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POHow do you swap /dev/sda with /dev/sdb?
 

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  1. POHow do you swap /dev/sda with /dev/sdb?
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    <p>Boy, this is such. a. trivial. question, and yet nobody seems to be able to answer it correctly.</p> <p><strong>How do you swap /dev/sda with /dev/sdb ?</strong></p> <p>Someone can suggest to use permanent labelling (eg. /dev/disk/by-* ), but despite the best intentions, this does <em>NOT</em> answer the question. Yes, permanent labellings works where you can use them, but if a program is hardcoded to use eg. /dev/sda, this question persists.</p> <p>To illustrate the problem further from what I found on the internet: <a href="http://ubuntuforums.org/showthread.php?t=1569238&amp;page=2" rel="noreferrer">http://ubuntuforums.org/showthread.php?t=1569238&amp;page=2</a> (Reminds me of 'Schadenfreude')</p> <p>This chap seems to have found the solution, just didn't share it (boo!): <a href="http://ubuntuforums.org/showthread.php?t=944515" rel="noreferrer">http://ubuntuforums.org/showthread.php?t=944515</a></p> <p>And tbh, I have a potential similar hazard. I use CloneZilla, and if a program asks: <code>Would you like to backup /dev/sda to /dev/sdb or /dev/sdb to /dev/sda ?</code>, guess how nervous I get knowing that linux seems to randomly assign disk orders. I haven't overwritten my data with my own backup yet, but this is just waiting to happen.</p> <p>What within Linux assigns /dev/sd* to disks, and how do you influence this process? Has this got something to do with udev (/etc/udev/, udevadm)? My OS is CentOS, but I need to know this also for Ubuntu and CloneZilla (<a href="http://clonezilla.org" rel="noreferrer">http://clonezilla.org</a>), and this problem occurs on all systems, so my guess is that this problem is not related to the distribution, but rather to the kernel, kernel modules, or something very close to the kernel. Please help!</p> <p><strong>------------------ EDIT: 25 August 2013</strong> After advising the link that ypnos gave, I read it all, tried out one command, and the kernel just 'vommitted' udev rules all over my screen. Then prompted for root password to allow maintenance, or exit for restart. This is evidence that this stuff is indeed not for the Novice person.</p> <p>I also looked it up a bit further. I don't understand how or when the linux kernel loads, but several messages on the internet indicate that the BIOS (!! believe it or not) is passing the list of booteable disks down to grub, which then uses the device.map file to assign which devices to which grub (hd*,<em>). Note that /dev/sd</em> have already been defined at this stage, as you can use permanent dev symlinks. These device maps seem to somehow pass on to the actual root file system. So is this a bootloader thing now?</p> <p>Going back to the udev as a potential solution, I found a bugreport on google <a href="http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=578826" rel="noreferrer">http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=578826</a>, that resulted in a resolution where it was DISadviced to change the udev NAME (that ultimately will become /dev/sd* as we know it).</p> <p>For the suggested udev MAN pages:</p> <pre><code>| The following keys can get values assigned: | | NAME | The name of the node to be created, or the name the network | interface should be renamed to. NOTE: changing the kernel-provided name of device nodes (except for network devices) is not supported and can result in unexpected behavior. Today, the kernel defines the device nodes names, and udev is expected to only manage the node's permissions and additional symlinks. </code></pre> <p>...But I went out to do it anyway in a slightly altered way.</p> <pre><code># vi /etc/udev/rules.d/00-corrections.rules KERNEL=="sd?", ATTRS{model}=="SAMSUNG SP0411N", NAME="sda" KERNEL=="sd??", ATTRS{model}=="SAMSUNG SP0411N", NAME="sda%n" KERNEL=="sda", ATTRS{model}!="SAMSUNG SP0411N", NAME="sdb" KERNEL=="sda?", ATTRS{model}!="SAMSUNG SP0411N", NAME="sdb%n" </code></pre> <p>Essentially, what it does is "If model is samsung, assign it NAME sda*. If model is not Samsung, but has been assigned sda*, assign it NAME sdb*." This rule was placed before all other rules as much as possible. Note that I am not sure about this, because there seem to be some 'invisible' rule files also, AND although you have renamed the devices, the kernel somewhere in 'kernel-loaded-memory', still might have the references wrong. This might be evident when you look at the /var/log/boot.log file, where in my case, the beginning it says:</p> <pre><code>%G Welcome to [0;36mCentOS[0;39m Starting udev: %G[60G[[0;32m OK [0;39m]Setting hostname UncleFloServer: [60G[[0;32m OK [0;39m]ERROR: asr: seeking device "/dev/sda" to 5999998795264 ERROR: ddf1: seeking device "/dev/sda" to 5999998795264 ERROR: ddf1: seeking device "/dev/sda" to 5999998664192 ERROR: hpt45x: seeking device "/dev/sda" to 5999998790144 ERROR: isw: seeking device "/dev/sda" to 5999998794752 ERROR: jmicron: seeking device "/dev/sda" to 5999998795264 ERROR: lsi: seeking device "/dev/sda" to 5999998795264 ERROR: nvidia: seeking device "/dev/sda" to 5999998794752 ERROR: pdc: seeking device "/dev/sda" to 137438913024 ERROR: pdc: seeking device "/dev/sda" to 137438920192 ERROR: pdc: seeking device "/dev/sda" to 137438927360 ERROR: pdc: seeking device "/dev/sda" to 137438934528 ERROR: sil: seeking device "/dev/sda" to 5999998795264 ERROR: via: seeking device "/dev/sda" to 5999998795264 Setting up Logical Volume Management: No volume groups found [60G[[0;32m OK [0;39m]Checking filesystems _CentOS-6.4-x86_: clean, 85517/655360 files, 662649/2621440 blocks /dev/sda1: clean, 56/65536 files, 33367/262144 blocks [60G[[0;32m OK [0;39m]Remounting root filesystem in read-write mode: [60G[[0;32m OK [0;39m]Mounting local filesystems: [60G[[0;32m OK [0;39m]Enabling local filesystem quotas: [60G[[0;32m OK [0;39m]Enabling /etc/fstab swaps: [60G[[0;32m OK [0;39m] </code></pre> <p>Here, my Samsung device is 40GB (that I would like as /dev/sda), and my big Areca Raid is 6TB (that I would like as /dev/sdb).</p> <p>Some remaining questions remain</p> <ol> <li><p>What do the errors mean? </p></li> <li><p>Are these errors the cause of the kernel, or the cause of a rule file still running before my 00-corrections.rules from udev?</p></li> <li><p>Do these errors indicate something data- threatening? The Areca partition mounted no problems on one of my folders in fstab.</p></li> <li><p>Is there a better, earlier device-assignment method?</p></li> </ol>
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    1. COActually, this does sound very close to what I need. But how to use it? http://en.wikipedia.org/wiki/Udev
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      1. POHow do you swap /dev/sda with /dev/sdb?
      1. USFlorian Mertens
    2. COSorry, I forgot to mention: The final described 00-corrections.rules from my Edit as of 25 Aug seemed to work, aside from the mentioned errors.
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      1. POHow do you swap /dev/sda with /dev/sdb?
      1. USFlorian Mertens
 

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