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. 17:03 ч., David Newall wrote:
> Hello Qu,
> 
> Thank you very much for helping me with this.
> 
> On 12/9/19 4:35 pm, Qu Wenruo wrote:
>> Would you please check how fast (or how slow in this particular case)
>> the related disks are?
>> To me, it really looks like just too slow devices.
> 
> I discover that you are correct about the underlying storage being
> slow.  Nikolay suggested that, too.
> 
> Although I mentioned that the filesystem is encrypted with luks on the
> VM, I didn't say that the underlying storage is connected via multipath
> iSCSI (with two paths) on the host server, and provided to the VM via
> KVM as Virtio disk, which should be fine, but, using dd (bs=1024k
> count=15) on the VM, I'm seeing a woeful 255KB/s read speed through the
> encryption layer, and 274KB/s from the raw disk.  :-(
> 
> On the host, I'm seeing 2MB/s via one path and 846KB/s via the other, so
> I think that's where I need to turn my attention.  (Time to benchmark,
> turn off one path, and speak to the DC management.)
> 
>> I see all dumps are waiting for write_all_supers.
>>
>> Would you please provide the code context of
>> write_all_supers.isra.43+0x977?
>>
>> I guess it's wait_dev_flush(), which is just really waiting for disk
>> writes.
> 
> Sorry, I don't understand what you mean by "code context".  Maybe the
> question is now moot.
> 
> Although it's now apparent that I've got a really slow disk, I still
> wonder if btrfs is holding a lock for an unnecessarily long time
> (assuming that it is btrfs holding the lock.)  I feel that having to
> wait tens of minutes to find the device names of mounted devices could
> never be intended, so there might be something that needs tweaking.
> 

With the kernel you are using that's how things were structured for
various reasons. Recent kernel versions do not take device_list_mutex
when printing the device name. So if you update your kernel to one which
contains :

88c14590cdd6 ("btrfs: use RCU in btrfs_show_devname for device list
traversal")

this particular problem would be gone. Looking at the history of that
commit this means kernel 4.17 at least.

<snip>

>