[mythtv-users] "background" balancing disks in a storage group
Brian J. Murrell
brian at interlinx.bc.ca
Wed Apr 21 22:07:07 UTC 2010
Given that one of the nice features of storage groups is to spread disk
i/o load among spindles, I wonder if there is anything (i.e.
transparently) balancing space amongst disks in a storage group to keep
the disks evenly available in space.
Imagine I have two disks where one was twice (or 3 or 4 times) as big as
the other. Further assume, for simplicity's sake that whenever I am
recording, it's typically on two or more tuners at a time (in fact, this
is not so outlandish an assumption: prime time television is just like
that -- all the good shows are on at the same time, competing with each
So, if we follow the assumption that you have two disks both starting
out empty with one twice as big as the other and two things recording
whenever there is any recording happening, pretty soon, the one disk
will get full and the other will only get half (or 1/3 or 1/4) full.
Ideally, while there is no recording going on (which is most of the
time), mythtv should be re-balancing the disks so that their absolute
(not percentage) free space is as close to equal as possible.
This would even make extreme cases such as having a single large disk
(i.e. to be the main recordings store) in a system and then adding
several much smaller disks just to act as temporary storage, solely to
increase i/o bandwidth. When bandwidth demands are low, the smaller
disks are emptied out into the large disk.
As I understand it, if a recording is moved from one disk in a storage
group to another by a process unknown to mythbackend, it will still find
it, by searching on all of the disks, so it's not entirely infeasible to
build such a re-balancing process outside of the backend altogether.
But such an "outboard" process would not have the knowledge that
mythbackend has about what is being recorded currently and to which
disk(s) and as such an outboard process would not be able to influence
mythbackend's storage weighting algorithm as it consumes bandwidth to
re-balance. Additionally mythbackend has knowledge of how far away a
future recording is allowing it to judge when to stop/pause the
re-balancing to provide recording with disk i/o.
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