-----Original Message-----
From: linux-bcache-owner@vger.kernel.org [mailto:linux-bcache-
owner@vger.kernel.org] On Behalf Of Gabriel de Perthuis
Sent: mercredi 29 janvier 2014 11:42
To: linux-bcache@vger.kernel.org
Subject: Re: Moving a backing device between 2 cachesets

On Tue, 28 Jan 2014 21:59:02 +0000, Patrick Zwahlen wrote:

quoted

Hi,

We're working on a 2-nodes pacemaker cluster that provides iSCSI LUNs

via

quoted

SCST. The LUNs are either software RAID arrays located in a shared

JBOD, or

quoted

DRBD ressources (active/passive).

We are adding bcache to the game with local SSDs (ie not shared, but
dedicated to each cluster node).

We are using write-through.

I need to evaluate the risk when moving a backing device (md) from

cacheset1

quoted

(on node #1) to cacheset2 (on node #2) and then back to cacheset #1.

Scenario
- md attached to cacheset1 and working (on node 1)
- md detached from cacheset1
- md stopped on node 1
- md started on node 2
- md attached to cacheset2 on node 2

At this point, cacheset1 is attached to nothing, but still has valid

blocks

quoted

"linked" to the backing md device

From bcache.h:

When you register a newly formatted backing device it'll come up
in passthrough mode, and then you can attach and detach a backing device
from
a cache set at runtime - while it's mounted and in use. Detaching
implicitly
invalidates any cached data for that backing device.

After flushing, detaching does two things:
- the backing device gets flagged as detached
- the backing device is removed from the cache set's metadata
(stored as uuid_entry in a special bucket; the entry is flagged
with a bogus uuid but not reused).  The offset in that uuids
array constitutes an id, local to the cache set, that is not
reused after detaching.

The second step invalidates the backing device's id in the cache set,
and indirectly invalidates all buckets that referenced it (through
bkey->inode in the bucket key).

I understand that we are safe, then. Right ? 

Thanks for your clarifications

quoted

- md detached from cacheset2
- md stopped on node 2
- md started on node 1
- md RE-attached to cacheset1 on node 1

At this point, I need to make sure that bcache will not serve "old"

blocks

quoted

that were linked to the backing device.

My understanding is that as we have attached the backing device to a

new

quoted

cacheset (#2) in-between, this will be "recorded" in the bcache

headers and

quoted

all the blocks that used to be valid in the first place won't be

served.

quoted

Can you please validate if this is safe or if we need to take special

care

quoted

about invalidating the original cacheset ?

quoted

Thanks a lot, - Patrick -

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