[DRBD-user] 3 simple questions for a nifty setup

[David]

Hi all,

Just thought I'd dig this one up out of the archives as it interests me to
know more on some progress, if it has been made:

> -----Original Message-----
> From: drbd-user-bounces at lists.linbit.com
> [mailto:drbd-user-bounces at lists.linbit.com]On Behalf Of paddy
> Sent: Friday, 1 July 2005 5:05 AM
> To: drbd-user at lists.linbit.com
> Subject: Re: [DRBD-user] 3 simple questions for a nifty setup
>
>
> Hi malte,
>
> I'm no expert, but I am doing similar things so I'll have a stab at
> some answers for you.
>
> On Thu, Jun 30, 2005 at 08:43:48PM +0200, ephigenie at g-house.de wrote:
> > Hello Everyone,
> >
> > I've managed to get drbd-07.11 to work with 2 sarge boxes,
> > but there a few questions open at this point :
> >
> > whenever i try to mount the drbd/0 device on the secondary,
>
> If I understand you correctly, the answer is: you shouldn't do that.
>
> http://wwnew.linux-ha.org/DRBD_2fFAQ#head-ff8f594c0d61552676dd75c5
> c2818546f737f712
>
> nevertheless I got the impression that you could ...
>
> > i get the following ...
> > but the device is not mounted (according to /proc/mounts & mtab etc..)
> > the mountpoint has no influence on that behaviour since when i try
> > an other one, it works ?
>
> sorry don't understand those last two lines (above).  Do you mean you can
> mount it okay on the primary ?
>
> > ---snipp----------
> > www-2:/# mount -t ext3 /dev/drbd/0 /ha/
> > mount: block device /dev/drbd/0 is write-protected, mounting read-only
> > mount: /dev/drbd/0 already mounted or /ha/ busy
> > ---snapp----------
>
> aren't mount(1) error message great !
>
> > the second one :
> >
> > i'd like to make some webspace ha - this should work i think
> > but as I'm lazy, i concidered using drbd for a mysql instance
> > but there's my question - are there any problems known with that ?
>
> Don't know if there are know problems but I have such a setup on
> a sarge testbed
> it seems to work (very simple testing), and I'd be happy to share
> my config with you.
>
> My assumptions are:
>
>   If drbd does what it says on the packet, then when you start
> mysql after failover
>   it shouldn't be any worse than if you restarted after, say, a
> power failure.
>
> I don't how big (or how long) problems might be if mysql do its
> equivalent of
> fsck, so that's a worry.  By the looks of it mysql replication is
> not without
> its complications and the cluster mode is out of the question for
> my application.
>
> Having said that, I plan to try out mysql replication and compare
> the results.


Paddy,

Have you made any more progress on your mysql install on top of DRBD yet? I
am interested in doing the same, but if I go the 'proper' mysql cluster way,
I need 3 machines, and I only have 2.


> Since IIUC mysql replication is asyncronous, drbd might even be
> better for some
> values of better.
>
> > for sure i will sort all writeaccess out and direct it to the master
>
> reads too, with drbd you're only going to have one mysql instance
> accessing the
> database at a time.  I've yet to figure out what happens at the
> client when the
> connection to the primary get lost, but I expect it'll be okay :)
>
> > but this seems to me more smart than using the mysql - replication
>
> I guess it depends on your load.  If you are doing a lot of reads
> then I imagine
> replication could be a big win.
>
> > Is there anyone who has this as a working setup or has
> experiences with that ?
> >
> > the third one :
> > at the moment there are only 2boxes, but a third and a forth a
> prepared ...
> > but i searched the half web for that and had nothing found -
> >
> > so is it possible to have 1 primary and 3 or more secondarys ?
> > What about choosing the next primary - can heartbeat do that job - or is
> > this just for the head-to-head config everywhere mentioned with
> 1 primary and 1
> > secondary ?
>
> my understanding is that drbd will only do 2 machines, although
> I'd be delighted
> to learn otherwise.  One of the temptations of mysql replication
> for me is the
> ability to form larger networks of replicas.

Is it possible to have more than 2 members of a resource group? I seem to
remember seeing such an example in this list sometime but I can't find it. I
think the config was something like:

resource r0 {

  on machine1 {
    device      /dev/drbd0;
    disk        /dev/hdc;
    address     192.168.1.1:7788;
    meta-disk   /dev/hda5 [0];
  }

  on machine2 {
    device      /dev/drbd0;
    disk        /dev/hdc;
    address     192.168.1.2:7788;
    meta-disk   /dev/hda5 [0];
  }

  on machine3 {
    device      /dev/drbd0;
    disk        /dev/hdc;
    address     192.168.1.3:7788;
    meta-disk   /dev/hda5 [0];
  }
}

So that, only 1 of machine1/2/3 may be primary at any given time, but the
effect is that of the device /dev/hdc being "mirrored" accross all three
PC's?

Could someone please correct me here if I have this wrong?

Regards,

David.

> There are other similar technologies that do different things.  I
> don't any
> experience with them, (so these are _not_ recommendations) but an
> ipcomplete
> list I'm sure:
>
>   GNBD/GFS comes to mind as an example of a clustered filesystem
>   where you can mount and write at more than one node.  I don't
> know whether
>   you can run multiple mysql servers against the same backend
> (perhaps with
>   a master for the writes ?), somehow I doubt it.  IIRC its in
> unstable but
>   not sarge (not stable upstream).
>
>   ENBD (looks similar to DRBD)
>
>   There's also stuff like AFS and coda.  Intermezzo looked
> interesting for a while there.
>
>   postgresql appears to have similar features to mysql.
>
> of course, its horses for courses.
>
> > thanks for your help
> >
> > malte
>
> hope this does help :)
>
> Regards,
> Paddy
> --
> Perl 6 will give you the big knob. -- Larry Wall
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