swap partition missing?

[Paul Howarth]

Hongwei Li wrote:
>>Hongwei Li wrote:
>>
>>>Hi,
>>>
>>>I just found that my fc3 system's swap partition is "gone".  When I
>>>installed the system, I set it to 1Gb.  The kernel is 2.6.10-1.766_FC3.
>>>I
>>>don't know when it started, but here are some information:
>>>
>>># free
>>>             total       used       free     shared    buffers
>>>cached
>>>Mem:       1035748     658792     376956          0     145380
>>>272540
>>>-/+ buffers/cache:     240872     794876
>>>Swap:            0          0          0
>>>
>>># cat /etc/fstab
>>>
>>># This file is edited by fstab-sync - see 'man fstab-sync' for details
>>>LABEL=/1                /                       ext3    defaults
>>>1 1
>>>none                    /dev/pts                devpts  gid=5,mode=620
>>>0 0
>>>none                    /dev/shm                tmpfs   defaults
>>>0 0
>>>LABEL=/home1            /home                   ext3
>>>defaults,usrquota
>>>      1 2
>>>LABEL=/opt              /opt                    ext3    defaults
>>>1 2
>>>none                    /proc                   proc    defaults
>>>0 0
>>>none                    /sys                    sysfs   defaults
>>>0 0
>>>LABEL=/tmp1             /tmp                    ext3    defaults
>>>1 2
>>>LABEL=/usr1             /usr                    ext3    defaults
>>>1 2
>>>LABEL=/var1             /var                    ext3
>>>defaults,usrquota
>>>      1 2
>>>LABEL=SWAP-hda7         swap                    swap    defaults
>>>0 0
>>>...
>>>
>>>I tried this:
>>>
>>># swapon -a
>>>swapon: cannot find the device for LABEL=SWAP-hda7
>>>
>>>The system log does not show anything related to this problem.  Can
>>>somebody tell me what may cause this problem?  how to check it further
>>>and
>>>fix it?
>>
>>Have you booted one of the LiveCD style distributions on this machine? I
>>have heard that one of them may remove the label from swap partitions.
>>
>>You might try changing the fstab entry:
>>LABEL=SWAP-hda7 swap swap defaults 0 0
>>to:
>>/dev/hda7 swap swap defaults 0 0
>>
>>and see if that works. If it does, you might want to relabel the
>>partition (when it's not being used), using (I think - the -L option is
>>undocumented):
>>
>># mkswap -L SWAP-hda7 /dev/hda7
>>
>>Paul.
>>
> 
> 
> Yes, it works.  Thank you very much!  Now, I have questions:
> 
> 1. What's the use of mkswap -L SWAP-hda7 /dev/hda7? I don't see any effect?

It *should* be setting the "filesystem label" of the partition to 
"SWAP-hda7" so that mkswap can find it by name rather than being needed 
to be told where exactly the swap partition is.

> 2. When I put the original entry back to /etc/fstab as:
> LABEL=SWAP-hda7         swap     swap    defaults 0 0
> 
> it does not work again.  I have to put /dev/hda7 in it.  How to let the
> original entry work -- the above mkswap ... does not have effect?

Was your system swapping to that partition at the time you did the mkswap?

 > Why do we need those LABEL=... in fstab?

The idea is that labels should be less of a moving target than device 
names. For example, lots of people found that their SATA drives moved 
from being /dev/hdX to /dev/sdX fairly recently. Labelling filesystems 
means that the OS can find them no matter what the device name is. This 
isn't without its problems (e.g. when moving disks between machines, 
resulting in multiple partitions with the same labels), but that's why 
they're there.

Labels for swap partitions are discussed at:
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=127892

Paul.