Mailinglist Archive: opensuse (4446 mails)

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Re: [opensuse] downloading with aria2c problem - checksum error, image destroyed!
  • From: "Carlos E. R." <robin.listas@xxxxxxxxxxxxxx>
  • Date: Fri, 8 Dec 2006 16:32:45 +0100 (CET)
  • Message-id: <Pine.LNX.4.64.0612081613310.1727@xxxxxxxxxxxxxxxx>
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The Friday 2006-12-08 at 13:44 -0000, B.Weber@xxxxxxxxxxxxx wrote:

> > It should not happen. Metalink is supposed to use methods to avoid
> > download errors. So they say, at least. I very much doubt I'll ever use it
> > again nor recomend it.
>
> Correct me if I'm wrong, but I don't think http provides for the
> possibility of checksuming individual chunks of a file. Which would
> explain the unusually high incidence of corrupt downloads I'm seeing among
> people using metalink enabled clients to download 10.2.
>
> If you download chunks from 50 different mirrors and just one of the
> mirrors has a corrupted copy, then your entire download will be corrupt,
> downloading from lots of mirrors greatly increases the chance of a corrupt
> download.

You may be right. :-/

I had a look inside the metalink file, in XML format. It does contain a
checksum, but of the whole thing, not the portions. Then it contains a
list of servers, with some usefull info like country. I don't see how it
can checksum the portions, unless that checksum info is downloaded from
somewhere else.

However, you can certainly have that chunk checksuming using http or ftp -
not because the protocol support it, which it doesn't, but because you
can do that on top of it using your own means - that's what I had thought
metalink did. It appears it doesn't :-(

It simple does a checksum at the end for the whole download.


> So downloading 4gb isos using a metalink enabled client using lots of
> mirrors does not seem like a good idea, especially as any single mirror
> will likely max out whatever consumer bandwidth you have.

Well, as to speed, it is very fast indeed: almost three times faster than
torrent, which is still running and has finished only 44% in over 12 hours
- - aria2c had finished yesterday in that time.

The idea is good: a mirror may be too busy to give you your full download
bandwidth, so the user has to try several measuring the speed and choose
the best one. The metalink client does that automatically.

So, speed is good, reliability is bad: probably worse than with single ftp
or http, certainly worse than torrent.

> Bittorrent enables downloads from many sources, but with bittorrent each
> chunk is checked, so you only have a small amount to re-download on
> corruption.

I know. I have used it to correct corrupted fpt downloads.

- --
Cheers,
Carlos E. R.
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