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Thread: dd hanging, not responding, IOWait is very high

  1. #1
    Join Date
    Jun 2012
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    Unhappy dd hanging, not responding, IOWait is very high

    Ubuntu 16.04, Lenovo T440p, 2x Seagate BUP Slim RD USB Hard disks

    Hello,

    I'm duplicating a 2TB USB hard disk as a backup ahead of a two month travel. I've used this technique for years:

    # pv /dev/sdb | dd of=/dev/sdc

    Once complete, I take an MD5SUM to assure myself the volumes are correct.

    Yesterday, I woke up to find it was hung at 520GB, with a transfer rate of 19KiB/s and the ETA timer frozen. CTRL-C, couldn't kill it. It didn't occur to me that kill -9 might kill it as I wasn't coffee'd up yet. I instead told the laptop to shutdown. It didn't. I had to force shutdown the laptop.

    Today, I tried again and this time it's at 1.17TiB and hung frozen at 19KiB/s with exactly 4:49:50 remaining. Kill -USR1 doesn't wake it up. Other terminal windows work fine and I ran top. No processes out of control, but the Wait parameter is steady between 70-90%. Forgot using a web browser. I was going to cut-paste some info, but typing lags by approximately 80-90 seconds before appearing on the screen. However, the Terminal works just fine. Nothing is logged in dmesg.

    I have not tampered with anything and I have an operable term window. I want to understand where the problem is. These tools are as old as I am and cannot accept that there is a bug in them. Where do I look to find out where the issue is? This is not a code red emergency, but I'd rather not leave these disks powered for days at a time.

    Thanks!

    Tim

  2. #2
    Join Date
    Mar 2010
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    Re: dd hanging, not responding, IOWait is very high

    When dd has issues, it doesn't fail gracefully. Use ddrescue instead. There might be a single, bad, sector, preventing dd from continuing. It might be unused.

    For making an image backup, using something like fsarchiver might be a better choice since it compresses stuff and if it understands the file system, only the actually used parts are included. Also, it can be restored to a smaller disk/partition unlike most other image-based tools.

    OTOH, if you have great backups already, using real backup tools, then those should be preferred over any image-based solution. This is the method I use. Backups take 2-5 minutes daily and restores to a working system take just 30-45min including a base install of the OS from the current ISO file. Anyways, that is probably an option for when you return. Getting everything into this backup method, but nothing extra, takes a few attempts. Took me about 5 failures before I got it correct on the first system. That was about 10 yrs ago. Used it last week to move a system to new hardware. Used it 3 months ago to move to a new laptop. About once a year, I do something really stupid and need to restore the entire system during an approved maintenance period.

    There is something about images which make people sleep better. The trade-off is backup downtime and huge storage requirements when compared to the alternatives. Once you master grub-install and update-grub with having the files in the correct places, all the boot "magic" doesn't matter anymore. It is very easy to get a system restored to working from efficient non-image backups.

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