tail: ensure db file is unique (#2070)

* tail: ensure db file is unique

Signed-off-by: Patrick Stephens <[email protected]>

* fix: resolve review comments

Signed-off-by: Patrick Stephens <[email protected]>

* fix: resolve review comments

Signed-off-by: Patrick Stephens <[email protected]>

---------

Signed-off-by: Patrick Stephens <[email protected]>

Author: patrick-stephens

pipeline/inputs/tail.md

@@ -22,7 +22,7 @@ The plugin supports the following configuration parameters:
 | `ignore_older`        | Ignores files older than `ignore_older`. Supports `m`, `h`, `d` (minutes, hours, days) syntax.                                                                                                                                                                                                                                                                                                                                                                     | Read all. |
 | `skip_long_lines`     | When a monitored file reaches its buffer capacity due to a very long line (`buffer_max_size`), the default behavior is to stop monitoring that file. `skip_long_lines` alter that behavior and instruct Fluent Bit to skip long lines and continue processing other lines that fit into the buffer size.                                                                                                                                                           | `off`     |
 | `skip_empty_lines`    | Skips empty lines in the log file from any further processing or output.                                                                                                                                                                                                                                                                                                                                                                                           | `off`     |
-| `db`                  | Specify the database file to keep track of monitored files and offsets.                                                                                                                                                                                                                                                                                                                                                                                            | _none_    |
+| `db`                  | Specify the database file to keep track of monitored files and offsets. Recommended to be unique per plugin.                                                                                                                                                                                                                                                                                                                                                                                          | _none_    |
 | `db.sync`             | Set a default synchronization (I/O) method. This flag affects how the internal SQLite engine do synchronization to disk, for more details about each option see [the SQLite documentation](https://www.sqlite.org/pragma.html#pragma_synchronous). Most scenarios will be fine with `normal` mode. If you need full synchronization after every write operation set `full` mode. `full` has a high I/O performance cost. Values: `extra`, `full`, `normal`, `off`. | `normal`  |
 | `db.locking`          | Specify that the database will be accessed only by Fluent Bit. Enabling this feature helps increase performance when accessing the database but restricts externals tool from querying the content.                                                                                                                                                                                                                                                                | `false`   |
 | `db.journal_mode`     | Sets the journal mode for databases (`wal`). Enabling `wal` provides higher performance. `wal` isn't compatible with shared network file systems.                                                                                                                                                                                                                                                                                                                  | `wal`     |
@@ -75,6 +75,8 @@ If no database file is present, positioning behavior depends on the value of `re
 - When `read_from_head` is `true`, the plugin reads from the beginning of the file.
 - When `read_from_head` is `false`, the plugin starts monitoring from the end of the file (classic "tail" behavior). This means that only new content written after Fluent Bit starts will be monitored.
 
+The database file essentially stores `inode=offset` so it should be unique per instance of the plugin, for example if you have two tail inputs then use two separate `db` files for each. That way each tail input can independently track its own state.
+
 ## Monitor a large number of files
 
 To monitor a large number of files, you can increase the `inotify` settings in your Linux environment by modifying the following `sysctl` parameters: