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

[Jeff Cody <jcody@redhat.com>]

On Fri, Nov 11, 2016 at 08:46:11PM +0100, Max Reitz wrote:
> On 09.11.2016 20:15, Jeff Cody wrote:
> > On Tue, Nov 08, 2016 at 08:14:58AM +0100, Markus Armbruster wrote:
> >> Max Reitz <mreitz@redhat.com> writes:
> >>
> >>> On 07.11.2016 09:20, Markus Armbruster wrote:
> >>>> Max Reitz <mreitz@redhat.com> writes:
> >>>>
> >>>>> On 03.11.2016 08:56, Markus Armbruster wrote:
> >>>>>> Max Reitz <mreitz@redhat.com> 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.
> >>>>
> >>>> I'm not trying to make you explain why it's okay to drop TFTP.  I'm
> >>>> trying to make you explain what exactly worked and what exactly didn't.
> >>>> Such explanations generally involve a certain degree of "why".
> >>>
> >>> Well, I'm trying to explain both. :-)
> >>>
> >>>> Your first paragraph provides a few more hints, but I'm still guessing.
> >>>> Here's my current best guess:
> >>>>
> >>>> * Commonly, images smaller than 256 KiB work, and larger images don't.
> >>>
> >>> Yes. Unless you set the "readahead" option to something different (it
> >>> just defaults to 256 kB), then it'll commonly work for that images up to
> >>> that size.
> >>>
> >>> Oh, and I just realized it's not called "readahead" for nothing: It gets
> >>> added to the size of the read operation, so if your first read operation
> >>> has a size of 1 GB... Well, then all of that will be correctly cached.
> >>> So both the size and the offset of the first read operation are significant.
> >>>
> >>>> * "Don't work" means the block layer returns garbled data.
> >>>
> >>> Right. It will be data from the image, but not from the offset you want.
> >>>
> >>>> * "Commonly" means when the first read is for offset zero.  Begs the
> >>>>   question when exactly that's the case.  You mentioned format probing.
> >>>>   What if the user specified a format?  It's okay not to answer this
> >>>>   question.  I'm not demanding exhaustive analysis, I'm fishing for a
> >>>>   better commit message.  Such a message may leave some of its questions
> >>>>   unanswered.
> >>>
> >>> Well, qcow2 will always start at offset zero anyway (because it reads
> >>> the header first). For raw images, the offset can be anywhere, but if
> >>> you're starting a VM from it, offset zero is obviously likely to be read
> >>> first, too.
> >>>
> >>> (And as a side note, the first read operation for qcow2 images will
> >>> always be 64 kB in size.)
> >>>
> >>> But, yes, for raw images the offset can be anywhere and if it is not
> >>> zero, the answer what works and what doesn't becomes a bit more complicated:
> >>>
> >>> <optional>
> >>> Suppose the first offset read from is 64k. curl will return data from
> >>> offset 0 anyway, so it's pretty much garbage. But if you then do another
> >>> read operation from 0, that will return correct data.
> >>>
> >>> If after that you try to read data from the area that has been covered
> >>> by both read operations... Then it depends on which buffer the curl
> >>> driver sees first, which is most likely the first one, i.e. you'll get
> >>> broken data again.
> >>> </optional>
> >>
> >> There's a lovely addition to your commit message struggling to get out
> >> of your reply.
> > 
> > I'm going to go ahead and apply the series; I think the relevant point
> > for the commit message is that TFTP is not usable and never has been.  If
> > Max has no objections, I'll pull some wording in from his reply here into
> > his commit message for patch 3, and squash all the patches into one.  
> > 
> > Max, any objections?
> 
> No, that sounds good to me. Thank you very much!
> 
> Max
> 




Thanks,

Applied to my block branch (squashed, and commit message amended):

git://github.com/codyprime/qemu-kvm-jtc.git block

-Jeff