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Document size limits in kv

Couchbase is based upon memcached which have a maximum document size of 1MB, but when we created the Coucbase style buckets we set this limit to 20MB which seemed very high at that time.

The maximum document size is specified on a "per bucket" basis, so you can have different limits for different buckets. This is useful if you have some buckets that store small documents and others that store large documents. It is passed as a parameter when creating the bucket via the max_item_size parameter (default is 20MB, and ns_server does not provide this parameter as of today).

We do however have some guardrails in place to protect the system:

  1. The server will disconnect a client trying to send a MCBP frame larger than a configured maximum size. This is set to 30MB by default, but can be changed via themax_packet_size configuration parameter in memcached.json.

  2. When we receive a frame on the wire which claims to be compressed we try to inflate the object to verify that is in fact compressed. If the inflated object is larger than a maximum size, we will reject the frame. This is currently hard coded to 30MB (see MB-67957)

There is also the dcp_backfill_byte_limit configuration parameter which is used to limit the size of the backfill that DCP will send to a client. I don't think one needs to increase this limit; it just means that things will run slower (It'll hit the max limit after inserting 1 item which exceeds the threshold).