[mythtv-users] Storing recordings on network share

[Travis Tabbal]

On Fri, Sep 23, 2011 at 4:48 AM, belcampo <belcampo at zonnet.nl> wrote:

>
> What are we talking about, in terms of large buffers, I mean. Even 6
> concurrent HD-streams of probably <= 20Mb/s only generate 120Mb/s being
> 15MB/s, with todays drives easily being capable of > 80MB/s.
>


Not under high random I/O load, which is what we get with Myth when dealing
with multiple recordings and such. Modern drives, even many SDDs, can't
sustain high random I/O loads at even close to 80MB/s. Not unless you go
with high-IOPS disks, then you start talking real money. Nothing you would
waste on recording TV. The streaming I/O capacity of a drive is meaningless
to us, as we never use it. On a mechanical HDD, seek time is close to 10ms,
that's 10ms that you can't do I/O. Start doing a bunch of those, and you
start seeing slowdowns.  Every block you write, particularly when sync() is
being called, means you seek to write it, then seek to the filesystem
structures and update those, then seek to do it again.. you quickly get to
where you're seeking more than reading/writing. That's where the
recommendation comes from to keep the database and the recordings separated,
the database can issue a LOT of seek calls.

And we're dealing with a stream here. A file copy can just wait to read the
next bit in, if we don't write fast enough, we corrupt the recording. The
codecs can fix some of that, but you drop enough data and you can't really
enjoy the recording as it will be all pixelated or choppy.

For just one example, look at the random I/O test at the bottom of this
article:

http://www.anandtech.com/show/3734/seagates-momentus-xt-review-finally-a-good-hybrid-hdd/3

>From the article:

"Random read/write performance is abysmal. You can't really make out the
numbers here but that's 0.7MB/s for reads and 0.3MB/s for writes compared to
40MB/s+ for the SSDs. It's the poor random access performance that
ultimately prevents the Momentus XT from feeling like an SSD most of the
time."

.3MB/s.....

That's a far rougher test than we would hit our drives with, but it shows
the huge performance drop from random I/O. Even the SSDs in their test only
got to 40MB/s.
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