59 Backup enhancements

Status: unimplemented proposals. The existing delta-file and hot-standby facilities are described in Backup and hot standby for the LSS. This chapter records possible extensions to those facilities.

Apply a delta while it is being written

The LSS could open each delta file for exclusive write access while allowing shared read access. LssApplyDeltas could then apply complete transactions to a standby before the delta file is closed, reducing replication latency.

Coalescing delta files

A utility could merge a sequence of delta files into one composite delta covering a range of checkpoint sequence numbers. It would verify that the output checkpoint identifier of each input matches the input checkpoint identifier of the next. The composite would inherit its first input identifier and its final output identifier, and could itself participate in a later merge.

A composite delta file could be named:

    nnnnnn-mmmmmm.lssdelta

where nnnnnn and mmmmmm are the first and last checkpoint sequence numbers. LssApplyDeltas could select an efficient combination of individual and composite deltas from the same directory.

Merging could omit intermediate versions of a serial element and deletions that supersede earlier writes. The result could be smaller than its inputs, but would no longer provide every intermediate snapshot. Repeated coalescing could retain fine-grained snapshots for the recent past and progressively coarser snapshots for older periods.