Re: kernel 3.17-rc3: task rsync:2524 blocked for more than 120 seconds

[Chris Mason <clm@fb.com>]

On 09/02/2014 09:31 PM, john terragon wrote:
> Rsync finished. FWIW in the end it reported an average speed of about
>  900K/sec. Without autodefrag there have been no messages about hung
> kworkers even though rsync seemingly keeps getting hung for several
> minutes throughout the whole execution.

So lets take a step back and figure out how fast the usb stick actually is.
This will erase your usb stick, but give us an idea of its performance:

dd if=/dev/zero of=/dev/<wherever you mkfs btrfs> bs=20M oflag=direct count=100

Note again, the above command will erase your usb stick ;)  Use whatever device name
you've been sending to mkfs.btrfs

The kernel will allow a pretty significant amount of ram to be dirtied before 
forcing writeback, which is why you're seeing rsync stall at seemingly strange 
intervals.  In the base of btrfs with compression, we add some worker threads between 
rsync and the device, and these may be turning the writeback into a somewhat 
more bursty operation.

-chris