Is it possible to read the data on a disk removed from a RAID1 array?

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doktor
doktor Posts: 14  Freshman Member
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This is a rather theoretical question, although I have a practical application in mind.

So, let's say we have a healthy RAID1 array consisting of 2 identical disks. We shut down the NAS and remove one of the 2 disks. What will happen?

1. If we power the NAS up, with only one disk in place, will it still work?
2. Is there a way of connecting the removed disk to a computer to access its data (read only, we don't want to write anything to it).
3. If we put the removed disk back into the NAS, will everything still be OK? Will it take some time to rebuild the array?

The practical application I have in mind is this: when upgrading the capacity of an array it is recommended (as always) to backup the data first. BUT, since the first step of upgrading is to remove one of the array's disks, if the stored data on the removed disk is accessible, then no backup is really required! All the data is in our hands already!

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  • Mijzelf
    Mijzelf Posts: 2,639  Guru Member
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    The whole idea of raid1 is that the data will survive a failure of a single disk. So when you remove one disk the NAS will boot fine, and the data will  be accessible. And you can also mount the removed disk in another system. Any Linux system will do.

    3. If we put the removed disk back into the NAS, will everything still be OK? Will it take some time to rebuild the array?
    Depends. Each member of the array keeps track of the number of changes made, and which time the last one happened. If those numbers are not the same, the array will not assemble, and you'll have to rebuild. (Which amount of time is only dependent on the size of the disk, and not the amount of data in the filesystem, as the raid manager doesn't know anything about the filesystem, and so always copies the whole partition, including unused space.)
    So if you mounted one half ro, and one half rw, I think they won't 'fit' anymore, as the rw is changed.

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  • Mijzelf
    Mijzelf Posts: 2,639  Guru Member
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    The whole idea of raid1 is that the data will survive a failure of a single disk. So when you remove one disk the NAS will boot fine, and the data will  be accessible. And you can also mount the removed disk in another system. Any Linux system will do.

    3. If we put the removed disk back into the NAS, will everything still be OK? Will it take some time to rebuild the array?
    Depends. Each member of the array keeps track of the number of changes made, and which time the last one happened. If those numbers are not the same, the array will not assemble, and you'll have to rebuild. (Which amount of time is only dependent on the size of the disk, and not the amount of data in the filesystem, as the raid manager doesn't know anything about the filesystem, and so always copies the whole partition, including unused space.)
    So if you mounted one half ro, and one half rw, I think they won't 'fit' anymore, as the rw is changed.

  • doktor
    doktor Posts: 14  Freshman Member
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    " Any Linux system will do."

    Is there maybe a utility that gives a Windows 10 PC a way to access the disk? What I have in mind is putting the disk into a USB HD controller and connect it to the PC through USB.

  • Mijzelf
    Mijzelf Posts: 2,639  Guru Member
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    I have no experience, but it seems (non-free) UfsExplorer will do the trick. But how about Linux in a virtual machine?
    What I have in mind is putting the disk into a USB HD controller and connect it to the PC through USB.
    There might be a problem with that approach. Some USB-Sata convertors do sectorsize translation for disks >2TiB. As the partition table addresses in sectors, that means the start and ends of the partitions do not fit anymore. Maybe UfsExplorer can handle that. In Linux (when new enough) you can create a loopdevice with different sectorsize to solve that.

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