Re: [PATCH 00/16] DRBD: a block device for HA clusters

[Lars Marowsky-Bree]

On 2009-05-05T17:57:15, Philipp Reisner <philipp.reisner@xxxxxxxxxx> wrote:

> Up to now we do not offer barrier support for the layers above us.
> That will follow sooner or later.
> 
> Here is an example, why it is not completely trivial:
> 
>   Imagine DRBD on top of a dm-linear on both nodes. When you start,
>   both dm-linear mappings sit on top of something that supports 
>   barriers itself. -- Then the user replaces the backing device
>   below the dm-linear on the secondary node with something that
>   does not support barriers.

The same problem exists essentially for md raid1 as well, and I'd not
consider it objectionable if you took a brutal approach:

>   When we get a write request with the BIO_RW_BARRIER flag set
>   in from the FS, we submit this locally, ship it over to the
>   peer and submit it there. Unfortunately it fails now with
>   ENOTSUP on the peer. 
> 
>   We can not ship that error back to the upper layer, because
>   our mirror is already inconsistent.

Disconnect the secondary with a loud error as to why (incompatible
change of the device below). (Re-)negotiate barrier capability at
connect time; then, resync.


Regards,
    Lars

-- 
SuSE Labs, OPS Engineering, Novell, Inc.
SUSE LINUX Products GmbH, GF: Markus Rex, HRB 16746 (AG Nürnberg)
"Experience is the name everyone gives to their mistakes." -- Oscar Wilde

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