On Fri, Jun 12, 2015 at 09:16:30PM +0800, Anand Jain wrote: > BTRFS_IOC_DEVICES_READY is to check if all the required devices > are known by the btrfs kernel, so that admin/system-application > could mount the FS. It is checked against a device in the argument. > > However the actual implementation is bit more than just that, > in the way that it would also scan and register the device > provided in the argument (same as btrfs device scan subcommand > or BTRFS_IOC_SCAN_DEV ioctl). > > So BTRFS_IOC_DEVICES_READY ioctl isn't a read/view only ioctl, > but its a write command as well.
The implemented DEVICES_READY behaviour is intentional, but not a good example of ioctl interface design. I asked for a more generic interface to querying devices when this patch was submitted but to no outcome. > Next, since in the kernel we only check if total_devices > (read from SB) is equal to num_devices (counted in the list) > to state the status as 0 (ready) or 1 (not ready). But this > does not work in rest of the device pool state like missing, > seeding, replacing since total_devices is actually not equal > to num_devices in these state but device pool is ready for > the mount and its a bug which is not part of this discussions. That's an example why the single-shot ioctl is bad - it relies on some internal state that's otherwise nontrivial to get. > Questions: > > - Do we want BTRFS_IOC_DEVICES_READY ioctl to also scan and > register the device provided (same as btrfs device scan > command or the BTRFS_IOC_SCAN_DEV ioctl) > OR can BTRFS_IOC_DEVICES_READY be read-only ioctl interface > to check the state of the device pool. ? This has been mentioned in the thread, we cannot change the ioctl that way. Extensions are possible as far as they stay backward compatible without changes to the existing users. > - If the the device in the argument is already mounted, > can it straightaway return 0 (ready) ? (as of now it would > again independently read the SB determine total_devices > and check against num_devices. We can do that, looks like a safe optimization. > - What should be the expected return when the FS is mounted > and there is a missing device. I think the current ioctl cannot give a good answer to that, similar to the seeding or dev-replace case. We'd need an improved ioctl or do it via sysfs which is my preference at the moment. _______________________________________________ systemd-devel mailing list systemd-devel@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/systemd-devel
