Re: [RFC PATCH 0/3] md: export internal stats through debugfs

[Hou Tao <houtao1@huawei.com>]

Hi,

On 2019/7/23 5:31, Song Liu wrote:
> On Tue, Jul 2, 2019 at 6:25 AM Hou Tao <houtao1@huawei.com> wrote:
>>
>> Hi,
>>
>> There are so many io counters, stats and flags in md, so I think
>> export these info to userspace will be helpful for online-debugging,
>> especially when the vmlinux file and the crash utility are not
>> available. And these info can also be utilized during code
>> understanding.
>>
>> MD has already exported some stats through sysfs files under
>> /sys/block/mdX/md, but using sysfs file to export more internal
>> stats are not a good choice, because we need to create a single
>> sysfs file for each internal stat according to the use convention
>> of sysfs and there are too many internal stats. Further, the
>> newly-created sysfs files would become APIs for userspace tools,
>> but that is not we wanted, because these files are related with
>> internal stats and internal stats may change from time to time.
>>
>> And I think debugfs is a better choice. Because we can show multiple
>> related stats in a debugfs file, and the debugfs file will never be
>> used as an userspace API.
>>
>> Two debugfs files are created to expose these internal stats:
>> * iostat: io counters and io related stats (e.g., mddev->active_io,
>>         r1conf->nr_pending, or r1confi->retry_list)
>> * stat: normal stats/flags (e.g., mddev->recovery, conf->array_frozen)
>>
>> Because internal stats are spreaded all over md-core and md-personality,
>> so both md-core and md-personality will create these two debugfs files
>> under different debugfs directory.
>>
>> Patch 1 factors out the debugfs files creation routine for md-core and
>> md-personality, patch 2 creates two debugfs files: iostat & stat under
>> /sys/kernel/debug/block/mdX for md-core, and patch 3 creates two debugfs
>> files: iostat & stat under /sys/kernel/debug/block/mdX/raid1 for md-raid1.
>>
>> The following lines show the hierarchy and the content of these debugfs
>> files for a RAID1 device:
>>
>> $ pwd
>> /sys/kernel/debug/block/md0
>> $ tree
>> .
>> ├── iostat
>> ├── raid1
>> │   ├── iostat
>> │   └── stat
>> └── stat
>>
>> $ cat iostat
>> active_io 0
>> sb_wait 0 pending_writes 0
>> recovery_active 0
>> bitmap pending_writes 0
>>
>> $ cat stat
>> flags 0x20
>> sb_flags 0x0
>> recovery 0x0
>>
>> $ cat raid1/iostat
>> retry_list active 0
>> bio_end_io_list active 0
>> pending_bio_list active 0 cnt 0
>> sync_pending 0
>> nr_pending 0
>> nr_waiting 0
>> nr_queued 0
>> barrier 0
> 
> Hi,
> 
> Sorry for the late reply.
> 
> I think these information are really debug information that we should not
> show in /sys. Once we expose them in /sys, we need to support them
> because some use space may start searching data from them.
So debugfs is used to place these debug information instead of sysfs.

It's OK for user-space tools to search data from these files as long as these
tools don't expect these information to be stable. And the most possible user
of these files would be test programs, and if some user-space tools may truly
expect some stable information from the debugfs file, maybe we should move
these information from debugfs to sysfs file.

Regards,
Tao

> 
> Thanks,
> Song
> 
> .
>