Re: Mount/df/PAM login hangs during rsync to btrfs subvolume, or maybe doing btrfs subvolume snapshot

[Nikolay Borisov <nborisov@suse.com>]

On 12.09.19 г. 9:11 ч., Nikolay Borisov wrote:
> 
> 
> On 12.09.19 г. 7:38 ч., David Newall wrote:
>> On 11/9/19 8:22 pm, Nikolay Borisov wrote:
>>>> I saved it tohttps://davidnewall.com/kern.1
>>> Nothing useful in that log, everything seems normal.
>>
>> Thank you, again, for your help.  I am grateful.
>>
>> If I understand the output, both df and mount are waiting for
>> show_mountinfo which is waiting for btrfs_show_devname which is waiting
>> to get a lock.  They have to wait to find the devname for ten minutes. 
>> Is that really normal?
> 
> It's normal for them to wait for the lock, it's not normal for the lock
> to be taken for 10 minutes.
> 
>>
>> I'm not saying that btrfs is behind it, but it does seem like something
>> is not right.
> 
> From my POV what's wrong is the fact that btrfs transaction commit is
> taking a long time. Is it possible that the underlying storage is
> exhibiting problems e.g. a dying disk/ssd?

Actually when the issue occurs again can you sample the output of echo w
> /proc/sysrq-trigger. Because right now you have provided 3 samples in
the course of I don't know how many minutes. So they just give a
momentarily glimpse into what's happening. E.g. just because we saw
btrfs transaction/btrfs_show_devname doesn't necessarily mean that's
what's happening (Though having the same consistent state in the 3 logs
kind of suggests otherwise).

> 
>>
>> I notice that there's a waiting btrfs-transation, too.  I don't know
>> what the transaction would be, and no doubt it's completely appropriate,
>> even though the use of btrfs at that point is merely one mount (and one
>> mount of ext2 over a sub-directory, probably no involvement by btrfs.)
>>
>> I see, too, that systemd is waiting for btrfs_show_devname.  It's a
>> pattern.
>>
>> I take your point about the kernel being somewhat old and accept that
>> I'm unlikely to get far without confirming the problem on a recent kernel.
>>
>>
>