RFC: dm-ssdcache
 

Hi all,

to through some more confusion in an already heated debate, here is
the 'dm-ssdcache' module I recently wrote.
And held a talk about at LinuxCon Europe 2011 :-)

Upon request I'll be generating a proper patch ...

Not saying it's perfect, and surely doesn't have all the nifty
features bcache has, but it should serve as a 'minimal'
implementation. And probaly will start yet another discussion,
if nothing else.

So, comments etc are welcome.

Cheers,

Hannes
--
Dr. Hannes Reinecke zSeries & Storage
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SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
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RFC: dm-ssdcache
 

On Fri, Mar 16 2012 at 11:38am -0400,
Hannes Reinecke <hare@suse.de> wrote:

> Hi all,
>
> to through some more confusion in an already heated debate, here is
> the 'dm-ssdcache' module I recently wrote.
> And held a talk about at LinuxCon Europe 2011 :-)

Thanks for posting.

Do you happen to have any slides you can share from your Linuxcon
Europe talk?
https://events.linuxfoundation.org/events/linuxcon-europe/reinecke

> Upon request I'll be generating a proper patch ...
>
> Not saying it's perfect, and surely doesn't have all the nifty
> features bcache has, but it should serve as a 'minimal'
> implementation. And probaly will start yet another discussion,
> if nothing else.
>
> So, comments etc are welcome.

Please expect some from me next week. And obviously we can continue
discussion at LSF.

Thanks again,
Mike

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RFC: dm-ssdcache
 

On Fri, 16 Mar 2012, Hannes Reinecke wrote:

> Hi all,
>
> to through some more confusion in an already heated debate, here is
> the 'dm-ssdcache' module I recently wrote.
> And held a talk about at LinuxCon Europe 2011 :-)
>
> Upon request I'll be generating a proper patch ...
>
> Not saying it's perfect, and surely doesn't have all the nifty
> features bcache has, but it should serve as a 'minimal'
> implementation. And probaly will start yet another discussion,
> if nothing else.
>
> So, comments etc are welcome.

Thanks for sharing this. I gave it a try with mixed but promising
resulting. I'll share my observations in case they are useful.

Writethrough mode worked as expected, as did 'readcache'. But I found the
caching would stop after a short time; data already in the cache was read
from there, but no new data written. dmesg no longer prints "flush start",
"flush done", but no other indication; possibly some lock is taken in some
way?

It took me a while to work out how to switch to LRU behaviour, as I had to
give the number of option arguments, for reference here it is:

dmsetup create test --table "0 625137282 ssdcache /dev/sdc1 /dev/sdb1 options 1 lru"

In writeback I found I was quickly getting ext4 reports of corruption.
Even just between formatting the device and using it. Possibly it
by-passes the cache a little too readily under some reasons (eg. large bio
size) when data is in the cache?

In all cases I zeroed the front of the cache device before use, but I
couldn't see quickly from the code whether the cache device is intended to
be persistent and if I need a better way to initialise it, I assume not?

"dmsetup remove" seems to take along time (several minutes), not spinning
on CPU or disk. But it does eventually complete. Possibly this is linked
to the non-caching issue and some kind of lock. But then that doesn't
explain why it eventually completes.

Thanks

--
Mark

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