The RADOS Gateway (RGW) presents the Ceph native object store via a S3 or swift-compatible interface, which are the two most popular object APIs for accessing object storage, with S3 being the dominant one, mainly due to the success of Amazon's AWS S3. This section of the book will primarily focus on S3.
The radosgw component of Ceph is responsible for turning S3 and swift API requests into RADOS requests. Although it can be installed alongside other components, for performance reasons, it's recommended to be installed on a separate server. The radosgw components are completely stateless and so lend themselves well to being placed behind a load balancer to allow for horizontal scaling.
Aside from storing user data, the RGW also requires a number of additional RADOS pools to store additional metadata. With the exception of the index pool, most of these pools are very lightly utilized and so can be created with a small amount of PGs, around 64 is normally sufficient. The index pools helps with the listing of bucket contents and so placing the index pool on SSDs is highly recommended. The data pool can reside on either spinning disks or SSDs, depending on the type of objects being stored, although object storage tends to be a fairly good match for spinning disks. Quite often, clients are remote and the latency of WAN connections offsets a lot of the gains to be had from SSDs. It should be noted that only the data pool should be placed on erasure-coded pools.
Handily, RGW will create the required pools the first time it tries to access them, reducing the complexity of installation somewhat. However, pools are created with their default settings, and it may be that you wish to create an erasure-coded pool for data-object storage. As long as no access has been made to the RGW service, the data pool should not exist after creation, and it can therefore be manually created as an erasure pool. As long as the name matches the intended pool name for the RGW zone, RGW will use this pool on first access, instead of trying to create a new one.