Mount exfat filesystems with the sync option by default

[kotarou3]

Removable drives are frequently formatted with the vfat and exfat filesystems

Currently vfat filesystems are mounted with the flush option by default: https://github.com/storaged-project/udisks/blob/060c9492d616f5b5287e834b50297e414c52f166/data/builtin_mount_options.conf#L4 This results in the write buffer being flushed frequently, so writes of large files end up being throttled to the disk's write speed (rather than RAM), resulting in accurate write speed indications in userspace and a speedy unmount

Flushing the write buffer frequently is especially important for removable drives with slow write speeds because:

• The user may "yank" the drive without unmounting, resulting in data loss if the write buffer isn't empty
• The user will have an indication of when the write will actually complete, and is likely to refrain from "yanking" until then
• Even if the drive is unmounted, the user might not wait for an unknown-length unmount (potentially ranging from minutes to an hour for sufficiently large buffers and sufficiently slow write speeds) to complete before "yanking"

Unfortunately, exfat doesn't support the flush option at the moment, so the closest would be sync, which flushes every write. So please consider updating the default mount options on exfat to use the sync mount option

[tbzatek]

I'm not totally convinced to use sync by default, it's very expensive. Of couse the current status is not good either.

As @vojtechtrefny pointed out, there might be a way to limit the dirty buffer for a particular device - see /sys/class/bdi/<bdi>/max_bytes. However, this feature has only been added in kernel 6.2 (the exfat filesystem driver was added in 5.7): https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=00df7d51263b46ed93f7572e2d09579746f7b1eb https://lore.kernel.org/all/20221119005215.3052436-6-shr@devkernel.io/