[DRBD-user] meta-disk

Chip Burke cburke at innova-partners.com
Thu Jan 5 15:45:06 CET 2006

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

Exactly. I was trying to replicate /boot with 'meta-data internal' which
would cause it to die being that the space wasn't available. I am slowly
getting my head around the whole process.

However, I can't seem to mount the drbd device. I edited my fstab from 

/dev/hda2 / ext3 defaults 1 1


/dev/drbd0 / ext3 defaults 1 1

and it does not like it. When I try to start drbd manually using 'service
brbd start' or 'drbdadm up all', I get an error that /dev/hda2 is already
mounted. So I am a bit confused as to how to actually mount the /dev/drbdx
device. Essentially I want the root file system to be identical between two
boxes for a failover situation. Am I headed in the right direction?

Thanks again for everyone's help. Once I am done with this, I am very
tempted to write a new HowTo for real novices such as myself.

Chip Burke

-----Original Message-----
From: Clive Eisen [mailto:clive at serendipita.com] 
Sent: Thursday, January 05, 2006 9:03 AM
To: cburke at innova-partners.com
Cc: drbd-user at lists.linbit.com
Subject: Re: [DRBD-user] meta-disk

Only if you told drbd to put the meta data on /boot

Not A Good Idea (tm).

Yes you mount the /dev/drbdx devices

If you are using linux-ha, you allow that to do the mounting.

Chip Burke wrote:

>Thank you very much for the very complete answer. I think I know what the
>issue is then. I have a separate /boot partition and it is only 100MB. The
>fact that it is only 100MB would not allow a 128MB file to be created. That
>makes sense. 
>My next question is, after read the linux-ha docs, it hints at this, but is
>not very explicit... In fstab am I to mount the new /dev/drbdx devices that
>are created instead of the normal /dev/hdx devices? 
>Thank you so much for your help,
>Chip Burke
>-----Original Message-----
>From: drbd-user-bounces at lists.linbit.com
>[mailto:drbd-user-bounces at lists.linbit.com] On Behalf Of Chris Procter
>Sent: Thursday, January 05, 2006 6:00 AM
>To: drbd-user at linbit.com
>Subject: Re: [DRBD-user] meta-disk
>On Wednesday 04 January 2006 20:07, Chip Burke wrote:
>>I think I am missing something very basic. Do I need to create a partition
>>on the system specifically for the meta-data or something else? I have
>>the drbd.conf notes about 10 times and I really don't understand what they
>As with all good questions the answer is "it depends".
>DRBD needs to store 128MB of metadata some where on the system to store the

>list of which blocks have changed (its stored on disk so it persists across

>reboots). This metadata can be stored in one of two places.
>It can be stored on the partition you are replicating (termed "internal" 
>metadata) in which case it basically takes over the last 128MB of the 
>partition, reducing the space you have to store data in and overwriting and

>data that is already stored there. This is good if you are starting fresh
>can overwrite the diskspace without risking the data or dont have room to 
>create a separate partition. 
>For this you put:
>meta-disk internal;
>in the drbd.conf for each machine (the "on [hostname]" section).
>Alternatively you can store the metadata on a separate partition
>metadata) in which case (obviously) you need to create a partition to use. 
>This partition needs to be at least 128MB for each DRBD device you are
>and is setup in the drbd.conf file as: 
>meta-disk [devicename] [[offset]];
>meta-disk /dev/hde5 [0];
>The number in brackets is an offset within the partition, so the above will

>use the first 128MB of the /dev/hde5 partion to store meta data, you could 
>then set up another drbd device (/dev/drbd1 say) with 
>meta-disk /dev/hde5 [1];
>which would use the second 128MB (obviously the partition would need to be
>least 256MB in size). You would then have the meta data for /dev/drbd0 in 
>0-127MB and for /dev/drbd1 in 128-256MB on /dev/hde5
>There are probably performance reasons for using one over the other, and
>are certainly issues with diskspace (if you dont have space to create a 
>seperate partion the internal is the way forward for example). In theory
>could configure drbd to use internal metadata on one machine and external
>the same device on the other machine. If you do this however you're on your

>own :-)
>Your error message implies that the meta-disk you are using is less then
>in size. I'm not sure if it defaults to something if you dont define it in 
>drbd.conf so its worth checking to make sure it is explicitly defined to 
>something sensible.
>Thats the way I understand it anyway, hope it helps.
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>drbd-user at lists.linbit.com
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>drbd-user at lists.linbit.com

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