Re: staging: writeboost: Add dm-writeboost

From: Mike Snitzer
Date: Tue Dec 09 2014 - 10:49:09 EST

On Tue, Dec 09 2014 at 10:12am -0500,
Joe Thornber <thornber@xxxxxxxxxx> wrote:

> On Mon, Dec 08, 2014 at 06:04:41AM +0900, Akira Hayakawa wrote:
> > Mike and Alasdair,
> > I need your ack
> Hi Akira,
> I just spent some time playing with your latest code. On the positive
> side I am seeing some good performance with the fio tests. Which is
> great, we know your design should outperform dm-cache with small
> random io.
> However I'm still getting v. poor results with the git-extract test,
> which clones a linux kernel repo, and then checks out 5 revisions, all
> with drop_caches in between.

Thanks for re-evaluating dm-writeboost performance Joe.

> It's fine to have different benefits of the caching software depending
> on the load. But I think the worst case should always be close to the
> performance of the raw spindle device.
> If you get the following work items done I will ack to go upstream:
> i) Get this test so it's performance is similar to raw spindle.
> ii) Write good documentation in Documentation/device-mapper/. eg. How
> do I remove a cache? When should I use dm-writeboost rather than
> bcache or dm-cache?
> iii) Provide an equivalent to the fsck tool to repair a damaged cache.

I agree with this TODO list. But I'd also add:
iv) perform full code review to catch various implementation issues,
any style nits, etc.

v) explore/implement read caching support (could the lack of read
caching be contributing to why the git_extract test is so poor?)

Item iv) would be a task for myself and anyone else interested in
getting dm-writeboost ready for inclusion. Akira, I can start working
on dm-writeboost code review once I complete review of the dm-dedup
target (my current priority) -- but realistically that likely won't be
until the new year.

BTW, I'm really not seeing much point putting dm-writeboost in staging.
I'd be happy to take dm-writeboost into drivers/md/ once the above list
is resolved. BUT if you'd still like dm-writeboost to go into staging
_without_ any of these 5 work items being completed I'll ack it but to
be very clear: dm-writeboost will not migrate out of staging until these
items are resolved.

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