I have one subvolume (rw) and 2 snapshots (ro) of it.

I just tested 'btrfs send <subvol> > /dev/null' and that also shows no 
IO after a while but also no significant CPU usage.
During this I tried 'ls' on the source subvolume and it hangs as well.
dmesg has some interesting messages I think (see attached dmesg.log)


On 9/6/18 5:48 PM, Chris Murphy wrote:
> On Thu, Sep 6, 2018 at 9:04 AM, Stefan Loewen <stefan.loewen@gmail.com> wrote:
>> Update:
>> It seems like btrfs-send is not completely hung. It somewhat regularly
>> wakes up every hour to transfer a few bytes. I noticed this via a
>> periodic 'ls -l' on the snapshot file. These are the last outputs
>> (uniq'ed):
>>
>> -rw------- 1 root root 1492797759 Sep  6 08:44 intenso_white.snapshot
>> -rw------- 1 root root 1493087856 Sep  6 09:44 intenso_white.snapshot
>> -rw------- 1 root root 1773825308 Sep  6 10:44 intenso_white.snapshot
>> -rw------- 1 root root 1773976853 Sep  6 11:58 intenso_white.snapshot
>> -rw------- 1 root root 1774122301 Sep  6 12:59 intenso_white.snapshot
>> -rw------- 1 root root 1774274264 Sep  6 13:58 intenso_white.snapshot
>> -rw------- 1 root root 1774435235 Sep  6 14:57 intenso_white.snapshot
>>
>> I also monitor the /proc/3022/task/*/stack files with 'tail -f' (I
>> have no idea if this is useful) but there are no changes, even during
>> the short wakeups.
> I have a sort of "me too" here. I definitely see btrfs send just hang
> for no apparent reason, but in my case it's for maybe 15-30 seconds.
> Not an hour. Looking at top and iotop at the same time as the LED
> lights on the drives, there's  definitely zero activity happening. I
> can make things happen during this time - like I can read a file or
> save a file from/to any location including the send source or receive
> destination. It really just behaves as if the send thread is saying
> "OK I'm gonna nap now, back in a bit" and then it is.
>
> So what I end up with on drives with a minimum read-write of 80M/s, is
> a send receive that's getting me a net of about 30M/s.
>
> I have around 100 snapshots on the source device. How many total
> snapshots do you have on your source? That does appear to affect
> performance for some things, including send/receive.
>
>