Impact of Backup Cursor for snapshot backups


Reading the notes on the technical-preview for snapshot backups for PBM:

What impact does opening the backup cursor have on the database? Can the database be accessed as normal while this cursor is open or is it expected that this blocks access?
“opens the $backupCursor

Opening backup cursor does not block db access.

Ok thank you, so the impact is that it stops changes being made to the files in each node, but the database remains online? You then copy out (with your own tools or method) and then finish which allows for changes to the files again?

I am just trying to understand how it keeps consistency.

PBM does physical/incremental backups using PSMDB $backupCursor pipeline (underly it is wiredtiger backup cursor).
When the $backupCursor is opened, the mongod still continues serving client requests (insert/update/delete operations). The backup will contain a data snapshot of a replset at the backup cursor open time + oplog with changes made during backup.

In other words, during backup, MongoDB works as usual: queries are “answered”, new documents are inserted, updates to existing documents are saved, and deleted documents are no more visible in collections. No performance impact.

Ok thanks for the clarification.