MicroSD Card performance variance on XO-1.5

Martin Langhoff martin.langhoff at gmail.com
Fri Aug 6 12:43:29 EDT 2010

On Fri, Aug 6, 2010 at 12:30 PM, Richard A. Smith <richard at laptop.org> wrote:
> Whats the qualifications on that?  I assume its raw RGB video?

No, it's compressed vid + uncompressed audio to be stitched later.

I have a confession to make: Many years ago I worked in video mixing
with mid-range gear (avid and media100, I *don't* miss you) and with
cheapskates PC-based rigs (which where hell on wheels compared to avid
and media100).

My take is that our CPU is perfectly capable of capturing and
compressing vid and audio and putting it down to disk, even on a
2MB/s-capable disk, helped a bit by (limited) linux kernel buffering.
Most importantly, we need to capture audio + vid together, or at least
timestamp them together. Otherwise drift is inevitable.

I also think our vm.dirty_* settings are wrong and likely causing our
current fill-buffer-and-stutter behaviour. We are using the defaults
and those are for spinning harddrives, with a significant cost to
spinning up the disk on a laptop -- hence long expire_centisecs and
writeback_centisecs and the assumption that once you've started
writing, it'll be written out fast. We have zero seek costs, zero
"spin disk up" costs, but slowish writes -- we have to start writing
buffers out ASAP.

But that's my (naive?) take. I haven't delved into the gstreamer madness.

 martin.langhoff at gmail.com
 martin at laptop.org -- School Server Architect
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