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

Tim Small tim at buttersideup.com
Mon May 16 15:53:46 CEST 2016


On 16/05/16 14:02, Tim Small wrote:
> # iostat -y -d 2 -x -p /dev/sdf /dev/dm-0 /dev/md2 /dev/bcache0

... and then mangled the word-wrapping.  Try again:

Here's a typical hand-edited excerpt from:

iostat -d 2 -x -y -m -p /dev/sdf /dev/dm-0 /dev/md2 /dev/bcache0

...

Device:    r/s     w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await
sdf     396.50   19.50   272.02     0.25  1340.38   138.44  346.09
sdf3    397.00    0.00   272.52     0.00  1405.83   130.05  338.40
dm-0      0.00  149.00     0.00   271.29  3728.81     0.01    0.04
md2       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

where:

sdf is the SSD (bcache cache device is sdf3)
dm-0 is dm-crypt backing device (bcache backing store)
md2 is the underlying device for dm-crypt
bcache0 is the bcache device.

According to the iostat manual page:

"avgrq-sz The average size (in sectors) of the requests that were issued
to the device."

dm-0 is described like this in the output of 'dmsetup table':

encryptedstore01: 0 46879675392 crypt aes-xts-plain64
0000000000000000000000000000000000000000000000000000000000000000 0 9:2
3072 1 allow_discards

Tim.


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