On Mon, Nov 10, 2025 at 5:31 PM Kevin Wolf <[email protected]> wrote: > > Am 10.11.2025 um 16:36 hat BALATON Zoltan geschrieben: > > On Mon, 10 Nov 2025, Kevin Wolf wrote: > > > Am 10.11.2025 um 14:42 hat Markus Armbruster geschrieben: > > > > Clément Chigot <[email protected]> writes: > > > > > > > > > On Mon, Nov 10, 2025 at 2:09 PM Markus Armbruster <[email protected]> > > > > > wrote: > > > > > > > > > > > > Clément Chigot <[email protected]> writes: > > > > > > > > > > > > > On Mon, Nov 10, 2025 at 11:13 AM Markus Armbruster > > > > > > > <[email protected]> wrote: > > > > > > > > > > > > > > > > Clément Chigot <[email protected]> writes: > > > > > > > > > > > > > > > > > This allows more flexibility to vvfat backend. The values of > > > > > > > > > "Number of > > > > > > > > > Heads" and "Sectors per track" are based on SD specifications > > > > > > > > > Part 2. > > > > > > > > > > > > > > > > > > Due to the FAT architecture, not all sizes are reachable. > > > > > > > > > Therefore, it > > > > > > > > > could be round up to the closest available size. > > > > > > > > > > > > > > > > > > FAT32 has not been adjusted and thus still default to 504 Mib. > > > > > > > > > > > > > > > > > > For floppy, only 1440 Kib and 2880 Kib are supported. > > > > > > > > > > > > > > > > > > Signed-off-by: Clément Chigot <[email protected]> > > > > > > > > > > > > > > > > [...] > > > > > > > > > > > > > > > > > diff --git a/qapi/block-core.json b/qapi/block-core.json > > > > > > > > > index 8a479ba090..0bcb360320 100644 > > > > > > > > > --- a/qapi/block-core.json > > > > > > > > > +++ b/qapi/block-core.json > > > > > > > > > @@ -3478,11 +3478,17 @@ > > > > > > > > > # (default: true) > > > > > > > > > # (since 10.2) > > > > > > > > > # > > > > > > > > > +# @fat-size: size of the device in bytes. Due to FAT > > > > > > > > > underlying > > > > > > > > > +# architecture, this size can be rounded up to the > > > > > > > > > closest valid > > > > > > > > > +# size. > > > > > > > > > +# (since 10.2) > > > > > > > > > +# > > > > > > > > > > > > > > > > Can you explain again why you moved from @size to @fat-size? > > > > > > > > > > > > > > Just to be sure, you mean in the above comment, in the commit > > > > > > > message or both ? > > > > > > > > > > > > Just to me, because I'm not sure I like the change, but that may > > > > > > well be > > > > > > due to a lack of understanding of your reasons. > > > > > > > > > > Naming `fat-size` instead of `size` ensures the parameter is only > > > > > recognized by the vvfat backend. In particular, it will be refused by > > > > > the default raw format, avoiding confusion: > > > > > "-drive file=fat:<path>,size=256M" results in a 504M FAT disk > > > > > truncated to 256M, raw format being implicit. > > > > > "-drive file=fat:<path>,fat-size=256M" is refused. "fat-size" is > > > > > unsupported by raw format. > > > > > > > > I figure throwing in format=raw to make raw format explicit doesn't > > > > change anything. Correct? > > > > > > > > > "-drive file=fat:<path>,format=vvfat,fat-size=256M" results in a > > > > > 256M FAT disk. > > > > > "-drive file=fat:<path>,format=vvfat,size=256M" is refused. "size" is > > > > > unsupported by vvfat format. > > > > > > > > If it was called @size, what behavior would we get? Just two cases, I > > > > think: > > > > > > > > 1. With raw format: > > > > > > > > -drive file=fat:<path>,size=256M > > > > > > You'd silently get a 504 MiB filesystem truncated to 256 MiB (i.e. a > > > corrupted file system). It's quite easy to forget format=vvfat, so > > > something that initially looks like it might be working is not a great > > > result for this user error. > > > > Why doesn't file=fat: imply format=vvfat? For what is the fat: part in > > file then? > > -drive is built pretty much on the assumption that you have an image > format that runs on top of a protocol. Format probing probes the image > format, not the protocol, while prefixes like fat: (or nbd:, http: etc.) > specify the protocol. > > So if you don't specify the format, we first open the protocol level > (which is vvfat) and then probing will detect that over this protocol, > we access a raw image. So it's mostly like saying format=raw. > > I think that format=<protocol driver> works is really more accidental, > but we can't change it now (and probably also don't want to). It results > in opening only the protocol layer and not stacking any format driver on > top of it. > > Options that you specify in -drive generally go to the top layer. So the > consequence in our case is that with format=vvfat, the option goes to > vvfat, but with format=raw (or unspecified format), it goes to the raw > forma driver. > > > I currently recommend using: > > > > -drive if=none,id=ufat,format=raw,file=fat:rw:/dir/to/export > > -device usb-storage,drive=ufat > > > > to my users which I got from somewhere but don't remember where and it > > seems to work but maybe not the best way to specify this. > > It's fine, and I might use the same one myself (though you should be > aware that fat:rw: is risky, it's full of bugs). > > But if you add an option like size=64M, it goes to the raw driver, which > will take whatever image you access on the protocol level and truncate > it at 64 MiB. > > If you want to give the size option on the vvfat level (and create a > filesystem that is actually only 64 MiB instead of truncating a larger > one), then obviously format=vvfat allows you to do that because then > there is no raw format layer to begin with. Or if you do have the raw > format layer, you can access options of the protocol layer by prefixing > "file.". So format=raw,file.size=64M would still pass the size option to > vvfat.
How is `file.size` working ? I've tried a similar syntax for other vvfat options (e.g `file.floppy` or the new `file.partitioned`) but those have no effect. Is this because there are fetched within the "filename" Wondering because I'mn ot a fan of the new ":unpartitioned:", I've added in patch 1. If it can simply be replaced by `format=raw,file.partitioned=false` or `format=vvfat,partitioned=false`. I think that would be far enough for its purpose. > So the command line does allow you to get the option to the right place, > it's just very easy to get confused about this and to specify the option > for the wrong layer. > > > After reading this thread I'm confused about how to use this > > correctly. Is there some documentation on this? There only seems to be > > some mentions in docs/system/qemu-block-drivers.rst.inc but all of > > them using older options: > > > > |qemu_system| linux.img -hdb fat:/my_directory > > |qemu_system| linux.img -fda fat:floppy:/my_directory > > |qemu_system| linux.img -fda fat:floppy:rw:/my_directory > > All of those are honestly fine, too, if you're happy with the defaults > and don't want to set more advanced options. > > I'll give you this bonus option if you want to be modern: > > -blockdev vvfat,node-name=ufat,dir=/my_directory,rw=on > -device usb-storage,drive=ufat > > But I don't think any of the other options is going away anytime soon. > > Kevin >
