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Unfortunately my internal hdd is probably going to die soon and I dont have money for a new one. But I have an external Hdd which I was using as a backup, so I thought of a solution. RAID

But I have some questiones, because I would like to get it right at the first place.

my internal Hdd      Extrernal Hdd (USB 2.0)                   
Seagate              Seagate
500 GB               1 TB       
5200 rpm             7200 rpm        

I want to mirror internal hdd on extrenal hdd and use the remaining 500 GB on external for backup (software raid 1) so that when my disc finally dies I will have another up to date disc ready.

first of all:

1) is it even possible? I have been studying about raids for some time and didnt found why this shouldnt work (just that the slower hdd will be bottleneck, but I dont care about that) But than I read somewhere that internal hdd is "dynamic" and external not, so it cant be used for booting up system (I dont really what is this about)

If yes than

2) do the partitiones in the raid need to be at the exactly the same place? I know they have to be the same size, but will it matter if I will make raid from eg /dev/sda1 primary part. at the beggining of disc and /dev/sdb5 logical part. at the end of disc (eg I wouldnt do that, just because I also want to have my win partition on internal hdd, but I dont want to raid it too. It has about 20 GB just for games and I dont want to put it at the end of disc, because of disc head travelling, I will rather have static data (movies, music, images... ) at the end of disc (I tried it in VBox and it worked fine)

3) is there any difference between having one big raid partition with lvm on top of it vs having separated raid 1 for every partition? (will lvm adds to cpu usage? latency? will be many raid partitiones a little safer than having one partition?)

EDIT: I found out that I can do cfdisk /dev/mdX and create another partitiones, so that would probably be the best solution, right? (also I am running gentoo linux)

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2 Answers 2

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  1. I think it could theoretically be possible, but it's going to be horribly slow. If your machine can boot from USB, you should be able to boot from an external disk, but that can be thrown off by additional usb disks (that steal the disk label before it gets it).

  2. Not in linux software raid. It can affect performance, but there is no need for it to be on the same place on the disk.

  3. It might be an issue if you have one single RAID partition and have bad sectors, but this is just a theory on my part.. I tend to make multiple smaller RAID partitions, as errors on that partition won't take out one half of your raid partition, just the part with errors.

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1) Is it even possible?

Yes, it is possible, but not trivial.

I have been studying about RAID for some time and did not found why this should not work (just that the slower HDD will be bottleneck, but I do not care about that)

The USB bus will be the bottleneck. An average speed for USB2 is about 30 MB/second. Both disks are likely to be much faster than that.

But than I read somewhere that internal HDD is "dynamic" and external not, so it cant be used for booting up system (I do not really what is this about)

A dynamic disk as in this ms way? On Gentoo?

If yes then

2) do the partitions in the RAID need to be at the exactly the same place?

No. Mdadm will happily create a mirror with two volumes as long as both are the same size.

3) is there any difference between having one big RAID partition with lvm on top of it vs having separated RAID 1 for every partition?

Both methods will work.

Creating a mirror spanning the whole disk is the simplest solution. Creating several smaller mdX devices is more work, but can be more flexible.

(will lvm add to CPU usage? Latency?

Any extra layer will add to latency.

I am not sure if that is significant or not when you also use a USB drive.

will be many raid partitions a little safer than having one partition?)

No. If the disk fails it fails. It does not care what is written on it, or how you see that data on it.


You did not ask how to do it, but it is probably best to:

  1. Get a backup all all your data on a disk you are not adding the the RAID. This will might require a third disk.
  2. Create the mirror on the external drive. Use the missing option to skip the internal drive.
  3. Format your volume(s) on the new volume on your '1 disk / mirror / degraded`
  4. Copy all the data to it.
  5. Test if it works. (Possibly discover that the bootloader needs tweaking)
  6. Add the internal HDD to the mirror.

If you are unsure about any step, first make a full disk backup. Clonezilla is a free tool which can do this.


Lastly, before you do all this:

  • Are you sure the disk is dying? Did you check the SMART values? Does the number of reallocated sectors increase?
  • If it is failing, would it be easier to just replace the HDD rather than using a second drive?
  • If you can not afford a second drive, consider swapping the drives (e.g. use the faster external drive in the computer and move the possible failing slow drive to the USB2 case.
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A dynamic disk as in this ms way? On Gentoo? - yes I was confused what were they talking about. Yeah thanks for the advices, I already planned how to do it, but I didnt thought about replacing the harddrivers, that would be a great idea, but I am afraid it wouldnt be possible (I have laptop (which I am practically using as desktop) and the external hdd is pretty big, and I am big noob in anything hardware-like) so I thought about a) using only external hdd over usb b) use them both in raid –  zahjin Jun 12 '13 at 17:30
    
and yes, SMART said it is time to replace the hdd, and it sometimes does that click noice when it moves its head to the remapped spare sector, but despite that it is still working (for a few months now) I will try zero the whole disk, mayby it isnt really dying and I am just really sensitive about the disc sound in quiet room, and that would help) –  zahjin Jun 12 '13 at 17:37

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