Under certain scenarios it is possible for logs or data to be ingested or created faster than the ability to flush it to some destinations. One such common scenario is when reading from big log files, especially with a large backlog, and dispatching the logs to a backend over the network, which takes time to respond. This generates backpressure leading to high memory consumption in the service.
In order to avoid backpressure, Fluent Bit implements a mechanism in the engine that restricts the amount of data that an input plugin can ingest, this is done through the configuration parameters Mem_Buf_Limit and storage.Max_Chunks_Up.
As described in the Buffering concepts section, Fluent Bit offers two modes for data handling: in-memory only (default) and in-memory + filesystem (optional).
The default storage.type memory buffer can be restricted with Mem_Buf_Limit. If memory reaches this limit and you reach a backpressure scenario, you will not be able to ingest more data until the data chunks that are in memory can be flushed. The input will be paused and Fluent Bit will emit a [warn] [input] {input name or alias} paused (mem buf overlimit) log message. Depending on the input plugin in use, this might lead to discard incoming data (e.g: TCP input plugin). The tail plugin can handle pause without data loss; it will store its current file offset and resume reading later. When buffer memory is available, the input will resume collecting/accepting logs and Fluent Bit will emit a [info] [input] {input name or alias} resume (mem buf overlimit) message.
This risk of data loss can be mitigated by configuring secondary storage on the filesystem using the storage.type of filesystem (as described in Buffering & Storage). Initially, logs will be buffered to both memory and filesystem. When the storage.max_chunks_up limit is reached, all the new data will be stored safely only in the filesystem. Fluent Bit will stop enqueueing new data in memory and will only buffer to the filesystem. Please note that when storage.type filesystem is set, the Mem_Buf_Limit setting no longer has any effect, instead, the [SERVICE] level storage.max_chunks_up setting controls the size of the memory buffer.
This option is disabled by default and can be applied to all input plugins. Please note that Mem_Buf_Limit only applies with the default storage.type memory. Let's explain its behavior using the following scenario:
Mem_Buf_Limit is set to 1MB (one megabyte)
input plugin tries to append 700KB
engine route the data to an output plugin
output plugin backend (HTTP Server) is down
engine scheduler will retry the flush after 10 seconds
input plugin tries to append 500KB
At this exact point, the engine will allow appending those 500KB of data into the memory; in total it will have 1.2MB of data buffered. The limit is permissive and will allow a single write past the limit, but once the limit is exceeded the following actions are taken:
block local buffers for the input plugin (cannot append more data)
notify the input plugin invoking a pause callback
The engine will protect itself and will not append more data coming from the input plugin in question; note that it is the responsibility of the plugin to keep state and decide what to do in that paused state.
After some time, usually measured in seconds, if the scheduler was able to flush the initial 700KB of data or it has given up after retrying, that amount of memory is released and the following actions will occur:
Upon data buffer release (700KB), the internal counters get updated
Counters now are set at 500KB
Since 500KB is < 1MB it checks the input plugin state
If the plugin is paused, it invokes a resume callback
input plugin can continue appending more data
Please note that when storage.type filesystem is set, the Mem_Buf_Limit setting no longer has any effect, instead, the [SERVICE] level storage.max_chunks_up setting controls the size of the memory buffer.
The setting behaves similarly to the above scenario with Mem_Buf_Limit when the non-default storage.pause_on_chunks_overlimit is enabled.
When (default) storage.pause_on_chunks_overlimit is disabled, the input will not pause when the memory limit is reached. Instead, it will switch to only buffering logs in the filesystem. The disk spaced used for filesystem buffering can be limited with storage.total_limit_size.
See the Buffering & Storage docs for more information.
Each plugin is independent and not all of them implements the pause and resume callbacks. As said, these callbacks are just a notification mechanism for the plugin.
One example of a plugin that implements these callbacks and keeps state correctly is the Tail Input plugin. When the pause callback is triggered, it pauses its collectors and stops appending data. Upon resume, it resumes the collectors and continues ingesting data. Tail will track the current file offset when it pauses and resume at the same position. If the file has not been deleted or moved, it can still be read.
With the default storage.type memory and Mem_Buf_Limit, the following log messages will be emitted for pause and resume:
With storage.type filesystem and storage.max_chunks_up, the following log messages will be emitted for pause and resume: