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vaibhavs 06-03-2009 05:58 AM

2 Identical Servers : Live Backup

I have 2 identical servers (identical MBD, Processor, RAM etc).
Both have FC9 installed.

What are my options to do a LIVE backup of Primary server on to the Secondary server.

Currently I use G4L (Ghost4Linux). But for this, I need to down both server. Remove the HDD from Sec server and install it in the Pri and then boot from G4L CD and then run the backup. For a 160GB HDD, it takes 155 minutes.

Next idea, which I have not used, is do a Folder by folder RSync of the full HDD.

Are there other tools which can help me do a full HDD backup without power-down ?


irishbitte 06-03-2009 06:32 AM

rsync is really your best bet, simply because once the first backup is made, every backup after the first is an 'incremental' backup, therefore your live backup time is alot less.

vaibhavs 06-03-2009 06:55 AM

When doing a full HDD sync, there are some files & folders which I should *NOT* rsync like :
ifcfg-eth0, /etc/sysconfig/network etc

Any other files which I *SHOULD NOT* rsync.


JulianTosh 06-03-2009 07:02 AM

Have you looked into iSCSI?

I've never used it, but it's a clustering technology that might help you get'r done.

JulianTosh 06-03-2009 07:06 AM

irishbitte's suggestion allows for exclude lists so you wont overwrite system specific files. It won't work well for "live" files such as databases though. iSCSI will replicate a file system while it's in use.

If you're just concerned about replicating relatively static files, rsync is a great answer. If you're trying to replicate any kind of files that are constantly changing, iSCSI is something that needs to be looked at.

vaibhavs 06-03-2009 07:25 AM

Well I need some aspects of both toola.

I would love to replicate the entire HDD, but the problem is the IP Address, MAC address, Hostname etc will also be replicated which is not good. I will have 2 PCs with same IPs, MACs, Hostname etc which will cause conflict.

Any idea how other Sys Admins handle this ??

I have 2 plans in mind:
Use RSync for Daily or hourly backup and use iSCSI (or something similar) on weekly basis.
Bt how do I handle the IP, MAC, hostname duplication issue ?

Please advice what others are doing for backup.

BTW: I could not find much info on iSCSI on google.
Pls help me with the url of the project.


vaibhavs 06-03-2009 07:28 AM

I just did some reading on iSCSI.
Its a bit complicated for my level of tech skills :(

Need something simpler like g4L but with "Live Backup".
Any other options ?


JulianTosh 06-03-2009 07:34 AM

iSCSI is not something you run once a week. It replicates disk writes as they happen in real-time.

vaibhavs 06-03-2009 07:38 AM

ah! A kind of a Raid between 2 PCs ?

JulianTosh 06-03-2009 07:50 AM

Yes. Think of it as tunneling SCSI disk writes through an IP tunnel. You can't get around having to maintain a unique OS for machines in a cluster - as far as I know. As long as both machines need to be "on", they they need their own unique configuration.

iSCSI is meant to be a poor man's SAN and replicate a separate-from-the-OS partition to a remote machine.

How you would implement this in your scenario:

1) Rebuild your two machines with the drive split up into an OS and data scheme.
2) Use iSCSI to replicate the data partition(s)

vaibhavs 06-03-2009 07:56 AM

Thx for the detailed explanation.
I understand this better now.

But what if there is a HDD failure of the main server which has the OS ?

okcomputer44 06-03-2009 07:59 AM

Just an idea.

What about the read only flag on the important files.

I mean make the first full sync to the another PC.
Then make all the important files under /etc/sysconfig, /etc/hosts, /etc/bind, etc.....(whatever you need) just to read only.(Take off the writable flag)

After that the rsync wont be able to rewrite the read only files.


JulianTosh 06-03-2009 08:15 AM

There are many methods for disaster recovery. Each one has pros and cons and specific purpose.

If you need to have an "instant on" disaster recovery process, then RAID0 or an off-line disk replication process is your answer. With those solutions, you have pros and cons:
1) RAID0 is "expensive" because you have to purchase twice as many drives. It's quick and painless because the backups are instant.
2) Backups are cheap as you just move the data off to some other media. cons: It's an administrative pain in the butt to restore the files. Time consuming depending on the type of data you're backing up. Can be easy or difficult to restore the data in the event of a disaster, depending on what method of backup you use - tar/rsync vs. disk imaging

It sounds to me like you should just take the drive out of one machine and use the RAID0 system. The rest of the other computer could be used for spare parts, perhaps.

irishbitte 06-03-2009 06:19 PM

Jeez, this thread took off in my absence! Anyway, if you are not replicating databases, what you do when setting up rsync is you specify the files and directories that you do want backed up, rather than those you don't.

For replicating complete snapshots, perhaps you should consider Logical Volume Management or LVM: is an idea of how this is done. I suggest testing testing testing if you are gonna use this method, but when it's working it's fantastic. An LVM snapshot can be taken at any stage, even as the system is live. The snapshot can be restored on another server, and if the main server goes down, the backup can be started almost immediately.

JulianTosh 06-03-2009 06:24 PM

Thanks, I'll have to check that out too!

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