacomo Mulas <gmulas@oa-cagliari.inaf.it>
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