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

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Date:	Tue, 5 May 2009 19:38:54 +0200
From:	Lars Marowsky-Bree <lmb@...e.de>
To:	Philipp Reisner <philipp.reisner@...bit.com>,
	Bart Van Assche <bart.vanassche@...il.com>
Cc:	James Bottomley <James.Bottomley@...senpartnership.com>,
	david@...g.hm, Willy Tarreau <w@....eu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Jens Axboe <jens.axboe@...cle.com>,
	Greg KH <gregkh@...e.de>, Neil Brown <neilb@...e.de>,
	Sam Ravnborg <sam@...nborg.org>, Dave Jones <davej@...hat.com>,
	Nikanth Karthikesan <knikanth@...e.de>,
	Kyle Moffett <kyle@...fetthome.net>,
	Lars Ellenberg <lars.ellenberg@...bit.com>
Subject: Re: [PATCH 00/16] DRBD: a block device for HA clusters

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