[dm-devel] Patch - writeback delay

[Steven Wilton]

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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