Prior to making a new snapshot, grab the (stored) transid of the
previous snapshot, and check if any files have been modified in the
source since that transid: btrfs sub find "${source}"
"${previous_transid}". If nothing is returned, then you don't have to
bother making the snapshot at all, otherwise after making the
snapshot, grab the transid via btrfs sub find "${new_snapshot}" -1,
and store it some place (even a dot file in the root of the snapshot
would work).
there might be small window of time where transid and snapshot could
be out of sync as we know them. since there is no atomic command which
provides both - snapshot and transid. As in the example below.
Assume tgw is the transaction group write which happens after we have
read the transaction group id.
---
sync; read current tran-id and compare
(new tgw occurs)
snapshot
new tgw occurs
sync; read current tran-id again and store
---
which will result in failing to take snapshot even if there are changes.
Certainly there will be some trade off, and below logic seems to be
more safer...
---
sync; read current tran-id and compare with previous
new tgw occurs
snapshot
new tgw occurs
store tran_id+2 (since tran_id gets added by two for a snapshot)
---
which might have a situation where we have two identical snapshot.
but a safer trade off.
thanks, Anand
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