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.

docs/distributed/README.md

@@ -14,9 +14,9 @@ Distributed MinIO provides protection against multiple node/drive failures and [
 
 A stand-alone MinIO server would go down if the server hosting the disks goes offline. In contrast, a distributed MinIO setup with _m_ servers and _n_ disks will have your data safe as long as _m/2_ servers or _m*n_/2 or more disks are online.
 
-For example, an 16-server distributed setup with 200 disks per node would continue serving files, even if up to 8 servers are offline in default configuration i.e around 1600 disks can down MinIO would continue service files. But, you'll need at least 9 servers online to create new objects.
+For example, an 16-server distributed setup with 200 disks per node would continue serving files, up to 4 servers can be offline in default configuration i.e around 800 disks down MinIO would continue to read and write objects.
 
-You can also use [storage classes](https://github.com/minio/minio/tree/master/docs/erasure/storage-class) to set custom parity distribution per object.
+Refer to sizing guide for more understanding on default values chosen depending on your erasure stripe size [here](https://github.com/minio/minio/blob/master/docs/distributed/SIZING.md). Parity settings can be changed using [storage classes](https://github.com/minio/minio/tree/master/docs/erasure/storage-class).
 
 ### Consistency Guarantees
 
@@ -38,14 +38,14 @@ __NOTE:__
 
 - All the nodes running distributed MinIO need to have same access key and secret key for the nodes to connect. To achieve this, it is __recommended__ to export access key and secret key as environment variables, `MINIO_ROOT_USER` and `MINIO_ROOT_PASSWORD`, on all the nodes before executing MinIO server command.
 - __MinIO creates erasure-coding sets of *4* to *16* drives per set.  The number of drives you provide in total must be a multiple of one of those numbers.__
-- __MinIO chooses the largest EC set size which divides into the total number of drives or total number of nodes given - making sure to keep the uniform distribution i.e each node participates equal number of drives per set.
+- __MinIO chooses the largest EC set size which divides into the total number of drives or total number of nodes given - making sure to keep the uniform distribution i.e each node participates equal number of drives per set__.
 - __Each object is written to a single EC set, and therefore is spread over no more than 16 drives.__
 - __All the nodes running distributed MinIO setup are recommended to be homogeneous, i.e. same operating system, same number of disks and same network interconnects.__
 - MinIO distributed mode requires __fresh directories__. If required, the drives can be shared with other applications. You can do this by using a sub-directory exclusive to MinIO. For example, if you have mounted your volume under `/export`, pass `/export/data` as arguments to MinIO server.
 - The IP addresses and drive paths below are for demonstration purposes only, you need to replace these with the actual IP addresses and drive paths/folders.
 - Servers running distributed MinIO instances should be less than 15 minutes apart. You can enable [NTP](http://www.ntp.org/) service as a best practice to ensure same times across servers.
 - `MINIO_DOMAIN` environment variable should be defined and exported for bucket DNS style support.
-- Running Distributed MinIO on __Windows__ operating system is experimental. Please proceed with caution.
+- Running Distributed MinIO on __Windows__ operating system is considered **experimental**. Please proceed with caution.
 
 Example 1: Start distributed MinIO instance on n nodes with m drives each mounted at `/export1` to `/exportm` (pictured below), by running this command on all the n nodes:
 
@@ -79,8 +79,7 @@ minio server http://host{1...4}/export{1...16} http://host{5...12}/export{1...16
 
 Now the server has expanded total storage by _(newly_added_servers\*m)_ more disks, taking the total count to _(existing_servers\*m)+(newly_added_servers\*m)_ disks. New object upload requests automatically start using the least used cluster. This expansion strategy works endlessly, so you can perpetually expand your clusters as needed.  When you restart, it is immediate and non-disruptive to the applications. Each group of servers in the command-line is called a pool. There are 2 server pools in this example. New objects are placed in server pools in proportion to the amount of free space in each pool. Within each pool, the location of the erasure-set of drives is determined based on a deterministic hashing algorithm.
 
-> __NOTE:__ __Each pool you add must have the same erasure coding set size as the original pool, so the same data redundancy SLA is maintained.__
-> For example, if your first pool was 8 drives, you could add further server pools of 16, 32 or 1024 drives each. All you have to make sure is deployment SLA is multiples of original data redundancy SLA i.e 8.
+> __NOTE:__ __Each pool you add must have the same erasure coding parity configuration as the original pool, so the same data redundancy SLA is maintained.__
 
 ## 3. Test your setup
 To test this setup, access the MinIO server via browser or [`mc`](https://docs.min.io/docs/minio-client-quickstart-guide).