Re: Persistent Write Back Cache

John Spray <john.spray@xxxxxxxxxx> · Wed, 04 Mar 2015 11:34:12 +0000



    On 04/03/2015 08:26, Nick Fisk wrote:

    
      To illustrate the difference a proper
        write back cache can make, I put a 1GB (512mb dirty threshold)
        flashcache in front of my RBD and tweaked the flush parameters
        to flush dirty blocks at a large queue depth. The same fio test
        (128k iodepth=1) now runs at 120MB/s and is limited by the
        performance of SSD used by flashcache, as everything is stored
        as 4k blocks on the ssd. In fact since everything is stored as
        4k blocks, pretty much all IO sizes are accelerated to max speed
        of the SSD. Looking at iostat I can see all the IO’s are getting
        coalesced into nice large 512kb IO’s at a high queue depth,
        which Ceph easily swallows.
         
        If librbd could support writing its cache
          out to SSD it would hopefully achieve the same level of
          performance and having it integrated would be really neat. 
      
    
    What are you hoping to gain from building something into ceph
    instead of using flashcache/bcache/dm-cache on top of it?  It seems
    like since you would anyway need to handle your HA configuration,
    setting up the actual cache device would be the simple part.

    
    Cheers,

    John

  
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