[mythtv-users] ANN: PlugMyth, a new MythTV distribution for Plug computers

[Simon Hobson]

Mike Perkins wrote:

>Every month or so we get a query here from someone who wants to put 
>everything on one disk and the response is always the same: unless 
>you have a small system and you don't record very much, it's not a 
>good idea.
>
>As you make recordings, the video stream is written to the partition 
>you identified as storage. At the same time, it is also writing 
>position information to the MySQL database. These two are going to 
>be on different parts of the disk.  You really /don't/ want to have 
>large amounts of head seek movement while you're writing a video 
>stream which you have no control over. Solution: put video storage 
>and OS files (including your database) on two separate spindles.

All I can say is - it works for me. Unless you have a heavy recording 
load, I can't see that it's going to be that big an issue.

First off, just how much traffic is there for the OS once the system 
is up and running ? Very little I'd imagine - and that leaves us with 
the database. Again, I'd hazard a guess that it isn't writing all 
that much data either.

Now lets look at the video streams. Typical SD digital streams (DVB) 
are in the order of about 2Mbps, or 1/4MByte/s. That means the system 
is going to take in the order of 4 seconds to fill a megabyte of 
buffer - more than enough to seek, write, and seek back again. Even 
if recording several streams (which I'll come to), that should not be 
beyond the abilities of the disk and OS caches to accommodate - and 
how big are disk and OS caches these days ?

But lets look at what happens in anything but a trivial situation 
(which would be writing one video stream and doing nothing else). Say 
we are recording two programs, while watching something else. That's 
two write streams, and one read stream - all of which are timing 
sensitive. By the law of averages, most of the time, the files are 
not going to be in similar areas of the disk - and so the disk is 
going to be doing "a fair amount of seeking" anyway. Yes, adding the 
OS, and especially db updates, may be enough to tip from "can cope" 
to "run out of buffers" - but I rather think the idea of PlugMyth is 
to deal with the small installations where lots of streams aren't 
intended.

For reference, my BE runs as a VM (under Xen) on a single core AMD64. 
Everything for the entire machine is on one 1.5TB disk (a WD 'Green' 
so not the highest performance drive available). It can cope with 
recording two programs while watching a recording, or recording three 
programs. It does however run out of steam if I try four streams (3 
recording plus playback) or 3 streams plus commflagging.

So on the basis of my experience, I'd say that for the sort of small 
installation this project is aimed at, having everything on one disk 
should not be a big issue. Maybe not ideal, but "good enough".

-- 
Simon Hobson

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