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

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Date:	Sun, 3 May 2009 07:36:36 -0700 (PDT)
From:	david@...g.hm
To:	James Bottomley <James.Bottomley@...senPartnership.com>
cc:	Willy Tarreau <w@....eu>,
	Bart Van Assche <bart.vanassche@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Philipp Reisner <philipp.reisner@...bit.com>,
	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>,
	Lars Marowsky-Bree <lmb@...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 Sun, 3 May 2009, James Bottomley wrote:

> Subject: Re: [PATCH 00/16] DRBD: a block device for HA clusters
> 
> On Sat, 2009-05-02 at 22:40 -0700, david@...g.hm wrote:
>> On Sun, 3 May 2009, Willy Tarreau wrote:
>>
>>> On Sat, May 02, 2009 at 09:33:35AM +0200, Bart Van Assche wrote:
>>>> On Fri, May 1, 2009 at 10:59 AM, Andrew Morton
>>>> <akpm@...ux-foundation.org> wrote:
>>>>> On Thu, 30 Apr 2009 13:26:36 +0200 Philipp Reisner <philipp.reisner@...bit.com> wrote:
>>>>>
>>>>>> This is a repost of DRBD
>>>>>
>>>>> Is it being used anywhere for anything?  If so, where and what?
>>>>
>>>> One popular application is to run iSCSI and HA software on top of DRBD
>>>> in order to build a highly available iSCSI storage target.
>>>
>>> Confirmed, I have several customers who're doing exactly that.
>>
>> I will also say that there are a lot of us out here who would have a use
>> for DRDB in our HA setups, but have held off implementing it specificly
>> because it's not yet in the upstream kernel.
>
> Actually, that's not a particularly strong reason because we already
> have an in-kernel replicator that has much of the functionality of drbd
> that you could use.  The main reason for wanting drbd in kernel is that
> it has a *current* user base.
>
> Both the in kernel md/nbd and drbd do sync and async replication with
> primary side bitmaps.  The main differences are:
>
>      * md/nbd can do 1 to N replication,
>      * drbd can do active/active replication (useful for cluster
>        filesystems)
>      * The chunk size of the md/nbd is tunable
>      * With the updated nbd-tools, current md/nbd can do point in time
>        rollback on transaction logged secondaries (a BCS requirement)
>      * drbd manages the mirror state explicitly, md/nbd needs a user
>        space helper
>
> And probably a few others I forget.

one very big one:

DRDB has better support for dealing with split brain situations and 
recovering from them.

David Lang
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