What is Buffer size used for; when should I change it?

Short answer

Buffer Size(kB) in the vService Limits profile is the size of the buffer that the vService uses per connection while processing requests and responses; the default is 128 kB, the range is 16–262,144 kB, and the field is required. It is increased when very large headers or large content pages are legitimately needed; as the value grows, memory usage per connection also increases. On a vService with WAF on, the buffer is the larger of Buffer size(kB) and the WAF's Max. Body Size to Process(kB) value. The change restarts the vService.

When Is It Changed?

Situation	Direction
The Backend Service legitimately sends very large response headers and the client receives 502	Increase
A large content page (maintenance, blocking page) cannot be displayed; the page must be about 2 kB smaller than the Buffer size(kB) value	Increase
There is memory pressure at a very high number of concurrent connections	Keep at the default (buffer and resource management)

Buffer Size in TR7

• Setting: Load Balancer → Profiles → vService Limits → right-click on the profile row → Edit (or Add) → Buffer Size(kB) → Save; the profile is selected in the Details → vService Limits box of the vService (how to add it to a vService). vServices with no profile assigned also run with 128 kB.
• Effect of the change: A Buffer size(kB) change restarts the vService and open connections are dropped; it is done in a maintenance window (When does a change I make take effect; does it go live as soon as I save it?). In a cluster, it is also sent to the peer device with the default auto-sync; if Manual Sync Mode is on, it is sent with Synchronize.
• vService type: HTTP and L7 TCP.
• WAF: If WAF is enabled on the vService, the effective buffer is the larger of Buffer size(kB) and the WAF's Max. Body Size to Process(kB) value (Max. Body Size to Process).
• Content pages: The content pages that TR7 displays must also fit in this buffer; a page that exceeds the limit prevents the vService configuration from being loaded (What is the Content Pages profile; how do I prepare my own HTML page?).
• WEB-CLI: lb limits update id <id> --maxBufferSize 256 (limits examples).

Common Misconceptions

• "Buffer size limits the size of files that can be uploaded." Request and response bodies pass through the buffer by streaming; large files are not limited by this value.
• "Increasing it is always good." Memory per connection increases, and since saving restarts the vService, a short interruption occurs.

How it is done in the documentation

• vService Limits · Buffer Size(kB)
• vService Limits · The Buffer and Resource Management
• vService Limits · Profile Add Form

Related questions

• What is the vService Limits profile; what are the default limits?
• What is the Content Pages profile; how do I prepare my own HTML page?
• When does a change I make take effect; does it go live as soon as I save it?