[DRBD-user] Changes from lower level block device

Digimer linux at alteeve.com
Tue Feb 7 13:51:22 CET 2012

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


On 02/07/2012 03:16 AM, Felix Frank wrote:
> Hi,
> 
> On 02/07/2012 08:36 AM, Jaap78 wrote:
>>
>> Hello,
>>
>> Normally DRBD writes changes from device /dev/drbd[x] to a lower level block
>> device (for example /dev/sda) and then submit it to the secondary node.
>>
>> Does DRBD also submit changes to the secondary node, if the change came from
>> the lower level block device (/dev/sda)?
> 
> no, it cannot possibly do that. It's crucial that the backing device is
> not touched once DRBD is operational. Otherwise you won't ever find out
> what blocks have diverged by writing through this "bypass".
> 
>> I am looking for a way to migrate a 8TB SAN to SAN storage server.
> 
> So your idea is to add DRBD metadata to your existing SAN and have any
> changes replicated from then on?
> 
> I fear there is no way to do this. You will need a downtime, but it
> should be possible to keep it as short as a couple of minutes:
> 1. add DRBD as you seem to be planning
> 2. shut down your services, unmount
> 3. do drbdadm create-md, go primary and bring the drbd up
> 4. mount (drbd instead of backing storage)
> 
> Your downtime comprises step 3, and can be done rather quickly (a bit of
> practice doesn't hurt for this).
> 
> This way you opt to set up a standalone primary at first. You can then
> take your time for setting up the peer. If you need to keep the initial
> sync short as well, some more planning is in order, so that ideally you
> can do some variation of truck based replication.
> 
> HTH,
> Felix

To clarify or add a bit to Felix's comments;

The meta data requires a certain amount of free space (the exact amount
can be calculated using the formula on the DRBD website docs). This will
required adding a bit of physical space, if possible, shrinking the FS a
certain amount, again if possible, or adding a new disk and configuring
external metadata.

-- 
Digimer
E-Mail:              digimer at alteeve.com
Papers and Projects: https://alteeve.com



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