Re: how btrfs uses devid?

[Hugo Mills <hugo@carfax.org.uk>]

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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/  |
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