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I have an Ubuntu 12.04 server on my old laptop. Its HDD (160 GB) is too small for the file storage I need, so I'd like to attach a couple of external HDDs to it (through USB ports). The data I'll keep there is rather valuable to me, so I'd like to organize a software RAID (RAID 1) on 2 external HDDs. The only solution for this problem I found on the net is described here (and it's said that using external drives is not recommended).

How should I do it then, and what is the difference compared to installing it on internal drives? Do I need external drives that are exactly the same (model), like for a hardware RAID? Or is it a really bad idea? If so, do I need not a RAID, but a backup solution or a NAS, attached to my server?

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migrated from serverfault.com May 21 '12 at 7:13

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macguild.org/raid.html in theory you could do it with floppy drives as well –  Journeyman Geek May 21 '12 at 9:14
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4 Answers

up vote 6 down vote accepted

One thing that is often overlooked on a home setup is that you usually would be better off without raid / mirroring. Unless you have a need to protect data that is less than 24 hours old, you would usually be better off using the second drive as storage for your backups of the primary drive. That way, assuming data doesn't change much (where you could get use out of incrementals), you could be protected against both physical failure and filesystem or other soft failures.

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Agreed. If the data is valuable, you don't want RAID, you want backups. RAID for (high) availability, backups for safety. –  Dave May 21 '12 at 2:28
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You can do this. I've seen it done with up to 8 drives on usb 2.0. The performance is really rubbish, but with md it's perfectly doable.

It's mostly the performance issue that'll make you want to go internal.

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I've never come across any solutions for doing this with hardware RAID, although I'm pretty certain it could be done with Software RAID as a USB HDD in Linux is treated the same as any other mounted storage device (In fact the link in the OP proves this).

The bigger issue you will run into is bandwidth, USB ports are notoriously slow and high latency when compared to internal connectors such as SATA and SAS. This will be compounded if you end up using a USB hub as suggested in the linked article. The best advice I can give here is; use USB 3.0/eSATA/Firewire if at all possible, as all of these are at-least on the same order of magnitude as SATA/SAS. If this isn't possible, try to spread the drives out amongst the different USB hubs on your PC/Laptop to alleviate any issues of shared bandwidth (on lower end chipsets, USB bandwidth tends to be shared between ports).

However, ultimately I feel like this is more of an academic exercise, the best case performance will still be poor and I personally fail to see any use cases for this that couldn't be solved using a backup solution or internal (hardware?) RAID.

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If you just want to use a bunch of disks with some form of redundancy as a single volume, you could use greyhole - which seems perfect for your useage case - it supports redundancy and lets you add and remove volumes at will.

ZFS or btrfs might also be options with raid style volume management options but neither is as simple as greyhole

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ah greyhole. possibly the worst named good project ever :-) –  Sirex May 21 '12 at 20:19
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