On 09.11.2016 20:15, Jeff Cody wrote: > On Tue, Nov 08, 2016 at 08:14:58AM +0100, Markus Armbruster wrote: >> Max Reitz <mre...@redhat.com> writes: >> >>> On 07.11.2016 09:20, Markus Armbruster wrote: >>>> Max Reitz <mre...@redhat.com> writes: >>>> >>>>> On 03.11.2016 08:56, Markus Armbruster wrote: >>>>>> Max Reitz <mre...@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
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