Re: [gentoo-user] android and mtp

[Alan McKinnon <alan.mckinnon@gmail.com>]

On Sat, 22 Dec 2012 10:01:37 -0200
luis jure <ljc@internet.com.uy> wrote:

> 
> on 2012-12-21 at 23:20 Neil Bothwick wrote:
> 
> > No, and there's a good reason for that.
> 
> well, i'm glad to know that there's a good reason to use MTP, because
> what i've read so far about it made me wonder...

It all becomes understandable when you figure out what MTP actually is.
It's Media Transfer Protocol, it's not eg Media Transfer Filesystem.
Wiki tells you some more:

http://en.wikipedia.org/wiki/Media_Transfer_Protocol

So it's a protocol for getting an entire file (originally usually an
mp3) from storage to a device that would play it. It makes sense to copy
the entire file to an mp3 player then play it, seeks would not involve
network (or bus) traffic.

Consider that http also doesn't let you usually transfer bits of
files. That protocol is also happier giving you all or nothing (yes,
you can do partial downloads, but that's not really a seek action,
it's a once-off action to say where the start of the copy happens from.




> 
> from the link mark sent earlier:
> 
> <quote>
> libmtp (and I assume the MTP protocol itself) doesn’t support seeking
> within a file or partial file reads or writes. You have to fetch or
> send the entire file. To simluate normal random access files, when a
> file is opened the entire file contents are copied from the device to
> a temporary file. Reads and writes then operate on the temporary
> file. When the file is closed (or if a flush or fsync occurs) then if
> a write has occurred since the file was last opened the entire
> contents of the temporary file are sent back to the device. This
> means repeatedly opening a file, making a small change, and closing
> it again will be very slow.
> 
> Renaming or moving a file is implemented by copying the file from the
> device, writing it back to the device under the new name, and then
> deleting the original file. This makes renames, especially for large
> files, slow. This has special significance when using rsync to copy
> files to the device. Rsync copies to a temporary file, and then when
> the copy is complete it renames the temporary file to the real
> filename. So when rsyncing to a jmtpfs filessystem, for each file,
> the data gets copied to the device, read back, and then copied to the
> device again. There is a true rename (but not move) supported by
> libmtp, but this appears to confuse some Android apps, so I don’t use
> it. Image files, for example, will disappear from the Gallery if
> they’re renamed. </quote>
> 



-- 
Alan McKinnon
alan.mckinnon@gmail.com