Thank you very much, sir. Could you confirm that devid and UUID_SUB will never be changed at any circumstances for a given device, unless it is reformatted by mkfs.btrfs?
2016-01-01 20:23 GMT+08:00 Hugo Mills <h...@carfax.org.uk>: > On Fri, Jan 01, 2016 at 08:16:28PM +0800, UGlee wrote: >> Dear all: >> >> If a btrfs device is missing, the command tool tells user the devid of >> the missing devices. >> >> I understand that each device (disk) in a btrfs volume has been >> assigned a uuid (UUID_SUB field in udevadm info output). If the device >> is missing, it's hard to tell user to input such uuid string in >> command line. So devid is for convenience. > >> In our product, we want to record all disk information of a volume in >> a file. If a disk is missing, not because it's broken, but because the >> user has so many disks and in some cases they may put back the wrong >> one. In this scenario, we can provide the disk information (such as >> serial number) to user and help them to check if they did something >> wrong. >> >> My question is: is the devid just an alias to sub uuid? for a given >> disk device, it is never changed during any btrfs operation, including >> add, remove, balance and replace? or it may be changed, and when? > > Actually, devid is the ID that the FS uses internally in the device > tree to identify them. It's not just a convenience -- it's the > "official" identifier for the device within the filesystem. > >> One more question just for curiosity. I checked the source code of >> btrfs-progs briefly. It seems that there is no data structure in >> superblock recording all sub-uuids or all devids for the volume, so >> how does btrfs figure out the missing devid? since they are not always >> sequential integers, for example, after one device is removed, the >> devid is simply removed and the devid of other device is not >> re-numbered. > > The devices that should be there (identified by devid) are listed > in the device tree. If one of those doesn't match up with a > currently-known device for that filesystem (as determined by btrfs dev > scan), then it's missing. > > Hugo. > > -- > Hugo Mills | I gave up smoking, drinking and sex once. It was the > hugo@... carfax.org.uk | scariest 20 minutes of my life. > http://carfax.org.uk/ | > PGP: E2AB1DE4 | -- 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
