Re: [PATCH 0/4] btrfs: DEV_STATS item related minor fixes

[Qu Wenruo <wqu@suse.com>]

在 2026/4/14 04:29, David Sterba 写道:
> On Tue, Apr 07, 2026 at 07:03:57PM +0930, Qu Wenruo wrote:
>> [BACKGROUND]
>> DEV_STATS (objectid, not type) items are storing the error counts for
>> each device. It's updated every time an error is hit, including
>> the following types:
>>
>> - READ errors
>> - WRITE errors
>> - FLUSH errors
>>    Above alls are all errors directly returned from the device.
>>
>> - CORRUPTION errors
>>    Aka, checksum mismatch.
>>
>> - GENERATION errors
>>    Metadata generation mismatch.
>>
>> [MINOR BUGS]
>> Recently when debugging an error reported about rejected dev-replace,
>> the device tree dump includes a DEV_STATS item for devid 0.
>>
>> I'm wondering if that DEV_STATS item will ever be deleted, but
>> unfortunately it will never be deleted.
>>
>> Normally it's not a big deal, as the DEV_STATS are normally all zeros.
>>
>> But if it's not, and a new dev-replace is started and interrupted, at
>> the next mount the replace target device will suddenly inherit all the
>> error records, giving end users false alerts about a completely good
>> device.
>>
>> [FIX]
>> The first 2 are manually removing the DEV_STATS items when dev-replace
>> and dev-removal finishes.
> 
> The device stat item is expected by btrfs-progs even if it's just zeros,
> otherwise the 'btrfs device stats' prints just 'no stats found'.

That's not true.

Firstly when btrfs is created, there is no device stats at all, it's at 
the first RW mount we create the dev status item when initializing the 
dev status.

Secondly, all device adding ioctl has the proper update to force a dev 
status update, thus they will properly have one no matter what.
And that's what the older kernel missing.

With those features guarding the dev status, it's pretty safe to remove 
the old dev status.

> This
> has a high potential to break any monitoring tools or scripts. So please
> don't delete the item, and create it empty in case it does not exist.

Please give me an example where it doesn't work.

The only situation where no dev stats item can be found is for the 
unpatched kernels with offline dev stats query.

After a new device is added, no new dev stats item is added (requires an 
error to create), thus it requires a new RW mount to create them.

But before that, there is no such stats item, and offline dev stats call 
will not find the item.

And device removal/replace won't change the situation.


For kernel GET_DEV_STATS ioctl it's not a problem at all, because kernel 
always grab the numbers from the in-memory structure which always exists.

Thanks,
Qu

> 
> For the wrong stats on the replaced device the stats should be zero from
> the beginning of the operation.