About Index Slices
The Master server is always created with one slice named default. Adding a slice is necessary when the size of the disk limits the number of documents that can be indexed or when its performance affects the query response time. A slice must be added on separate disks on the Maser server.
Note: When all slices on your Master server are getting full, consider federating search on two or more Coveo instances (see About Geographically Distributed Indexing).
Index slices facts
A slice can typically contain up to 40 million documents. One Coveo server can typically host up to two slices and contain up to 80 million documents. These numbers can however vary depending on a number of factors.
Each slice should be created on a separate dedicated disk set.
Slice files can also be hosted on a storage area network (SAN).
Slices are filled in a distributed fashion so they grow with the same number of documents.
Note: When you add one slice, because the default slice is getting close to the limit, new documents will be added only to the new slice until it reaches the number of documents contained in the default slice. The documents are then distributed evenly in the two slices.