A.5. Changed default values

In DRBD 8.4, several drbd.conf default values have been updated to match improvements in the Linux kernel and available server hardware.

A.5.1. Number of concurrently active Activity Log extents (al-extents)

al-extents' previous default of 127 has changed to 1237, allowing for better performance by reducing the amount of metadata disk write operations. The associated extended resynchronization time after a primary node crash, which this change introduces, is marginal given the ubiquity of Gigabit Ethernet and higher-bandwidth replication links.

A.5.2. Run-length encoding (use-rle)

Run-length encoding (RLE) for bitmap transfers is enabled by default in DRBD 8.4; the default for the use-rle option is yes. RLE greatly reduces the amount of data transferred during the quick-sync bitmap exchange (which occurs any time two disconnected nodes reconnect).

A.5.3. I/O error handling strategy (on-io-error)

DRBD 8.4 defaults to masking I/O errors, which replaces the earlier behavior of passing them on to upper layers in the I/O stack. This means that a DRBD volume operating on a faulty drive automatically switches to the Diskless disk state and continues to serve data from its peer node.

A.5.4. Variable-rate synchronization

Variable-rate synchronization is on by default in DRBD 8.4. The default settings are equivalent to the following configuration options:

DRBD 8.4 default options for variable-rate synchronization. 

resource test {
  disk {
    c-plan-ahead 20;
    c-fill-target 50k;
    c-min-rate 250k;
  }
  ...

A.5.5. Number of configurable DRBD devices (minor-count)

The maximum number of configurable DRBD devices (previously 255) is 1,048,576 (220) in DRBD 8.4. This is more of a theoretical limit that is unlikely to be reached in production systems.