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

Andrew Martin amartin at xes-inc.com
Tue Feb 18 20:14:18 CET 2014

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

----- 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.

Thanks for the help!


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