[mythtv-users] Shared Firewire Drive??

[Graham Wood]

On Tue, Apr 18, 2006 at 10:05:35AM -0400, Bryan wrote:
> Does this seem feasible? I don't see an issue where both backend systems
> writing to the same file at the same time due to the way myth handles the
> master slave setup.
I'm not aware of anything in the myth setup that would allow it to work
this way without filesystem corruption.  The most obvious problem is if
both backends are trying to record at the same time - they will be using
cached data to decide which inodes to allocate to which recording, and
will therefore most likely record over the same places creating
immediate corruption.  You're going to need to use a cluster filesystem
to do this (something like GFS), which will affect your throughput when
writing to two files in the same directory...

> I am concerned with the speed and access rates of a
> firewire drive? Is there anyone using it to write recorded shows to it? I
> will be recording at max, 4 shows at 4500 bit rate at a time.
The drive itself is very likely to be a standard drive - so therefore
raw throughput is unlikely to be a problem - the issue is in the
filesystem overhead.

Assuming the "4500 bit" is not refering to 4.5kbit, you're looking at
18Mbit raw throughput - which shouldn't be a problem in itself.
However, getting a filesystem to allocate the correct inodes fast enough
when you're writing to 4 files in the same directory is going to be an
issue.

You've basically got 2 options:
1. Use something like GFS and hope that it can keep up - if you can keep
the two machines writing to different directories on the same
filesystem, then this /should/ be OK.... But you'd be best trying it
first to make sure.
2. Use a different partition for each backend - so that although they
are using the same disk, there's no contention for the filesystem.

Obviously (1) gives you the advantage of shared space, and (2) gives you
the advantage of reduced setup/overhead in use.

Both are reliant on the drive supporting 2 simultaneous computers
accessing it, and I believe that not all firewire chipsets do - so
you'll need to check that too.

Basically the answer is that it is probably doable, but not as easily as
you seem to be suggesting.