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Using a USB external HDD as a mirror

Posted on 2004-09-29
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Last Modified: 2013-12-16
I have a RH Linux  6.2 server that has two internal IDE HDD.  HDD-1 is 60bg and HDD-2 is 140gb.  I have purchased two usb external HDD that are equal or greater storage capacity.  I would like to use the usb drives to mirror each of the internal drives.  The server chassis does not have any open bays for placement of an internal IDE drive, this is why I am having to use USB external drives.  The server MB does have 2 open usb ports, but to be honest they have never been used.  I only assume they are enabled in the bios.  If not, I can do that.
(1) can this be done? if so,
(2) what is the procedure for getting the server to mirror the drives, such as commands, editing the specific files, mounting the usb drives, etc.
Please be very specific with answers as I am, by no means, a linux person, but do have some experience.  Windows is by choice. Anyway, I will thank all in advance for any detailed answers posted.  Mucho points for this.
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Question by:EaglePress
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by:blkline
ID: 12182316
When you say mirror, do you mean a bootable mirror?    Please explain exactly what you are trying to accomplish and we'll go from there.

Barry
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by:EaglePress
ID: 12183354
Mirror drive. RAID 1
A seperate drive of equal storage capacity that is a redundant drive for the main IDE drive.  Data writes/reads from both devices at the same time.  A RAID-1 device (e.g., /dev/md0) maintains an exact copy (mirror) of the files in a given partition (e.g. /dev/hda2) on a separate partition (e.g./dev/hdc2). The Linux RAID code mirrors partitions, not entire disks. The partitions that make up a RAID device set should be on separate hard disks.
maybe this helps.  But, yes the mirror drives will be attached all the time, and must mount upon boot.
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blkline earned 2000 total points
ID: 12184117
Okay.

To start with, I'm not sure why RAID-1 is not acceptable because if you are mirroring all partitions on both drives then you have everything you need for an exact copy.  In fact, if the drives are identical and are partitioned identically then you have, in essence, an exact duplicate of your drive.     The FS type for a software RAID partition is fd and for Linux ext2 is 83.  That's about the only difference.

To enable software mirroring on an already active system is not a problem but it can be somewhat challenging the first go but I've done it a number of times.  What I don't know, however, is how far advancedthe code for that is in RH6.2.    The boxes I did it with were 7.3 and higher.    You'd have to check on it.

One thing I'm fairly sure of and that is USB support is minimal, at best, in RH6.2.   So I'm not even sure that you can do much with those drives on the a stock kernel.  You didn't mention which kernel you are running.

Another option would be to use a hardware RAID controller but I think it's probably out of the question for you as you said you didn't have the space.

Barry

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