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 |
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