In cases where a cluster is degraded, we do not uphold our consistency
guarantee and we will write fewer erasure codes and rely on healing
to recreate the missing shards.
In some cases replacing known bad disks in practice take days.
We want to change the behavior of a known degraded system to keep
the erasure code promise of the storage class for each object.
This will create the objects with the same confidence as a fully
functional cluster. The tradeoff will be that objects created
during a partial outage will take up slightly more space.
This means that when the storage class is EC:4, there should
always be written 4 parity shards, even if some disks are unavailable.
When an object is created on a set, the disks are immediately
checked. If any disks are unavailable additional parity shards
will be made for each offline disk, up to 50% of the number of disks.
We add an internal metadata field with the actual and intended
erasure code level, this can optionally be picked up later by
the scanner if we decide that data like this should be re-sharded.
Current implementation requires server pools to have
same erasure stripe sizes, to facilitate same SLA
and expectations.
This PR allows server pools to be variadic, i.e they
do not have to be same erasure stripe sizes - instead
they should have SLA for parity ratio.
If the parity ratio cannot be guaranteed by the new
server pool, the deployment is rejected i.e server
pool expansion is not allowed.
Harshavardhana
Klaus Post
WangYuMu