[FR] New module for `sysfs-block`

[RichiH]

As per our chat, sysfs-block would be useful as an optional module to learn more about the underlying block devices.

Documentation at https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-block

/sys/block/<disk>/queue/logical_block_size is a hard requirement as the defintion of "block" in this sysfs and for Linux as a whole comes from there

[RichiH]

Fields I would need are:

• /sys/block/<disk>/stat
  • 1 - reads completed successfully
  • 5 - writes completed
  • 9 - I/Os currently in progress
• /sys/block/<disk>/queue/logical_block_size

[SuperQ]

The stat metrics already exist:
1 - node_disk_reads_completed_total
5 - node_disk_writes_completed_total
9 - node_disk_io_now

You might be interested in the iostat Grafana dashboard I made:
https://gitlab.com/gitlab-org/grafana-dashboards/-/blob/master/dashboards/node-iostat.json

That one might be a good one to add to the node_exporter mixin.

The logical block size data is new, we would have to add a parser for that in prometheus/procfs.

[SuperQ]

We should also probably parse /sys/block/<disk>/queue/physical_block_size.

[SuperQ]

It wasn't documented where in the kernel this comes from, but but apparently the logical sector size is hard-coded in the kernel somewhere. This might be different from the "logical block size" value.

See the original reason that we assume 512 byte sectors here: #180

[dswarbrick]

It wasn't documented where in the kernel this comes from, but but apparently the logical sector size is hard-coded in the kernel somewhere.

It's defined in linux/blkdev.h

/*
 * The basic unit of block I/O is a sector. It is used in a number of contexts
 * in Linux (blk, bio, genhd). The size of one sector is 512 = 2**9
 * bytes. Variables of type sector_t represent an offset or size that is a
 * multiple of 512 bytes. Hence these two constants.
 */
#ifndef SECTOR_SHIFT
#define SECTOR_SHIFT 9
#endif
#ifndef SECTOR_SIZE
#define SECTOR_SIZE (1 << SECTOR_SHIFT)
#endif

(see also https://elixir.bootlin.com/linux/latest/ident/SECTOR_SIZE)

Note that there are also ioctls for getting the sector size of a device (BLKSSZGET, BLKPBSZGET).