[lvm-devel] Reg cache target

[Lakshmi Narasimhan Sundararajan]

Hi Team!
A very good day to you.

I have a lvm cache setup with 14TB cache on a origin device that is 110TB.
The cache was configured with 1MB cache blocks, and 100MB migration
bandwidth/smq policy/writeback. This setup has a large memory and high cpu
count, so we relaxed the allocation/cache_pool_max_chunks to
accommodate this. The setup was stable until the cache got full of dirty
blocks.
Under some cases, cache target seems to block submitted IO and only do
migration and all IO that's incoming seems to be very slow.
Does the cache target have a scenario where this may happen when the cache
is full of dirty blocks?

It looks like the migration bandwidth was underprovisioned as the cache got
full of dirty blocks. Now I am trying to flush the cache and bring down the
dirty block count on a live setup. Below are some options I tried and would
like feedback to know what's the best way to bring down the dirty blocks
without taking down the node.


1/ increase migration threshold to a larger value 1600 MB.
2/ change cache policy to cleaner

Do these change take effect on the fly?
I do not seem to see the dirty block count drop significantly.

Can you please point me how to bring down the dirty block count immediately
on a live setup?

Thanks for your help.
Regards
LN
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