Kevin Wolf <[email protected]> writes: > Am 05.11.2025 um 08:08 hat Markus Armbruster geschrieben: >> Kevin Wolf <[email protected]> writes: >> >> [...] >> >> > To me it looks a bit like what we really want is an enum for floppy >> > sizes (though is there any real reason why we have only those two?), but >> > an arbitrary size for hard disks. >> > >> > Without the enum, obviously, users could specify 1440k and that would do >> > the right thing. Maybe special casing whatever 1.44M and 2.88M result >> > in and translating them into 1440k and 2880k could be more justifiable >> > than special casing 1M and 2M, but it would still be ugly. >> > >> > Markus, do you have any advice how this should be represented in QAPI? >> >> Still want answers here? > > Yes, I'm still not sure how we could best represent both hard disk and > floppy sizes in vvfat in a way that isn't completely counterintuitive > for users, that also isn't just arbitrary magic and that works on the > command line. > > Unless the need for different sizes has gone away, but I don't think we > found any other solution for the problem that would not require a > configurable disk/file system size?
Let me recap the problem. Please correct my misunderstandings, if any. Hard disks can have almost arbitrary sizes. Almost, because it still needs to be a multiple of the block size. Floppy disks have one of a small set of well-known sizes. I vaguely recall that we generally derive the device's actual size from the backend's size. Some devices reject certain sizes. For instance, SD cards require a power of 2. Most devices seem to accept anything. I can create an IDE, SCSI, or floppy disk backed by a raw image of one byte. I have no idea how it would behave. As is, the vvfat backend can only do certain sizes, configurable with parameters @floppy and @fat-type. They work for floppies, but not for SD cards, since they're not powers of two. Instead of deriving size and CHS from @floppy and @fat-type, Clément proposes to specify the size (and derive fat-type and CHS[*]?). In QMP, we specify the size in bytes. This is fine regardless of size; management applications don't mind sending things like "size": 1474560. In HMP and the command line, big byte sizes are inconvenient. That's why we support suffixes there. size=256M is a fine way to pick an SD card's size. The size suffixes seem inconvenient for floppies, though. For instance, 2 heads * 80 tracks * 18 sectors * 512 bytes = 1474560 bytes = 1440 KiBytes, but size=1.44M does not work: 1.44 MiBytes = 1509949.44 Bytes. However, size=1440K does. This leads me to suggest to simply stick to numeric size, and use appropriate suffixes. These are obvious enough for anything but floppies. So advise users "use K for floppies"[**]. If this isn't good enough, I can help you explore fancier parts of QAPI, such as alternate types. [*] I guess we could support specifying an optional fat-type in addition to size, and derive only CHS then. [**] Even for a hypothetical floppy with an odd number of 512 byte sectors: .5K works, because .5 * 1024 is an integer.
