[DRBD-user] Considerations for using bcache on top of DRBD

[Dan Barker]

> -----Original Message-----
> From: drbd-user-bounces at lists.linbit.com [mailto:drbd-user-
> bounces at lists.linbit.com] On Behalf Of Andrew Martin
> Sent: Tuesday, February 18, 2014 2:14 PM
> To: Berto Bermudez
> Cc: drbd-user at lists.linbit.com
> Subject: Re: [DRBD-user] Considerations for using bcache on top of DRBD
> 
> ----- Original Message -----
> > From: "Berto Bermudez" <berto at momar.com>
> > To: "Andrew Martin" <amartin at xes-inc.com>, drbd-user at lists.linbit.com
> > Sent: Monday, February 17, 2014 2:06:33 PM
> > Subject: RE: [DRBD-user] Considerations for using bcache on top of DRBD
> >
> > You didn't indicate where in your setup the SSDs would sit. I would
> suggest
> > you look at also replicating the ssd contents using drbd and layering
> bcache
> > on top of that, so that in the event of failover you don't have the
> penalty
> > of cold caches.
> >
> > HDDs <-- md/raid <-- LVM <-- DRBD <-- bcache |<-- ext4
> >                                               SSD<-- DRBD <-- bcache |
> >
> > There was a talk about something similar, but using flashcache
> > http://www.youtube.com/watch?v=l910kiEuHOM
> >
> > Berto
> > Momar, Inc.
> Hi Berto,
> 
> I was intending to keep the SSDs as the top layer, above DRBD. If I
> understand
> correctly, you're suggesting creating a separate DRBD device on top of the
> SSD and then passing that DRBD device to bcache as the cache device, thus
> propagating the cache changes between nodes?
> backing device: HDDs <-- md/raid <-- LVM <-- DRBD <-- bcache <-- ext4
> cache device: SSDs <-- md/raid <-- DRBD <-- bcache
> 
> Could this create a problem where during failover the cache device is held
> open
> and thus can't be promoted to primary on the other node? In writeback mode
> this
> would prevent the device from being usable until the cache device could be
> failed over successfully.

In the Video, Florian discusses this. You put both the SSD and the HDD in the same resource (Requires version 8.4). In that way, the HDD and SSD remain linked, as intended. Putting them in separate resources, a la version 8.3, would be a mess as you note.

Dan