dm-cache as a writeback cache that streams all writes to cache, and then updates origin

dm-cache as a writeback cache that streams all writes to cache, and then updates origin

[Gary]

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I haven't tested the MQ policy since I'm on kernel 4.6, I am on SMQ. I have
a spinning disk as the origin that is falling behind as a mysql slave. I
added an ssd writeback cache but I can still see a a large amount of random
writes going to the spinning disk on iotop. The filesystem is XFS. Origin
is 2tb, but the dataset I am working with is only 305gb. The cache is 40gb
total.

dmsetup status shows

cached: 0 3906766976 cache 8 330/1663 512 163686/163686 1781562 5956880
6792625 24895886 18847 18847 3398 1 writeback 2 migration_threshold 2048
smq 0 rw -

a minute later shows

cached: 0 3906766976 cache 8 330/1663 512 163686/163686 1781956 5959500
6794474 24908493 18850 18850 3396 1 writeback 2 migration_threshold 2048
smq 0 rw -

I was looking over the archives of dm-cache and came across
https://www.redhat.com/archives/dm-devel/2015-September/msg00090.html

I understand that dm-cache does not just blindly stream all writes to the
cache and then update the origin. Is there some way I can enable that
though (even if it involves changing code)? The hotspot feature is
impractical for this workload and is of little help it seems. I know the
SSD taking writes by itself can keep up, I just can't dedicate that much
SSD space to this right now. I have to turn barriers off to make the hard
disk keep up and I'd rather not do that.

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Re: dm-cache as a writeback cache that streams all writes to cache, and then updates origin

[Edward Thornber]

On Fri, Jul 22, 2016 at 06:56:47PM -0700, Gary wrote:
> I understand that dm-cache does not just blindly stream all writes to the
> cache and then update the origin. Is there some way I can enable that
> though (even if it involves changing code)? 

No, sorry.  I really wish I'd called it dm-hotspot.

- Joe