Am 12.02.2015 um 18:18 schrieb Charles Arnold <carn...@suse.com>: >>>> On 2/12/2015 at 03:23 AM, Kevin Wolf <kw...@redhat.com> wrote: >> Am 12.02.2015 um 11:09 hat Peter Lieven geschrieben: >>>> Am 12.02.2015 um 11:06 schrieb Kevin Wolf: >>>> Am 12.02.2015 um 11:02 hat Peter Lieven geschrieben: >>>>>> Am 12.02.2015 um 10:58 schrieb Kevin Wolf: >>>>>> Am 12.02.2015 um 10:23 hat Peter Lieven geschrieben: >>>>>>>> Am 10.02.2015 um 15:53 schrieb Kevin Wolf: >>>>>>>> Am 10.02.2015 um 15:00 hat Peter Lieven geschrieben: >>>>>>>>>> Am 10.02.2015 um 14:54 schrieb Kevin Wolf: >>>>>>>>>> Am 10.02.2015 um 14:42 hat Jeff Cody geschrieben: >>>>>>>>>>> On Tue, Feb 10, 2015 at 02:34:14PM +0100, Kevin Wolf wrote: >>>>>>>>>>>> Am 10.02.2015 um 12:41 hat Peter Lieven geschrieben: >>>>>>>>>>>>> Am 09.02.2015 um 17:09 schrieb Kevin Wolf: >>>>>>>>>>>>>> The CHS calculation as done per the VHD spec imposes a maximum >>>>>>>>>>>>>> image size of ~127 GB. Real VHD images exist that are larger than >>>>>>>>>>>>>> that. >>>>>>>>>>>>>> >>>>>>>>>>>>>> Apparently there are two separate non-standard ways to achieve >>>>>>>>>>>>>> this: You could use more heads than the spec does - this is the >>>>>>>>>>>>>> option that qemu-img create chooses. >>>>>>>>>>>>>> >>>>>>>>>>>>>> However, other images exist where the geometry is set to the >>>>>>>>>>>>>> maximum (65536/16/255), but the actual image size is larger. >>>>>>>>>>>>>> Until now, such images are truncated at 127 GB when opening them >>>>>>>>>>>>>> with qemu. >>>>>>>>>>>>>> >>>>>>>>>>>>>> This patch changes the vpc driver to ignore geometry in this case >>>>>>>>>>>>>> and only trust the size field in the header. >>>>>>>>>>>>>> >>>>>>>>>>>>>> Signed-off-by: Kevin Wolf <kw...@redhat.com> --- >>>>>>>>>>>>>> >>>>>>>>>>>>>> Peter, I'm replacing some of your code in the hope that the new >>>>>>>>>>>>>> approach is more generally valid. Of course, I haven't tested if >>>>>>>>>>>>>> your case with disk2vhd is still covered. Could you check this, >>>>>>>>>>>>>> please? >>>>>>>>>>>>> I checked this and found that disk2vhd always sets CHS to 65535ULL >>>>>>>>>>>>> * 16 * 255 independed of the real size. >>>>>>>>>>>>> >>>>>>>>>>>>> But, as the conversion to CHS may have an error its maybe the best >>>>>>>>>>>>> solution to ignore CHS completely and always derive total_sectors >>>>>>>>>>>>> from footer->size unconditionally. >>>>>>>>>>>>> I had a look at what virtualbox does and they only rely on >>>>>>>>>>>>> footer->size. If they alter the size or create an image the write >>>>>>>>>>>>> the new size into the footer and recalculate CHS by the formula >>>>>>>>>>>>> found in the appendix of the original spec. >>>>>>>>>>>>> >>>>>>>>>>>>> Check vhdCreateImage, vhdOpen in >>>>>>>>>>>>> http://www.virtualbox.org/svn/vbox/trunk/src/VBox/Storage/VHD.cpp >>>>>>>>>>>>> >>>>>>>>>>>>> The original spec also says that CHS values purpose is the use in >>>>>>>>>>>>> an ATA controller only. >>>>>>>>>>>> The problem with just using footer->size back then when I >>>>>>>>>>>> implemented this was that from the perspective of a VirtualPC guest >>>>>>>>>>>> run in qemu, the size of its hard disk would change, which you >>>>>>>>>>>> don't >>>>>>>>>>>> want either. Going from VPC to qemu would be ugly, but mostly >>>>>>>>>>>> harmless as the disk only grows. But if you use an image in qemu >>>>>>>>>>>> where the disk looks larger and then go back to VPC which respects >>>>>>>>>>>> geometry, your data may be truncated. >>>>>>>>>>> I believe the vpc "creator" field is different if the image was >>>>>>>>>>> created by Virtual PC, versus created by Hyper-V ("vpc" and "win", >>>>>>>>>>> respectively, I think). Perhaps we could use that to infer a guest >>>>>>>>>>> image came from VirtualPC, and thus not use footer->size in that >>>>>>>>>>> scenario? >>>>>>>>>> Right, I think we discussed that before. Do you remember the outcome >>>>>>>>>> of >>>>>>>>>> that discussion? I seem to remember that we had a conclusion, but >>>>>>>>>> apparently it was never actually implemented. >>>>>>>>>> >>>>>>>>>> Would your proposal be to special-case "vpc" to apply the geometry, >>>>>>>>>> and >>>>>>>>>> everything else (including "win", "d2v" and "qemu") would use the >>>>>>>>>> footer >>>>>>>>>> field? >>>>>>>>> That sounds reasonable. In any case we have to fix qemu-img create >>>>>>>>> to do not create out of spec geometry for images larger than 127G. >>>>>>>>> It should set the correct footer->size and then calculate the >>>>>>>>> geometry. >>>>>>>> Do I understand correctly that you just volunteered to fix up that >>>>>>>> whole >>>>>>>> thing? ;-) >>>>>>> I knew that this would happen ;-) >>>>>>> >>>>>>> Regarding the C/H/S calculation. I was just wondering if we should >>>>>>> not set this to maximum (=invalid?) for all newly created images. >>>>>>> That is what disk2vhd does. >>>>>> CHS is what Virtual PC relies on. So I guess if you did that, you >>>>>> would render images unusable by it. Are you sure that disk2vhd does this >>>>>> always? I would have thought that it only does it for large images. >>>>> At least 2.0.1 (latest available version) does this as well as the version >>>>> that I used when I added the hack for d2v creator. >>>>> >>>>> Virtual PC would not be able to use images we create with qemu-img create >>>>> if we use footer->size (which I suppose to reanme to footer->cur_size, >>>>> btw) >>>>> to calculate bs->total_sectors because we might write data to the end of >>>>> the image which gets truncated in CHS format. >>>> These kinds of problems are why I'd like to keep CHS and size always >>>> consistent when creating an image with qemu-img. >>> >>> Okay, then I would vote for your RFC patch + fixing qemu-img create >>> to not generate out of spec CHS values and just set maximum which >>> then would make vpc_open use footer->size. >> >> Really the RFC patch or what we discussed above ("vpc" creator = CHS, >> everything else = footer->size)? Once I know what we prefer, I'll send >> the real patch. >> >> As for heads > 16, that would essentially mean reverting 258d2edb. >> Should be easy to do, the harder part is probably the commit message >> explaining why it's helpful and safe. Note that the commit message of >> 258d2edb claims that it's not out of spec. I _think_ we can do the >> revert with a good explanation, but I'll leave that to you. >> >> (CCed Charles who wrote that commit) > > IIUC, the plan is to revert my old commit and use the footer->size field to > describe images greater than 127 GB. This change would break other tools > from Virtual PC, Xens vhd-util and maybe others from reading images greater > than 127 GB because the head field would be forced back to using 16 and > these tools won't know to check the footer->size field. Is there any > reason not to keep the original commit and still use the footer->size field?
do you have a Pointer to a spec that is newer than 2006? the one i have describes CHS calculation up to 65535 x 16 x 255 sectors. that is set as Maximum if total sectors is higher. I would do the same when writing a footer. in vhd_open I would derive total_sectors from C x H x S except for the case that it is exactly 65535 x 16 x 255. In this case I would take footer->size / 512. Virtualbox does it that way and at the comment from Stefan in the commit message for your Patch suggest that you observed a similar behaviour for HyperV. Peter > > A purist would argue that heads must be 16 for true ATA emulation but allowing > up to 255 doesn't seem to matter and the VHD spec does support up to 2 TB. > > - Charles >
