[lug] Mystery Disk Use, Fedora 23

Zan Lynx zlynx at acm.org
Fri Sep 23 22:08:20 MDT 2016

On 9/23/2016 9:06 PM, stimits at comcast.net wrote:
> Hi,
> I was planning on trying to make a RAID1 setup and then cover it with 
> LVM, but I only got as far as creating the RAID1 setup (it isn't 
> mounted, but I can verify I can format it or mount it, so on). Before 
> I created the RAID1 on two new disks there was nothing strange. After 
> I created this, I can see constant hard drive activity, it never ends. 
> It isn't updatedb, it isn't fsck.

Ah an easy one! This is almost certainly the MD driver running in the 
kernel, doing a RAID sync. You can use the drives while they 
synchronize, but the array won't be "clean" until it finishes. You 
should be able to view the status in /proc/mdstat if I remember that 

It's all low priority disk IO and should not cause any problems. You can 
raise the disk IO priority if you want it to finish faster, although I 
don't remember the trick for that.

> I don't know how to remove my "/dev/md0" now that I've created it, but 
> I know I can't leave this constant hard drive access churning away, it 
> worries me (and I don't know which drive it is...I have many).
Really, it shouldn't be a problem. It just takes a while to write 4 TB 
(or whatever you have) at low priority.
> Is there something like top, but for watching disk I/O? I see nothing 
> using significant CPU, and I've tried fuser on any partition's device 
> special file which is not supposed to be mounted, and see nothing. 
> What is a good tool to find out what's is consuming disk activity?
I like "atop" to see processes. I like "iostat -xk /dev/sd[a-z] 1' for 
just general IO stats.

> What is a good way to remove a RAID1 device (there is no 
> "/etc/mdadm.conf" and "/dev/md0" persists across boots).
Use the mdadm tool I think. There isn't any file for it, the RAID layout 
is written onto the disks during setup.

                 Knowledge is Power -- Power Corrupts
                         Study Hard -- Be Evil

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