AW: [DRBD-user] Replica of existing ext3 fs

Martin Bene martin.bene at icomedias.com
Fri Mar 26 09:03:28 CET 2004


> 1. My drbd.conf has:
> 	sync-min    = 500k
> 	sync-max    = 1M
> 
> And I need to replicate a 10Gb fs via a 100Mbits link. Is 
> there some kind of formula to estimate the time needed for a 
> first sync?

The sync-max of 1M limits drbd to using about 1/10 of your 100Mbit link;
this will dramatically slow down your resync. If there's nothing else on
the link, you'll want to set that above the max physical throughput of
the medium, say 20M. If it's a shared link you'll want to reserve some
bandwith for other applications, but hopefully you'll still be able to
give more than 1M to drbd. 

Also, be aware that the sync-max will only aply to resync on connect -
during normal operations drbd will use whatever bandwith is available.

When using the full bandwidth of a 100M link, you'll usually get drbd
throughput of 10-12MB/sec, so your 10G resync would take ~ 15 minutes.
With your setup using only 1M, synctime would obviously be about 10
times that.

These times are assuming no other IO on the drbd device during resync -
drbd resync will reduce ist throughput to allow other applications
reasonable access.

> 2. What's the best solution: (to avoid downtime, loss of data, ...)
> 
> 	1. Umount the fs, setup drbd, remount using new device?
> 	2. Full backup, create a new fs on top of drbd and restore?

Drbd 0.6.x doesn't put any metadata on the device, so there's no reason
to do a backup/restor cycle provided the device size on the 2nd machine
is at least as big as your existing device. As a best solution I'd
recommend 
	3. Full backup, unmount fs, setup drbd, remount using new
device.

Bye, Martin




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