Support variable server pools (#11256)

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.

cmd/erasure-metadata.go

@@ -334,7 +334,7 @@ func writeUniqueFileInfo(ctx context.Context, disks []StorageAPI, bucket, prefix
 // Returns per object readQuorum and writeQuorum
 // readQuorum is the min required disks to read data.
 // writeQuorum is the min required disks to write data.
-func objectQuorumFromMeta(ctx context.Context, er erasureObjects, partsMetaData []FileInfo, errs []error) (objectReadQuorum, objectWriteQuorum int, err error) {
+func objectQuorumFromMeta(ctx context.Context, partsMetaData []FileInfo, errs []error, defaultParityCount int) (objectReadQuorum, objectWriteQuorum int, err error) {
 	// get the latest updated Metadata and a count of all the latest updated FileInfo(s)
 	latestFileInfo, err := getLatestFileInfo(ctx, partsMetaData, errs)
 	if err != nil {
@@ -343,13 +343,13 @@ func objectQuorumFromMeta(ctx context.Context, er erasureObjects, partsMetaData
 
 	dataBlocks := latestFileInfo.Erasure.DataBlocks
 	parityBlocks := globalStorageClass.GetParityForSC(latestFileInfo.Metadata[xhttp.AmzStorageClass])
-	if parityBlocks == 0 {
-		parityBlocks = dataBlocks
+	if parityBlocks <= 0 {
+		parityBlocks = defaultParityCount
 	}
 
 	writeQuorum := dataBlocks
 	if dataBlocks == parityBlocks {
-		writeQuorum = dataBlocks + 1
+		writeQuorum++
 	}
 
 	// Since all the valid erasure code meta updated at the same time are equivalent, pass dataBlocks