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The machine in question is equipped with 4x ssd, and running Debian wheezy 7.8, linux kernel 3.2.0-4, mdadm v3.2.5. Currently I'm using a RAID10 consisting of all these ssd. I've to increase the storage capacity, ssd are to expensive for this, and so thought about getting rid of two ssd, instead putting two hdd inside, and splitting the RAID10 into two RAID1, one made out of the ssd, the other out of the hdd.

I've searched the Internet a lot these past days, but not really found anything useful regarding this. Could someone tell me how to convert a RAID10 into one (or two) RAID1?

I'm quite unsure if this is possible at all, because, for example, Neil Brown, one of the mdadm developers, wrote 2009 [1] (which is a while back, sure) about reshaping:

RAID0 and RAID10

These arrays cannot be reshaped at all at present.

Could someone tell me if this is possible nowadays and if so, how to do this?

Thanks in advance!

[1] http://neil.brown.name/blog/20090817000931#4

Edit 1: At [2], someone was asking if converting raid10 to raid0 is possible, getting three answers, one of them being "Yes.", one being "No." - quite confusing.

[2] Can I convert a raid10 array to raid0?

Edit 2: Using mdadm /dev/md0 --grow --level=1 (in a virtual test machine) gives mdadm: RAID10 can only be changed to RAID0. So it seems the way to go is converting to RAID0, and afterwards to RAID1; but don't like this idea really, because it's quite risky... Anyone knows of a better, safer, process?

Edit 3: It's not possible to convert a RAID0 to RAID1, trying to do so gives mdadm: Impossibly level change request for RAID1. So it seems one would then have to create a new array with level 1, made up of the two devices currently serving the RAID0.

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  • Backup/restore?
    – hookenz
    Feb 10, 2015 at 18:39
  • I don't think converting RAID10 to RAID1 will ever be possible that way, as it will always break a stripe into 2 or more separate parts. You're essentially willing to fit a capacity of n times RAID1 onto a single RAID1. Backup & restore are your only option. Feb 19, 2015 at 20:17

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I don't know in general but in your particular case you can't reshape because mdadm can't fit the size of your RAID10 device in any RAID1 device built from your disks. Let's say that your SSDs are 100GB each. This means that your RAID10 device has a size of 200GB (your data maybe a lot less but note that mdadm works on a device level not filesystem level). RAID1 devices built from 100GB disks can be at most 100GB in size so reshaping is out of the question.

The sad news are that you must add your HDDs, configure them in RAID1, copy all the data there, reconfigure the 2 SSDs in RAID1 and finally move some of the data from the HDDs RAID to the SSDs RAID.

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