[dm-crypt] bcache gets stuck flushing writeback cache when used in combination with LUKS/dm-crypt and non-default bucket size

Eric Wheeler bcache at lists.ewheeler.net
Fri May 20 01:15:49 CEST 2016


On Mon, 16 May 2016, Tim Small wrote:
> Hi Eric,
> 
> On 15/05/16 10:08, Tim Small wrote:
> > On 11/05/16 02:38, Eric Wheeler wrote:
> >> Ming Lei's patch got in to 4.6 yet, but try this:
> >> >   https://lkml.org/lkml/2016/4/5/1046
> >> > 
> >> > and maybe Shaohua Li's patch too:
> >> >   http://www.spinics.net/lists/raid/msg51830.html
> 
> > I'll give them both a go...
> 
> I tried both of these on 4.6.0-rc7 without change to the symptoms (cache
> device continuously read).  Then I tried also disabling
> partial_stripes_expensive prior to registering the bcache device as per
> your instructions here:
> 
> https://lkml.org/lkml/2016/2/1/636
> 
> and that seems to have improved things, but not fixed them.

What is your /sys/class/X/queue/limits/io_opt value? (requires the sysfs 
patch)

Caution: make these changes at your own risk, I have no idea what other 
side effects that might when modifying io_opt and dc->disk.stride_width, 
so be sure this is a test machine.

You could update my sysfs limits patch to set QL_SYSFS_RW for io_opt and 
shrink it or set it to zero before registering.  

or,

bcache sets the disk.stripe_size at initialization, so you could just 
force this to 0 in cached_dev_init() and see if it fixes that:

-bcache/super.c:1138    dc->disk.stripe_size = q->limits.io_opt >> 9;
+bcache/super.c:1138    dc->disk.stripe_size = 0;

It then uses stripe_size in the writeback code:

writeback.c:299:        stripe_offset = offset & (d->stripe_size - 1);
writeback.c:303:                              d->stripe_size - stripe_offset);
writeback.c:313:                if (sectors_dirty == d->stripe_size)
writeback.c:357:                                        stripe * dc->disk.stripe_size, 0);
writeback.c:361:                                       next_stripe * dc->disk.stripe_size, 0),
writeback.h:20: do_div(offset, d->stripe_size);
writeback.h:34:         if (nr_sectors <= dc->disk.stripe_size)
writeback.h:37:         nr_sectors -= dc->disk.stripe_size;

Speculation only, but I've always wondered if there are issues when opt_io!=0.

Are you able to test one or the other or both methods?

--
Eric Wheeler


> 
> The cache device is 120G, and dirty_data had got up to 55.3G, but has
> now dropped down to 44.5G, but isn't going any further...
> 
> The cache device is being read at a steady ~270 MB/s, and the backing
> device (dm-crypt) being written at the same rate, but the writes aren't
> flowing down to the underlying devices (md RAID5, and SATA disks).  I'm
> guessing that these writes are being refused/retried, and are maybe
> failing due to their size (avgrq-sz showing > 4000 sectors on the
> backing device)?  Disabling the partial stripes expensive maybe just
> resulted in a few GB of small writes succeeding?
> 
> # iostat -y -d 2 -x -p /dev/sdf /dev/dm-0 /dev/md2 /dev/bcache0
> Linux 4.6.0-rc7+  16/05/16        _x86_64_        (2 CPU)
> 
> Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s
> avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
> sdf               0.00     0.00  413.00    0.00 281422.00     0.00
> 1362.82   143.18  338.31  338.31    0.00   2.42 100.00
> sdf1              0.00     0.00    0.00    0.00     0.00     0.00
> 0.00     0.00    0.00    0.00    0.00   0.00   0.00
> sdf2              0.00     0.00    0.00    0.00     0.00     0.00
> 0.00     0.00    0.00    0.00    0.00   0.00   0.00
> sdf3              0.00     0.00  413.00    0.00 281422.00     0.00
> 1362.82   143.18  338.31  338.31    0.00   2.42 100.00
> dm-0              0.00     0.00    0.00  138.50     0.00 280912.00
> 4056.49     0.00    0.01    0.00    0.01   0.01   0.20
> md2               0.00     0.00    0.00    0.00     0.00     0.00
> 0.00     0.00    0.00    0.00    0.00   0.00   0.00
> bcache0           0.00     0.00    0.00    0.00     0.00     0.00
> 0.00     0.00    0.00    0.00    0.00   0.00   0.00
> 
> Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s
> avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
> sdf               0.00     6.00  412.00    1.50 281806.00    32.00
> 1363.18   135.19  314.09  314.78  124.00   2.42 100.00
> sdf1              0.00     6.00    0.00    1.50     0.00    32.00
> 42.67     4.10  124.00    0.00  124.00 388.00  58.20
> sdf2              0.00     0.00    0.00    0.00     0.00     0.00
> 0.00     0.00    0.00    0.00    0.00   0.00   0.00
> sdf3              0.00     0.00  412.00    0.00 281806.00     0.00
> 1367.99   131.10  314.78  314.78    0.00   2.43 100.00
> dm-0              0.00     0.00    0.00  138.50     0.00 282388.00
> 4077.81     0.00    0.01    0.00    0.01   0.01   0.20
> md2               0.00     0.00    0.00    0.00     0.00     0.00
> 0.00     0.00    0.00    0.00    0.00   0.00   0.00
> bcache0           0.00     0.00    0.00    0.00     0.00     0.00
> 0.00     0.00    0.00    0.00    0.00   0.00   0.00
> 
> Cheers,
> 
> Tim.
> 


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