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Saturday, September 18, 2010

Linux software RAID and mdadm

Similarly, we can stop RAID devices, fail them, assemble them, and so forth. Mastering the mdadm utility takes time, but it gives you a lot of power and freedom when it comes to using Linux software RAID.
Fail/remove device
If you want to break apart a RAID, you will first have to fail its members and then remove them. On active partitions used by the operating systems, this may not be possible in-vivo and will require a reboot.
And when we print /proc/mdstat, we can now see that our targeted RAID device no longer uses both its members. The information in square brackets has changed. We only have one used device, with the other set as faulty. [2/2] now shows [2/1] and [UU] now shows [U_], indicating the second device (listed on the right) is no longer being used, as it has been set as faulty.
In reality, this might happen should the disk die or a partition get corrupted for some reason.
After that we can remove it ... and then, for fun, re-add it. Here's a screenshot showing the recovery progress after sdb1 was re-added. The RAID device is being rebuilt, with the data from sda1 being cloned over to sdb1.
man page for mdadm.
In general, there are several things you should consider before deploying RAID. These concern the cost, safety, performance, but also software compatibility.
You may need to check whether your imaging software, if you're using one, that is, is compatible with RAID devices. Just like when we configured partitions with GParted, we had to manually change the Inode size for our Ubuntu installation to make the partitions compatible with Acronis True Image software, which requires Inode size of 128 bytes.
With RAID, similar adjustments may be required.
Make sure you can use your backup and rescue tools effectively and reliably on systems deployed on RAID devices. Make sure you do not create impossible setups where you have no ability to change things save for a complete reinstall.
Working with RAID is an advanced task, but it can be managed.
Alongside the knowledge you have gained on working with the GRUB bootloader and GParted partitioning software, the RAID tutorial should give you the skills needed required to safely and with confidence wade into the waters of Linux system administration.
Have fun.

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