Slow writing of dirty data (bcache)

Slow writing of dirty data (bcache)

[Larkin Lowrey]

I'm seeing an odd behavior after updating to kernel
3.13.5-202.fc20.x86_64 from 3.12.7-300.fc20.x86_64.

I have a bcache device (bcache2) which backs a raid5 array (md2) that
has 160+MB of dirty data. The dirty data is being written at a rate of 1
page per second (8 sectors). At this rate it'll take 11 hrs to finish. I
like to keep this array spun down when inactive but the drives can't
spin down since they're being written to once a second.

# cat /sys/block/bcache2/bcache/writeback_rate
512

# cat /sys/block/bcache2/bcache/writeback_rate_debug
rate:           512/sec
dirty:          161M
target:         40.8G
proportional:   -35.2M
derivative:     0
change:         -35.2M/sec
next io:        624ms

This never happened when running 3.12.7. Is there anything I can do
about this?

--Larkin

Re: Slow writing of dirty data (bcache)

[Sitsofe Wheeler]

On Mon, Mar 10, 2014 at 11:34:45AM -0500, Larkin Lowrey wrote:
> 
> has 160+MB of dirty data. The dirty data is being written at a rate of 1
> page per second (8 sectors). At this rate it'll take 11 hrs to finish. I
> like to keep this array spun down when inactive but the drives can't
> spin down since they're being written to once a second.
> 
> This never happened when running 3.12.7. Is there anything I can do
> about this?

I believe this is intended behaviour - as the cache approaches the
writeback_percent level of fullness the rate of writeback will increase.
It is possible to override the writeback_rate by echoing a new value
into it but it will also change itself so you may find the rate falls
again...  Perhaps that can be avoided by echoing 0 into
writeback_percent too (see
http://evilpiepirate.org/git/linux-bcache.git/tree/Documentation/bcache.txt?h=bcache-dev#n265)...

-- 
Sitsofe | http://sucs.org/~sits/

Re: Slow writing of dirty data (bcache)

[Larkin Lowrey]

This solved my immediate problem. Thank you very much.

Should I keep this array at 0 from now on if I want it to spin down when
idle?

--Larkin

On 3/10/2014 12:12 PM, Kent Overstreet wrote:
>
> echo 0 > writeback_percent...
>
> On Mar 10, 2014 6:07 PM, "Larkin Lowrey" <llowrey@nuclearwinter.com
> <mailto:llowrey@nuclearwinter.com>> wrote:
>
>     I'm seeing an odd behavior after updating to kernel
>     3.13.5-202.fc20.x86_64 from 3.12.7-300.fc20.x86_64.
>
>     I have a bcache device (bcache2) which backs a raid5 array (md2) that
>     has 160+MB of dirty data. The dirty data is being written at a
>     rate of 1
>     page per second (8 sectors). At this rate it'll take 11 hrs to
>     finish. I
>     like to keep this array spun down when inactive but the drives can't
>     spin down since they're being written to once a second.
>
>     # cat /sys/block/bcache2/bcache/writeback_rate
>     512
>
>     # cat /sys/block/bcache2/bcache/writeback_rate_debug
>     rate:           512/sec
>     dirty:          161M
>     target:         40.8G
>     proportional:   -35.2M
>     derivative:     0
>     change:         -35.2M/sec
>     next io:        624ms
>
>     This never happened when running 3.12.7. Is there anything I can do
>     about this?
>
>     --Larkin
>
>
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