Re: When ceph synchronizes journal to disk? / read request

[Dieter Kasper <d.kasper@kabelmail.de>]

Hi Gregory,

another interesting aspect for me is:
How will a read-request for this block/sub-block (pending between journal and OSD)
be satisfied (assuming the client will not cache) ?
Will this read go to the journal or to the OSD ?

Best Regards,
-Dieter

On Tue, Mar 05, 2013 at 05:33:13AM +0100, Xing Lin wrote:
> Hi Gregory,
> 
> Thanks for your reply.
> 
> On 03/04/2013 09:55 AM, Gregory Farnum wrote:
> > The "journal [min|max] sync interval" values specify how frequently
> > the OSD's "FileStore" sends a sync to the disk. However, data is still
> > written into the normal filesystem as it comes in, and the normal
> > filesystem continues to schedule normal dirty data writeouts. This is
> > good ? it means that when we do send a sync down you don't need to
> > wait for all (30 seconds * 100MB/s) 3GB or whatever of data to go to
> > disk before it's completed.
> 
> I do not think I understand this well. When the writeahead journal mode 
> is in use, would you please explain what happens to a single 4M write 
> request? I assume that an entry in the journal will be created for this 
> write request and after this entry is flushed to the journal disk, Ceph 
> returns successful. There should be no IO to the osd's disk. All IOs are 
> supposed to go to the journal disk. At a later time, Ceph will start to 
> apply these changes to the normal filesystem by reading from the first 
> entry at which its previous synchronization stops. Finally, it will read 
> this entry and apply this write change to the normal file system. Could 
> you please point out where is wrong in my understanding? Thanks,
> 
> >> >I am running 0.48.2. The related configuration is as follows.
> > If you're starting up a new cluster I recommend upgrading to the
> > bobtail series (.56.3) instead of using Argonaut ? it's got a number
> > of enhancements you'll appreciate!
> 
> Yeah, I would like to use bobtail series. However, I started to make 
> small changes with Argonaut (0.48) and had ported my changes once to 
> 0.48.2 when it was released. I think I am good to continue with it for 
> the moment. I may consider to port my changes to bobtail series at a 
> later time. Thanks,
> 
> Xing
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