[mythtv-users] "TFW, Error: Write() -- IOBOUND" 8 minutes long!

[Kevin Kuphal]

On Wed, Mar 16, 2011 at 11:56 PM, Douglas Peale
<Douglas_Peale at comcast.net>wrote:

> I have been having picture breakups in my recordings lately. Looking in my
> backend logs I noticed that they seemed to correlate
> to "TFW, Error:Write() -- IOBOUND" errors. Most of them are short, on the
> order of seconds, but this one:
>
> 2011-02-21 22:56:51.280 TFW, Error: Write() -- IOBOUND begin
> remaining(4865) free(0) size(4194304) cnt(1)
> 2011-02-21 22:57:00.738 Player(0): Timed out waiting for free video
> buffers.
> 2011-02-21 23:05:02.603 TFW, Error: Write() -- IOBOUND end
>
> is over 8 minutes long, and corresponded with the missing ending of a
> recording.
>
> The system is an i7 processor with 6 GB of ram and a 1.5 TB SATA drive. The
> only slightly unusual thing I am doing on my system
> is starting the commercial flagging as soon as the recording starts. The
> assumption being this should reduce disk accesses since
> the data should still be in the disk cache when the commercial flagger
> tries to read it.
>
> I assume the normal behavior would be worse since it would start commercial
> flagging the moment the recording ended, the
> recording would be bigger than the disk cache, so the beginning of the file
> would already have been dumped from the cache, and
> if a second recording had started, the second recording would be fighting
> with the commercial flagger for disk bandwidth.
>
> Am I making a bad assumption here? Should I just go out and get a SSD for
> the system, and use the existing drive only for
> recordings?
>
> Is there any way to get MythTV to buffer to RAM when the disk drive gets
> bottlenecked? 6GB of ram should be good enough for a 30
> minute ring buffer.
>

I had no end of problems when I had the OS, swap, database, and recordings
all on the same spindle.  SATA is not the best for heavier random access
loads and I found that having recordings separate from OS/database was good
enough but having the DB on it's own disk also was best (3 devices).

Kevin
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