Re: Curious problem: btrfs device stats & unpriviliged access

[Hans van Kranenburg <Hans.van.Kranenburg@mendix.com>]

On 10/08/2018 06:37 PM, Holger Hoffstätte wrote:
> On 10/08/18 17:46, Hans van Kranenburg wrote:
> <snip>
>> fs.devices() also looks for dev_items in the chunk tree:
>>
>> https://github.com/knorrie/python-btrfs/blob/master/btrfs/ctree.py#L481
>>
>> So, BOOM! you need root.
>>
>> Or just start a 0, ignore errors and start trying all devids until you
>> found num_devices amount of them that work, yolo.
> 
> Since I need to walk /sys/fs/btrfs/ anyway I *think* I can just look
> at the entries in /sys/fs/btrfs/<fsid>/devices/ and query them all
> directly.

But, you still need root for that right? The progs code does a RO open
directly on the block device.

-$ btrfs dev stats /dev/xvdb
ERROR: cannot open /dev/xvdb: Permission denied
ERROR: '/dev/xvdb' is not a mounted btrfs device

stat("/dev/loop0", {st_mode=S_IFBLK|0660, st_rdev=makedev(7, 0), ...}) = 0
stat("/dev/loop0", {st_mode=S_IFBLK|0660, st_rdev=makedev(7, 0), ...}) = 0
open("/dev/loop0", O_RDONLY)            = -1 EACCES (Permission denied)

But:

-# btrfs dev stats /dev/xvdb
[/dev/xvdb].write_io_errs    0
[/dev/xvdb].read_io_errs     0
[/dev/xvdb].flush_io_errs    0
[/dev/xvdb].corruption_errs  0
[/dev/xvdb].generation_errs  0

> The skeleton btrfs_exporter already responds to http requests and
> returns dummy metrics, using the official python client library.
> I've found a nice little python sysfs scraper and now only need to
> figure out how best to map the btrfs info in sysfs to useful metrics.
> The privileged access issue would only have come into play much later,
> but it seems I can avoid it after all, which is great.
> I'm also (re-)learning python in the process, so that's the actual
> thing slowing me down..

:)

-- 
Hans van Kranenburg