[mythtv-users] MythTV Installer Usability: An Analysis

f-myth-users at media.mit.edu f-myth-users at media.mit.edu
Fri Nov 4 04:15:39 EST 2005

    Date: Fri, 04 Nov 2005 18:32:51 +1000
    From: ffrr <ffrr at tpg.com.au>

    Is this because the ringbuffer file seems to continually grow?  I have 
    noticed this.  Why does it do that? If you are not paused, surely the 
    file size should remain stable?

Yes.  The ringbuffer grows monotonically -up to its configured limit-
and then (I assume!) stops growing.  It's not clear to me why the
installer defaults to a partition sufficient to hold 5.3 hours' worth
for a 200GB disk, though; that seems like an awfully large buffer.
(I've cursed my TiVo's half-hour buffer, but it's seemed to me like
a couple of hours should be plenty.  Maybe it's 'cause Myth doesn't
allow you to dump the ringbuffer into an active recording [e.g., you
see something on TV you didn't know would be on, and decide you want
to save the entire thing, including what's already been aired], and
without this capability you tend to need a much larger ringbuffer?
I dunno.  I -do- hope that this capability is added someday; it's
been a real lifesaver on my TiVo.)

The problem is that the eventual upper bound of the ringbuffer, at
least with the sizes I got by default (and, apparently some others
in the KnoppMyth forum) is a couple GB -larger- than the size of the
partition the installer allocated for it, and Myth handles this...


To say the least.

Obviously, the two halves of the installer need to talk to each other,
to the pick the right sizes, or at least the ringbuffer-defining half
needs to actually look at the damned partition and make the intelligent
guess that (a) its default should not be bigger, and (b) the user
should not be allowed to -make- it any bigger, either.

In the current case, the installer defaults to behavior that will
cause the machine to livelock and explode if the user has the bad
luck to leave the UI on the wrong screen and go to bed.  Whoops.

P.S.  The current installer's default of such a large ringbuffer and
ext2 (ext3?) also has the problem that trying to switch away from live
TV back into the UI can take a -very- long time---apparently the
ringbuffer is deleted immediately (hey! but what if I wanted to finish
watching something after doing something in the UI! now it's -gone-!),
but deleting 12GB in that filesystem takes something like a second per
GB, at least, even on fast hardware, because of all the disk thrashing
that's required.  The first time this happened to me (while testing
the above behavior), I was wondering if the machine was -again- wedged,
until the UI finally reappeared.  Other filesystems (XFS?  JFS?  I don't
remember) are apparently much faster to delete, although using those
for the ringbuffer now means the kernel & installer need to know about
more filesystems & have tools to manipulate them, etc etc, more
complexity, yadda yadda...  Easy enough for a custom install;
questionable (maybe) for something like KnoppMyth.  I'd be happy
to be proven wrong.

(And, y'know, I've deleted very large files on identical hardware and
didn't seem to wait so long.  Either the uncertainty was making it
feel like forever, or Myth is doing something -else- weird while
that's going on which is making the whole process take even longer.
Maybe I'll set up another test someday, but not tonight.)

More information about the mythtv-users mailing list