[dm-devel] dm-thin metadata: can two device_id's have the same transaction_id ?

[Zdenek Kabelac]

Dne 12.3.2016 v 00:14 Eric Wheeler napsal(a):
>
> Hello all,
>
> For thin volumes shown in the lvm VG metadata backups in
> /etc/lvm/backup/pool, can any two device_id's have the same
> transaction_id, ever?
>
> Otherwise, can I assume that the device_id and transaction_id are both
> unique, or is only the tuple (transaction_id,device_id) unique?
>
> For example:
>
> my-thin-vol {
>          id = "M8fkBP-pFe8-Z6K0-0f7N-HAFM-Mv2v-KOTmM3"
>          status = ["READ", "WRITE", "VISIBLE"]
>          flags = ["ACTIVATION_SKIP"]
>          creation_host = "hv1.ewheeler.net"
>          creation_time = 1452374892      # 2016-01-09 13:28:12 -0800
>          segment_count = 1
>
>          segment1 {
>                  start_extent = 0
>                  extent_count = 3072     # 48 Gigabytes
>
>                  type = "thin"
>                  thin_pool = "data-pool"
>>>>>>>>>>>>>     transaction_id = 9843
>                  device_id = 5735
>                  origin = "backup2.ewheeler.net_14-drbd"
>          }
> }
>

Hi

It's rather for linux-lvm list.

Transaction_id is 24bit number - and like may be possible to wrap
although it has not been much tested in such corner case.
(I should probably add some tests around this for lvm2...)

Device_id lvm2 does not recycle ATM - so it just always seeks for
largest and picks +1.  But it doesn't mean we could not introduce
recyling at some point.

However at this moment we rather try to use unique numbers to simplify 
possible recovery steps - it's fairly simpler.

Regards

Zdenek