On Sat, Jan 27, 2018 at 14:12:01 -0700, Chris Murphy wrote: > doesn't count devices itself. The Btrfs systemd udev rule defers to > Btrfs kernel code by using BTRFS_IOC_DEVICES_READY. And it's totally > binary. Either they are all ready, in which case it exits 0, and if > they aren't all ready it exits 1. > > But yes, mounting whether degraded or not is sufficiently complicated > that you just have to try it. I don't get the point of wanting to know > whether it's possible without trying. Why would this information be
If you want to blind-try it, just tell the btrfs.ko to flip the IOCTL bit. No shortcuts please, do it legit, where it belongs. >> Ie, the thing systemd can safely do, is to stop trying to rule everything, >> and refrain from telling the user whether he can mount something or not. > > Right. Open question is whether the timer and timeout can be > implemented in the systemd world and I don't see why not, I certainly It can. The reasons why it's not already there follow: 1. noone created udev rules and systemd units for btrfs-progs yet (that is trivial), 2. btrfs is not degraded-safe yet (the rules would have to check if the filesystem won't stuck in read-only mode for example, this is NOT trivial), 3. there is not way to tell the kernel that we want degraded (probably some new IOCTL) - this is the path that timer would use to trigger udev event releasing systemd mount. Let me repeat this, so this would be clear: this is NOT going to work as some systemd-shortcut being "mount -o degraded", this must go through the kernel IOCTL -> udev -> systemd path, i.e.: timer expires -> executes IOCTL with "OK, give me degraded /dev/blah" -> BTRFS_IOC_DEVICES_READY returns "READY" (or new value "DEGRADED") -> udev catches event and changes SYSTEMD_READY -> systemd mounts the volume. This is really simple. All you need to do is to pass "degraded" to the btrfs.ko, so the BTRFS_IOC_DEVICES_READY would return "go ahead". -- Tomasz Pala <go...@pld-linux.org> -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
