Re: dm-cache module

[Heinz Mauelshagen <heinzm@redhat.com>]

Olivier,

some initial impressions:

I've spent an hour porting the source code to 2.6.34-rc4 and doing first
tests.

Those unveil, that it only works with cache block size equal to page
size (4KB here). With large cache block sizes it doesn't cache at all.

That may well be the result of a porting flaw but I have to have a
closer look to tell. It works with quite decent performance with that
block size. Code looks like heavily derived from the dm-cache source and
overly complicated.

Destroying a flashcache mapping with a large dirty cache flushs the
whole dirty content out which is not suitable to practical dm needs
requiring a mapping to be suspendable quickly during reconfiguration.

Regards,
Heinz

On Wed, 2010-04-28 at 10:21 +0200, Heinz Mauelshagen wrote:
> On Wed, 2010-04-28 at 03:33 +0200, Olivier B. wrote:
> > On 22/03/2010 16:05, Heinz Mauelshagen wrote:
> > > On Sun, 2010-03-21 at 00:56 -0400, Ming Zhao wrote:
> > >    
> > >> Hi Mike,
> > >>
> > >> Thank you very much for your advice!
> > >>
> > >> I can revise dm-cache code and resubmit it as you suggested. I would
> > >> also love to know Heinz's progress on his implementation and work with
> > >> him if there anything I could contribute.
> > >>      
> > > Hi all,
> > >
> > > this is a list of the functions of my dm-hstore
> > > device-mapper target implementation:
> > >
> > > o caches reads and writes keeping persistent state metadata.
> > > o writes back in order to enhance streaming performance
> > >    on fragmented access pattern.
> > > o can run on top of readonly original device
> > > o if so, writes back any dirty areas when set readwrite
> > >    (useful for tests)
> > > o readonly<->  readwrite access changes supported via message interface
> > > o initializes metadata for extents in cache in the background
> > >    in order to fasten cache construction
> > > o supports cache resizing via message interface or constructor
> > > o keeps metadata persistent by default
> > > o stores CRCs with metadata for integrity checks
> > > o stores versions with metadata to support future metadata migration
> > >
> > > Test features only:
> > > o transient cache
> > > o cache write through
> > >
> > >
> > > Provides very good performance on SSD cache backing stores.
> > >
> > > Has been shelved for a while because of other priorities so I need to
> > > rebase it to the actual kernel.
> > >
> > > Regards,
> > > Heinz
> > >
> > >    
> > Hi,
> > 
> > Facebook have just released "FlashCache" : 
> > http://github.com/facebook/flashcache
> > in the documentation we can read :
> 
> > Flashcache is built using the Linux Device Mapper (DM), part of the 
> > Linux Storage Stack infrastructure that facilitates building SW-RAID and 
> > other components.
> > 
> > So it's an other implementation of the same concept, no ?
> 
> Yes, it looks like it. I'll have a closer look how it differs form mine.
> 
> Heinz
> 
> > 
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> 
> 
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