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.
