xvda[0-9]+ disks (not partitions) are ignored by default

Host operating system: output of uname -a

Linux 5.3.18-24.64-default

node_exporter version: output of node_exporter --version

node_exporter, version (branch: , revision: )
build user:
build date:
go version: go1.14.7

node_exporter command line flags

none

Are you running node_exporter in Docker?

no

What did you do that produced an error?

default config does not collect disk stats

What did you expect to see?

all disk devices should be collected by default

What did you see instead?

No disk stats

Since #13 got merged, xvda[0-9]+ are ignored. However, it is not correct to assume that those are always partitions. Xen allows a machine to inform "virtual partition", which are mapped to dedicated devices (it makes expanding a filesystem trivial).

This is a normal xvda-disk with partitions (possibly on what that PR was based on):

# ls /sys/block/xvda* -l
lrwxrwxrwx 1 root root 0 Mai 17 14:46 /sys/block/xvda -> ../devices/vbd-51712/block/xvda
# grep xvda /proc/diskstats 
 202       0 xvda 26438 52 1897369 17235 4263982 53800 47477992 3542063 0 103104 3557922
 202       1 xvda1 22 0 176 7 0 0 0 0 0 7 7
 202       2 xvda2 118 0 13522 94 129 3 30326 274 0 42 367
 202       3 xvda3 26268 52 1880583 17122 4263853 53797 47447666 3541789 0 103052 3557536

And this is a "virtual-partitions" machine:

# ls /sys/block/xvd* -d1
/sys/block/xvda1
/sys/block/xvda2
# grep xvda /proc/diskstats 
 202       1 xvda1 66701 3 13340088 570748 1833 254 308680 72 0 32440 674096
 202       2 xvda2 101 0 4448 0 0 0 0 0 0 4 4

xvda[0-9]+ should not be excluded by default only based on their name. At least in Linux, this is the easy but wrong way to check if a device is a partition or a disk. node_exporter could check if /sys/block/ exists (not partition) of if /sys/dev/block/:/partition file exists (not a disk).

If collector.diskstats.ignored-devices, by default, only excludes partitions, I think a new option like:
"collector.diskstats.ignore-partitions (default: true)" should be used instead, leaving "ignored-devices" empty by default.

Start at the diskstats collector and its collector.diskstats.ignored-devices defaults, then compare /proc/diskstats with /sys/block and /sys/dev/block partition information described in the report. Done means virtual xvda[0-9]+ devices are collected while actual partitions remain excluded by the default behavior.