[dm-devel] [PATCH 2/2] - writeback delay

[Steven Wilton]

I just realised that I had also made a change to dm-cache-target.c 
(attached) to make sure the policy set_dirty function is called every 
time we write to a cache block to ensure that the writeback time us 
updated for a hot block.

Steve

On 03/03/16 22:25, Steven Wilton wrote:
> Hi,
>
> Please find attached a patch for the dm-cache mq policy that adds 
> another config option of writeback_delay which adds a fixed delay for 
> any write hits before a writeback will be performed.  I have tested 
> the patch on a development system, and confirmed that the writeback 
> occurs after the configured time, and does not appear to cause any 
> issues.  I also confirmed that overwriting the same block delays the 
> writeback indefinitely, while also avoiding the copy operation from 
> SSD to HDD (which is my reason for creating the patch).
>
> I am interested to get confirmation that this patch does not have any 
> glaring errors, and getting it merged if it is considered useful.  The 
> DEFAULT_WRITEBACK_DELAY option can be set to 0 if you want to merge 
> the code without affecting the behaviour of existing deployments.
>
> The patch is available as a git commit here:
> https://github.com/eskyuu/linux/commit/565a6e57fa5a55bdd6656ae89a28543a9d871f52 
>
>
>
> The dm-cache policy could be improved further to defer writebacks 
> until the cache is getting full (high/low watermarks), or trying to 
> keep sequential dirty blocks together so we write them back 
> sequentially.  I am happy to develop and test along these lines if 
> needed.
>
>
> I can confirm that this patch does make removing the cache more 
> difficult, since the cache removal code waits for all writeback to 
> complete before removing the cache.  I had a look at the lvm2 code, 
> and I'm not sure if the cleaner policy is meant to be applied before 
> waiting for the writebacks to complete.  At the worst case, the 
> user-space program could be modified to set the writeback delay to 0, 
> which would mirror the existing behaviour.
>
> regards
>
> Steven
>
>
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