[LU-19525] FLR-EC: Prevent stranding of parity mirror

• Type: Technical task
• Resolution: Unresolved
• Priority: Medium
• Labels: FLR-EC FLR-EC-RO
• Severity: 3
• Assignee: Marc Vef
• Reporter: Patrick Farrell

Description

Erasure coding parity mirrors require a corresponding data mirror with matching extents. A parity mirror is a mirror that contains only parity components and protects the data components of another data mirror. Data and parity mirrors have a 1-1 relationship. When data mirrors are deleted or split off, the parity mirror should not be left behind without the matching data mirror. This is what we call "stranding". To avoid this: if a data component is selected for splitting (to a separate file) or for deletion via lfs mirror split, the corresponding parity mirror is selected as well. This should be the default behavior.

Nevertheless, there may be cases where this "stranding" is useful. This means lfs mirror split should include a flag that prevents splitting the parity mirror when its data mirror is selected. If a parity mirror is selected, only the parity mirror is split off, and the data mirror remains. Note, for lfs mirror split (without -d) this means that a new file is created containing only the parity mirror.

All lfs mirror command changes are documented in LU-19548.

Issue Links

• is related to: LU-19527 FLR-EC: Never select EC mirror as primary for write or allow setting PREFER flags
• is related to: LU-19548 FLR-EC: support for other lfs mirror commands
• is related to: LU-19545 FLR-EC: Tight binding between erasure code and parity mirrors