Disk cloning redux

Kirchner, Kenneth W CTR USA NETCOM/9TH SC A ken.kirchner at us.army.mil
Tue Feb 3 16:51:12 CST 2009


Sounds like you just played a digital form of Russian roulette. :-)

Although, you did say 100% useable and not 100% reliable... I think I would at least drop to init level 1 if the server had any data I had any concern over.

Glad it worked for you, though.

Ken
-----Original Message-----
From: linux-poweredge-bounces at dell.com [mailto:linux-poweredge-bounces at dell.com] On Behalf Of Sid Young
Sent: Tuesday, February 03, 2009 3:25 PM
To: linux-poweredge at lists.us.dell.com
Subject: RE: Disk cloning redux


Just some more thoughts on this, we recently DD'd our live production system to our Mirror server (identical hardware and disk config), then reboot onto a CD rescue disk to do an e2fsck on the mirror box. After that we changed the IP config to give it back its original IP and a clean reboot made it 100% useable.

Sid


dd will defenetly not work in your case, since you are cloning a live/online system!

**** dd will sequentially read you device and write it to another device. ****

Scenario: 
dd have passed 100MB into the source device, then your application update/write to data 50 MB into the device. The updated information will never be replicated, and you have an unconsistent and useless filesystem on your target device.

To use dd you would have to take the system offline, unmount the filesystems to make sure no changes are made. Remember that even mounting the partition read only updates the information about when filesystem was last mounted.

You have 1 option and I doubt you have the functionality needed...

1. Disk Controller system or LVM system that can do a point-in-time SnapShot, then track changes since the point-in-time SnapShot. 
This would be the process:
- Flush you filesystem buffers.
- Take a point-in-time snapshot of source drive.
- Replicate snapshot to target drive.
- Once replicated flush filesystem buffers again.
- Use tracked changes information to replicate changes to target drive.
- Replicating the changes should be faster this time and, after repeating the step n number of times you should be able to take source filesystem offline and do a quick last transfer of changes and get to a fully synchronized state.

If you only need to clone the system once I suggest you take the system offline for maintenance, do the cloning. 

But remember! 
What use is that clone to you in 2 weeks? Or 1 month?

If you have to take the system offline again in once every few weeks or months to make sure you cloned data is recent enough to meet your DR recovery point objective this is probably not going to be the route to go anyway.


I would recommend going back to the thinking box and figure out What do I want to protect against? 
- Drive failure? (In this case, get RAID Mirroring)
- DBA having a bad day and drops the wrong SQL table, someone delete a file by mistake? (Use Backup or/and SnapShots or/and clones) Etc.


--
Harald Jensås






_______________________________________________
Linux-PowerEdge mailing list
Linux-PowerEdge at dell.com
http://lists.us.dell.com/mailman/listinfo/linux-poweredge
Please read the FAQ at http://lists.us.dell.com/faq

-----------------------------------------------------------------------------
General Notice and Disclaimer:
GXS, Access Business Information, UrbisPro, RealtyPro, WebMap, SearchAus, Inspex, Aussearch, OnlineLegal, Open Practice and OP Solutions are wholly owned entities of GlobalX Information Services Pty Ltd (GIS) ABN 00 073 436 414. GIS believes that the information contained in this document is accurate when issued. However, e-mail transmission cannot be guaranteed to be secure or error-free as information could be intercepted, corrupted, lost, destroyed, arrive late or incomplete, or contain viruses. Therefore GIS does not warrant that such information is accurate, reliable, complete or up to date, and, to the fullest extent permitted by law, disclaims all liability of GIS and its Associates for any loss or damage suffered by any person by reason of the use by that person of, or their reliance on, any information contained in this document or any error or defect in this document, whether arising from the negligence of GIS or its Associates or otherwise.

Confidential Communication:
Should this email contain confidential within the subject line, then this e-mail message and any attachments contain copyright material of GIS and/or information that is confidential and subject to privilege. If you are not the intended recipient of this e-mail, please contact GIS immediately by return e-mail or by telephone on 1300 362 585 or +61 2 9429 8900. In this case, you must not read, print, disseminate, copy, store or act in reliance on this e-mail or any of the accompanying attachments; and you must destroy all copies of this e-mail message. This notice should not be removed


_______________________________________________
Linux-PowerEdge mailing list
Linux-PowerEdge at dell.com
http://lists.us.dell.com/mailman/listinfo/linux-poweredge
Please read the FAQ at http://lists.us.dell.com/faq



More information about the Linux-PowerEdge mailing list