blkio

Blkio refers to the Block I/O cgroup controller in the Linux kernel. It is part of the legacy cgroup v1 subsystem and provides a way to limit and account for block device I/O on a per-cgroup basis. By attaching processes to a dedicated blkio cgroup, an administrator can cap throughput or I/O operations for specific devices and gather usage statistics.

In cgroup v1, blkio supports throttling and weighting. Throttling restricts the rate of I/O on individual devices, using files such as blkio.throttle.read_bps_device and blkio.throttle.write_bps_device for bandwidth, and blkio.throttle.read_iops_device and blkio.throttle.write_iops_device for IOPS. The limits are specified per device using a major:minor identifier and a value representing the limit. Weighting assigns a relative share of I/O bandwidth among siblings via blkio.weight and blkio.weight_device, allowing proportional distribution when multiple cgroups contend for I/O.

Accounting data is reported through blkio.stat, which shows reads and writes in bytes and operations, by device and in total. The configuration and statistics are typically exposed under the cgroup filesystem, often mounted at paths such as /sys/fs/cgroup/blkio.

With the advent of cgroup v2, the blkio controller has been superseded by the unified io controller. In v2, I/O limits are configured via files like io.max, and the legacy blkio interface is not central; some tooling may expose compatibility files for older software.

Usage contexts include containers, virtualization, and systems that require performance isolation. Limitations include uneven device support for throttling across hardware, complexity in multi-device scenarios, and changing semantics between cgroup generations.