Mark Watts m.watts at eris.qinetiq.com
Wed Feb 3 10:38:40 CET 2010

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

On Tue, 2010-02-02 at 20:51 -0800, Qwerty-1 wrote:
> Hi Mark,
> Thanks, what i understood that, to replicate the data locally we need RAID
> (RAID helps to decide which data need to be replicated).

Not quite.

Hardware RAID (for example) as performed by a dedicated controller, will
take disks and use them in their entirety. The Operating System will see
*one* disk per Logical Disk you create.

Software RAID can work at either disk level or partition level (where
not all of the disk is used). The Operating System will see all of the
component disks, along with a "RAID device" which you actually format,
mount and store data on.

In both cases the replication is automatic and total. You don't get to
choose whether data is processed or not.

RAID is mostly used by folks who want to mitigate the risk of a disk
failure. Some RAID levels use more than one disk and as a side-effect
they may provide more performance.

> But if we want to
> replicate the date over network then we need DRBD with RAID. In later case,
> DRBD will help to transfer data over network (other machine) by making TCP
> connectiong and RAID will help to decide which data needs to be replicated.
> Please correct me if i am wrong.

DRBD is another layer in the storage stack, responsible for ensuring
that any data written to a DRBD block device is replicated to a remote
node, thus allowing for certain types of high-availability and/or

For example, you might have the following (ignoring boot partitions):

/dev/sda        # disk #1
/dev/sdb        # disk #2 
  /dev/md0      # Linux software raid-1 mirror
    /dev/drbd0  # DRBD Replicated filesystem

You would then create your filesystem (ext3 or whatever)
using /dev/drbd0.

Assuming you've setup another node in a similar way, and DRBD is working
correctly, any data you write to /dev/drbd0 will be both replicated to
the remote node, and also mirrored on the local disks.

In this situation, RAID will be protecting you against disk failure, and
DRBD will be protecting you against machine failure.

http://www.drbd.org/users-guide/ is the best place to learn more about



Mark Watts BSc RHCE MBCS
Senior Systems Engineer, Managed Services Manpower
QinetiQ - Delivering customer-focused solutions
GPG Key: http://www.linux-corner.info/mwatts.gpg
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