[mythtv-users] Two-backends with two-frontends on one machine?

Billy Macdonald billymacdonald at gmail.com
Wed Jul 11 23:44:09 UTC 2007

On 7/11/07, Jay Mallar <surfing at jaymallar.com> wrote:
> On 7/11/07, billymacdonald at gmail wrote:
>  > Michael T. Dean <mtdean[at]thirdcontact.com> wrote:
>  > On 7/11/07, Michael T. Dean <mtdean[at]thirdcontact.com> wrote:
>  > > On 07/11/2007 11:27 AM, Steven Adeff wrote:
>  > > > 1) RAID 1 seems like overkill, why not go with RAID5? I've got a 6
>  > > > drive RAID 5 device that keeps up with my 4 HD tuner backend with two
>  > > > HD capable frontends no problem.
>  > >
>  > > And, in the future (i.e. 0.21 and up), RAID may in fact work against
>  > > you. With Storage Groups, you can set up your system to write each
>  > > recording to a separate disk (provided enough disks), thereby
>  > > significantly reducing fragmentation and seek issues. I have 4 HDTV
>  > > capture cards and more drives than capture cards and have configured
>  > > Myth so that it will only write two shows to the same disk if
> that's the
>  > > only way to record the show (i.e. if all the other disks are full).
>  > > After using it this way for some time, I can say I'll never use a
>  > > multiple-disk (RAID or LVM with multiple physical volumes)
> configuration
>  > > with Myth again.
>  >
>  > RAID 1&5 provide redundancy that I don't think your solutions
>  > addresses. Your solution seems to be solving more of a problem that
>  > isn't there IMO. I have 4 HDtuners, and 3 pvr250's. All 7 tuners can
>  > record to my LVM volume (not striped, 3 disks) and I can play back an
>  > HD recording at the same time. And these are cheap IDE drives that I
>  > got on sale at Best Buy. Now being able to store recordings in more
>  > than one location does provide flexibility which I believe is a good
>  > thing. But I'm not sure it's really needed for performance.
> RAID will work against me with MythTV 0.21 and up??  That seems a bit
> bizarre.  I see your point of using Storage groups to assign recordings
> to different disks, but in my mind, I don't have to want to mess with
> which tuners use which disks.  I want to set up a RAID 1 mirror (or
> several) and link them via LVM so MythTV sees one partition  (From what
> I can tell, LVM is a bit more flexible than Storage Groups.)
> I'm not sure I buy the argument that using Storage Groups significantly
> reduces fragmentation and seek issues.  I guess your point is the less
> you use a disk, the less chance of it being fragmented, but I think
> that's a red herring, at least with popular Linux file systems.  I've
> yet to have a need, in all my years of Linux, to defrag a Linux disk.
> What about MythTV would make me want to start?
> Your machine interests me tho - 4 HD, 3 SD tuners are putting out around
> 35MB/s of data by themselves.  Nice.  And you're telling me you can
> actually use all those tuners, plus decode and watch HD streams, on the
> same machine, without any artifacts?  Do you channel skip?   What kind
> of hardware are you using (MB, CPU, tuners in particular)?
> --
> -Jay

My backend is actually pretty crappy.  I think it's something like an
Athlon 2000 with 512 ram, nah 756 ram.  It's actually worse than it
used to be as I had a mobo fail and put in an old one.  To give you a
frame of reference, it doesn't have USB2.0.  I do think one of my
drives is SATA, but the other two are just PIDE.  I don't ever record
that much naturally.  I just do it to make sure it works after messing
with the system.  Though I can get 4 HD recordings at once on Thursday

The frontend is remote, the backend is headless, so that helps.  I
have to slow down any mythcommflagging that happens on the backend, or
playback and recording will suffer.  I do this by running a background
process that sends a SIGSTOP and then a SIGCONT ever x milliseconds.
Yeah, that's probably not ideal (especially since it uses killall),
but it works.  Causes the flagging to take longer, so we never expect
a show to be flagged the day it's recorded.  That's not our biggest
prerogative now that we learned that 3,skip will skip 3 minutes.

I actually have 2 frontends.  One of the frontends that is on an SDTV
is the designated transcoding PC.  The backend never does transcoding
(slow CPU).  The transoding PC will pause the mythtranscode job
(SIGSTOP again) if the backend is recording anything on the HD tuners.
 It queries the http backend status to check this every minute I

Oh, and the bitrate for HD is in bits, xMb/s, but the IDE standards
were in bytes, MB/s.  So even a UDMA33 drive, 33MB/s = 264Mb/s.  But
to be honest, I've been pleasantly surprised that my drives have been
able to keep up with the additional HD tuners.

My world is not an ideal world, but it is functional.


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