Node-exporter bug - expose the size of the disk in node_disk_info metrics

[anulika-asama]

[anulika-asama]

Goal

Expose block device capacity as metadata on the existing node_disk_info metric, without adding a separate gauge metric.

Files changed

File	Change
collector/diskstats_linux.go	Read disk size from sysfs; add size_bytes label
collector/diskstats_linux_test.go	Update golden node_disk_info expectations
collector/fixtures/sys.ttar	Add sys/block/sda/size fixture for unit test

Code changes (diskstats_linux.go)

1. New label on node_disk_info

Added size_bytes to the label set:

[]string{"device", "major", "minor", "path", "wwn", "model", "serial", "revision", "rotational", "size_bytes"}

2. Read size in Update()

Uses existing procfs helper (no raw sysfs parsing):

size, err := c.fs.SysBlockDeviceSize(dev)

Real path	Fixture path (tests)
/sys/block/<device>/size	fixtures/sys/block/<device>/size

• File content: sector count (512-byte sectors)
• SysBlockDeviceSize() returns bytes (sectors × 512)
• Label value: decimal string via strconv.FormatUint(size, 10)

3. Error handling

Situation	Behavior
Size file missing	size_bytes="", scrape continues
Permission / I/O error	size_bytes="", debug log
Size file present	size_bytes="<bytes>"

Same pattern as optional sysfs fields (e.g. rotational when queue stats are missing).

Why a label, not a separate metric

node_disk_info is already an info-style metric (value 1, metadata in labels). Disk size is static device metadata, consistent with serial, path, rotational, etc.

Fixture changes (sys.ttar)

Minimal test coverage: one device only.

collector/fixtures/sys/block/sda/size

Content: 1953525168 (sectors) → 1000204886016 bytes in test expectation.

Other devices in the test have no size fixture → size_bytes="".

Repack workflow

make collector/fixtures/sys/.unpacked
echo "1953525168" > collector/fixtures/sys/block/sda/size
chmod 644 collector/fixtures/sys/block/sda/size
make update_fixtures   # or repack sys.ttar only

Note: repacking sys.ttar may show a noisy git diff (entry reordering); tree diff should show only sda/size added.

Test changes (diskstats_linux_test.go)

All 15 node_disk_info lines in the golden output include size_bytes because the new label is emitted on every series:

• sda → size_bytes="1000204886016"
• All others → size_bytes=""

How to test

Unit test

make collector/fixtures/sys/.unpacked collector/fixtures/udev/.unpacked
go test ./collector/ -run TestDiskStats -count=1 -v
## Production behavior

- Device filter unchanged (`vda`, `vdb`, etc. — not partitions/loop devices).
- On normal Linux hosts, `/sys/block/<device>/size` exists for block devices → expect populated `size_bytes`.
- Example host (`metrics1`): `vda` and `vdb` should expose size after deploying this build.

## Example metric output

```text
node_disk_info{device="vda",major="253",minor="0",...,rotational="0",size_bytes="1000204886016",...} 1

Out of scope (not changed)

• No new Prometheus metric (node_disk_size_bytes gauge)
• No changes to device filter
• No fixture size files for vda, nvme0n1, etc. (can be added later)
• No changes to udev or /proc/diskstats reading