Linux – Which two disks can be safely removed from raid 10

linuxraid

which two disks can I remove from RAID 10 without breaking the array?

near-copies means that there is raid like this?

raid0(raid1(sdaX, sdbX), raid1(sdcX, sddX))

= sdb and sdd can be removed

cat /proc/mdstat

Personalities : [raid1] [raid10]
md2 : active raid10 sda3[0] sdd3[3] sdc3[2] sdb3[1]
      19550976 blocks 64K chunks 2 near-copies [4/4] [UUUU]

md1 : active (auto-read-only) raid10 sdc2[0] sdd2[3] sda2[2] sdb2[1]
      7823488 blocks 64K chunks 2 near-copies [4/4] [UUUU]

Best Answer

Given that you have 4 drives and are using near copies your RAID10s degenerate into RAID1+0, i.e., a stripe of mirrors. See, e.g., this posting which is straight from the horse's (Neil Brown's) mouth.

However, for your RAID10s the drives are ordered differently:

 md2: sda3[0] sdb3[1] sdc3[2] sdd3[3]
 md1: sdc2[0] sdb2[1] sda2[2] sdd2[3]

This means you could lose sdb and sdd and be OK but you could not lose any other combination of 2 drives.

Related Solutions

RAID1 – How to Fail a Drive Marked as Removed

You shouldn't need to fail them. Since they should have already been failed when you first noticed the issue and the RAID members are now removed. There are just a few steps to get it back up and running.

Setup partitions on the replacement disk. These partitions should be identical in size to that of the failed and currently active disk, and should be marked as partition type "Linux RAID Autodetect" (0xFD). You can simplify this by copying the partition table with sfdisk.
```
sfdisk -d /dev/sdb | sfdisk /dev/sda
```
If the disk has been used before then you may want to ensure that any existing softRAID information is removed before you begin again.
```
mdadm --zero-superblock /dev/sda
```
Install an MBR onto the new disk so that it is bootable. Do this from the grub shell. Assumes that /dev/sda is the first disk.
```
root (hd0,0)
setup (hd0)
quit
```

Add new partitions back to the arrays.

mdadm --add /dev/md0 /dev/sda1
mdadm --add /dev/md1 /dev/sda3
mdadm --add /dev/md2 /dev/sda2

Monitor the status of their reconstruction by viewing /proc/mdstat. You can automate this with.
```
watch -n10 cat /proc/mdstat
```

Linux – Rebuild Linux raid1 after os reinstall

Yes, you need to start the array in degraded mode using sdc drive. Then, you can re-add the failed drive sdd.

I usually use the command mdadm with option --assemble to assemble and start the array in degraded mode using one disk only. Something like this:

$ mdadm --assemble /dev/md1 /dev/sda2

You may need to use -f option to force starting degraded array.

If that works fine, you can proceed by re-adding the replaced drive.

$ mdadm --re-add /dev/md1 /dev/sdb2

I used these commands many times before and did not experience any data loss. They worked smoothly when the drives are OK.

Precaution: Please backup your disks before running such commands to avoid any possible data loss.

Best Answer

Related Solutions

RAID1 – How to Fail a Drive Marked as Removed

Linux – Rebuild Linux raid1 after os reinstall

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