Re: dm-thin metadata: can two device_id's have the same transaction_id ?

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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

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