So I've been considering some NOCOW files (for VM disk images), but some questions arose. IS there a "1COW" (copy on write only once) flag or are the following operations dangerous or undefined? (1) The page https://btrfs.wiki.kernel.org/index.php/FAQ (section "Can copy-on-write be turned off for data blocks?") says "COW may still happen if a snapshot is taken." Is that a "may" or a "will", e.g. if I take a snapshot and then start the VM will the file in the snapshot still be frozen or will it update as I alter the VM? Does the read-only-or-not status of the snapshot matter in this outcome? e.g. what does "may" mean in that section? (2) If you copy a file using "cp --reflink" and the destination is in a directory marked NOCOW, what happens? How about when the resultant file is modified in place? (3) when using a watever.qcow2 virtual machine image that does copy-on-write in the VM (such as QEMU) is it better, worse, or a no-op to have the NOCOW flag set on the file? All the advice on this matter I can find in Google seems to be "VM images bad, but will be addressed soon" and its old enough that I don't know if "soon" has come to pass. It seems like there is a 1COW flag implicit somewhere. Just curious. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html