[LU-10995] DoM2: allow MDT-only filesystems

• Status: Open
• Project: Lustre
• Type: Improvement
• Priority: Minor
• Reporter: Andreas Dilger
• Assignee: WC Triage
• Resolution: Unresolved
• Labels: DoM2

Description

We don't yet support all-MDT systems, but I'm sure it is only a matter of time until we do. I think the main limitation is that we don't allow clients to use the filesystem until the MDS has connected to at least one OST, to prevent clients just getting errors when trying to use the filesystem (before DoM).

We probably also need some testing to ensure we allow DoM file creates without OST components even if no OSTs are connected. We might also need to lift the 1GB dom_stripesize limit in that case.

Finally, we might need to handle MDT and OST on the same node better during recovery, but it might also just work since the MDT always uses the same connection UUID.

Issue Links

• is related to: LU-10176 Data-on-MDT phase II
• is related to: LU-11471 IO Errors during failover with very ...
• is related to: LU-11023 OST Pool Quotas
• is related to: LU-11421 DoM: manual migration OST-MDT, MDT-MDT
• is related to: LU-11425 Support quota for DoM

Comments

[Mikhail Pershin]

Copied from LU-11471 to have all related notes in one ticket:

With the advent of Data-on-MDT we will at some point want to allow filesystems with only MDTs to be created. At that point, this check has to be removed.

As a starting point, we could add a tunable that allows this behavior to be selected by the admin - return an error if no OSTs are available, or cause the client to block and wait for an OST to become available. I think in the case where an OST was previously available, but they are temporarily offline due to failover, the client should block. If the file being created has a DoM component at the start, then it should not block.