[dm-devel] DM-Cache Writeback & Direct IO?

[Christoph Nelles]

Hello Joe,

I updated to the latest kernel from Ubuntu mainline (4.2.0-040200rc1) 
and will rerun the same fio command over the night. Currently it shows 
the same behaviour. The fio command i use is
fio --name=test --filename=/dev/mapper/dmcache  --rw=randwrite 
--filesize=16G  --bs=64k --ioengine=libaio --direct=1 --iodepth=1000

After 4 runs, the status for the device mapper looks like this:
dmcache: 0 6442450943 cache 8 6029/4161600 512 2283/2913051 1517 119 
25120 1294780 0 2283 1484 1 writeback 2 migration_threshold 2048 smq 0 rw

Regards

Christoph



Am 08.07.2015 um 16:48 schrieb Joe Thornber:
> On Wed, Jul 08, 2015 at 11:03:42AM +0200, Christoph Nelles wrote:
>> Hi,
>>
>> Can the dm-cache writeback cache buffer direct IO or goes direct IO
>> always immediately to the origin device?
> Yes, the device mapper layer knows nothing about direct IO vs page
> cache IO.
>
>> I am currently performing tests and see that random direct write IO
>> never hits the cache, even after overwriting the same region over
>> and over.
> Could you try with the latest kernels and the new smq policy please?
>
> - Joe
>
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