[DRBD-user] Any way to jump over initial sync ?

[David Bruzos]

Hi,
 I have lots of experience skipping the initial sync with ZFS zvols and drbd 8.4.x.  I have been using the "drbdadm -- --clear-bitmap new-current-uuid <RESOURCE>" for years and never had a problem.  This is why it works:
 
 1. New ZFS volumes are guaranteed to return only zero data for unwritten blocks, so two new volumes are always in sync, if they have not been written to.
 2. Also, if you have a VM base image on two hosts, you can clone the volumes in the image on each host and also skip the initial sync, because both clones will be identical.  Of course, I am asuming that the base image was replicated via ZFS streams.
 3. LVM volumes and most other volume types will not work well, because they don't guarantee new volumes to be zero-filled.  However, depending on your use case, it is often better to zero-fill your volumes manually (E.G. cat /dev/zero >/dev/vg/vol0) and skip the sync.  It does not seem reasonable, but given the storage and network characteristics at play, it could be much much better than doing an actual DRBD sync.
4. If in doubt, run a drbd verify until you feel confidence in your process.

* Typical way to skip the sync (at least this is my proven method):

    # drbdadm create-md <resource>  -  (do this on both nodes)
    # drbdadm up <resource>  -  (do this on both nodes)
    # drbdadm -- --clear-bitmap new-current-uuid <resource>  -  (do this on secondary node)
    # drbdadm primary <resource>  -  (do this on primary node)
    # cat /proc/drbd  -  (Enjoy!)

    In my opinion, having to replicate multi-tb volumes is an incredible waste of time and resources, if it can be safely avoided.  I've talked to many people that patiently wait while their giant 4 TB VM volumes do their initial sync and hog their environment's I/O in the process...

I hope this helps you and others out there who are looking for a better way...

David

-- 
David Bruzos (Systems Administrator)
Jacksonville Port Authority
2831 Talleyrand Ave.
Jacksonville, FL  32206
Cell: (904) 625-0969
Office: (904) 357-3069
Email: david.bruzos at jaxport.com

On Tue, Aug 28, 2018 at 04:18:54PM -0400, digimer wrote:
> On 2018-08-28 12:59 PM, Julien Escario wrote:
> > Hello,
> > Just wanted to know : is there a way to get rid of initial sync with linstor and
> > zfs backend ?
> > Right now, I have a 1TB volume to create and initial sync is veeeery long.
> >
> > I think it's mostly due to unavailability of thinly provisionned ZFS resources
> > but perhaps is there a way to suspend resync and ask the system to simply
> > consider both resources as sync'ed ?
> >
> > Best regards,
> > Julien
> 
> In 8.4, you could do 'drbdadm new-current-uuid --clear-bitmap <res>'. Be 
> aware though; The bits won't match until/unless a given block is 
> written. So a 'verify' will be problematic.
> 
> digimer
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> drbd-user at lists.linbit.com
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