[stable/redis-ha] feature: adding option to use existing persistent volume

[bih-goncalves]

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What this PR does / why we need it:

Enables a user to mount a pre-existing persistent volume claim as volume to redis-ha pods to store all redis cluster data. This allows users to store the redis cluster data on any storage backend that they desire to use. We are using a pre-existing Gluster storage class.

Special notes for your reviewer:

Checklist

• [ ] DCO signed
• [X] Chart Version bumped
• [X] Variables are documented in the README.md
• [X] Title of the PR starts with chart name (e.g. [stable/mychartname])

co-author: @robbie558

[DandyDeveloper]

@bih-goncalves I wonder if this is the best implementation? This assumes the PVC in question is a RWX supported volume.

I'll have a think before merging this and offer some suggestions.

[Robbie558]

Thanks for considering @DandyDeveloper. Given that the PVC could be written to by multiple pods, would it not need to be RWX?

[DandyDeveloper]

@Robbie558 I think this is my concern here. Why do we have 3 instances writing to a single PVC? Are they not all writing to the same db file / fs?

Does this ruin the point of HA as corruption will potentially damage the cluster irreversibly?

[DandyDeveloper]

Closing as I haven't heard back from the original contributor.