[LU-19527] FLR-EC: Never select EC mirror as primary for write or allow setting PREFER flags

• Type: Technical task
• Resolution: Fixed
• Priority: Medium
• Fix Version/s: Lustre 2.18.0
• Labels: FLR-EC FLR-EC-RO
• Severity: 3
• Assignee: Patrick Farrell
• Reporter: Patrick Farrell

Description

A parity mirror should never be selected by the as primary for writes by the metadata server, since it is not a valid destination for normal data writes.  For a mirror which is partially parity and partially data mirrors, this still applies, because it is paired with a data only mirror and that's where the writes should be sent.

We should also not allow setting read or write prefer flags on a parity containing component, since it's not a valid source by itself.  We CAN allow setting the read prefer flag on a data component in a parity containing mirror.

Issue Links

• is related to: LU-19481 FLR-EC: Basic do-no-harm lov IO support
• is related to: LU-19525 FLR-EC: Prevent stranding of parity mirror
• is related to: LU-19545 FLR-EC: Tight binding between erasure code and parity mirrors