Re: [Qemu-devel] [PATCH for-2.8? 0/3] block/curl: Drop TFTP "support"

[Max Reitz]

On 03.11.2016 08:56, Markus Armbruster wrote:
> Max Reitz <address@hidden> writes:
> 
>> See patch 3 for the reason why we have actually never supported TFTP at
>> all (except for very small files (i.e. below 256 kB or so)).
> 
> Care to explain why it works "for very small files" in a bit more
> detail?  PATCH 3 gives a "does not support byte ranges" hint, but to go
> from there to "very small files", you need to know more about how the
> block layer works than I can remember right now.

Our curl block drivers caches data and uses a readahead cache, which by
default has a size of 256 kB. Therefore, if the start of the file is
read first (which it usually is, if just for format probing), then the
correct data will be read for that size.

Yes, you can adjust the readahead size. No, I cannot guarantee that
there are no users that just set readahead to the image size and thus
made it work. I can't really imagine that, though, because at that point
you can just copy the file to tmpfs and have the same result.

Also, if I were a user, I probably wouldn't use 256 kB images, and thus
I would just notice tftp to be broken. I don't think I would experiment
with the readahead option to find out that it works if I set it to the
image size and then just use it that way. I definitely think I would
give up before that and just copy the file to the local system.

Max

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