[TriLUG] [newbie][swap] why does `mount` not match /etc/fstab?

[David Black]

Your current swap partition may not have a matching UUID (as called for in /etc/fstab), e.g. when you moved it from wubi to physical partitions.

The easiest likely fix is to set the UUID of the swap partition to match the original one by running
  mkswap -U 2e73b542-1cb7-439a-b6a7-046dc4011372 /dev/<swap_partition_device>
Then you should be able to run 'swapon -a' again and have it attach.

Running 'mount' by itself is the same as 'cat /etc/mtab'.  To make sure I used strace to verify this on a Kubuntu 9.04 box.

The /dev/disk/by-uuid directory contains symlinks to the actual partition devices.  You're seeing the *requested* mount in /proc/mounts (initially by UUID), and in /etc/mtab the device the requested symlink points to.

Last, swaps don't show up in any of the above because they are not mounted filesystems, but just a set of blocks available to the kernel VM layer.   Try 'cat /proc/swaps' to see status for those.

Dave

----- "Tom Roche" <Tom_Roche at pobox.com> wrote:

> My everyday-use box is a 2007 Z61t ThinkPad with winxp preload.
> I wubi-ed xubuntu-9.04 and recently started using that on a daily
> basis. I was able to suspend without problems (and faster than
> winxp),
> so I didn't notice that wubi was unable to hibernate (as designed).
> Then my battery died :-( To get hibernation, I made "real" partitions
> (for / and swap) and migrated wubi to them ... but I still can't
> hibernate, because
> 
> > PM: Cannot find swap device, try swapon -a.
> 
> So I'm debugging that, and meanwhile noticing the following:
> 
> $ cat /etc/fstab
> > # <file system>                            <mount point>  <type>  
> <options>                    <dump>  <pass>
> > proc                                       /proc          proc    
> defaults                        0      0
> > UUID=a2aed1eb-0801-4780-bcdf-48fcd8f164b8  /              ext3    
> defaults,errors=remount-ro      0      1
> > #/host/ubuntu/disks/boot                    /boot          none    
> bind                            0      0
> > UUID=2e73b542-1cb7-439a-b6a7-046dc4011372  none           swap    
> sw                              0      0
> > # mounts of original NTFS partitions
> > # mount /c read-only in case of windows hibernation?
> > UUID=F8FCC29BFCC25412                      /media/Preload ntfs-3g 
> defaults,locale=en_US.UTF-8,ro  0      0
> > UUID=136C9F91B524AE31                      /media/PMagic1 ntfs-3g 
> defaults,locale=en_US.UTF-8     0      0
> > # internal mounts for compatibility with cygwin, modeled after /boot
> mount above     
> > /media/Preload                             /c             none    
> bind                            0      0
> > /media/PMagic1                             /e             none    
> bind                            0      0
> > /media/PMagic1/tlroche/backups/t_drive     /t             none    
> bind                            0      0
> 
> $ cat /proc/mounts 
> > rootfs / rootfs rw 0 0
> > none /sys sysfs rw,nosuid,nodev,noexec 0 0
> > none /proc proc rw,nosuid,nodev,noexec 0 0
> > udev /dev tmpfs rw,mode=755 0 0
> > /dev/disk/by-uuid/a2aed1eb-0801-4780-bcdf-48fcd8f164b8 / ext3
> rw,errors=remount-ro,data=ordered 0 0
> > tmpfs /lib/init/rw tmpfs rw,nosuid,mode=755 0 0
> > fusectl /sys/fs/fuse/connections fusectl rw 0 0
> > varrun /var/run tmpfs rw,nosuid,mode=755 0 0
> > varlock /var/lock tmpfs rw,nosuid,nodev,noexec 0 0
> > tmpfs /dev/shm tmpfs rw,nosuid,nodev 0 0
> > devpts /dev/pts devpts rw,nosuid,noexec,gid=5,mode=620 0 0
> > /dev/sda1 /media/Preload fuseblk
> ro,nosuid,nodev,user_id=0,group_id=0,allow_other,blksize=4096 0 0
> > /dev/sda5 /media/PMagic1 fuseblk
> rw,nosuid,nodev,user_id=0,group_id=0,allow_other,blksize=4096 0 0
> > /dev/sda1 /c fuseblk
> ro,nosuid,nodev,user_id=0,group_id=0,allow_other,blksize=4096 0 0
> > /dev/sda5 /e fuseblk
> rw,nosuid,nodev,user_id=0,group_id=0,allow_other,blksize=4096 0 0
> > /dev/sda5 /t fuseblk
> rw,nosuid,nodev,user_id=0,group_id=0,allow_other,blksize=4096 0 0
> > securityfs /sys/kernel/security securityfs rw 0 0
> 
> $ mount
> > /dev/sda6 on / type ext3 (rw,errors=remount-ro)
> > tmpfs on /lib/init/rw type tmpfs (rw,nosuid,mode=0755)
> > proc on /proc type proc (rw,noexec,nosuid,nodev)
> > sysfs on /sys type sysfs (rw,noexec,nosuid,nodev)
> > varrun on /var/run type tmpfs (rw,nosuid,mode=0755)
> > varlock on /var/lock type tmpfs (rw,noexec,nosuid,nodev,mode=1777)
> > udev on /dev type tmpfs (rw,mode=0755)
> > tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev)
> > devpts on /dev/pts type devpts (rw,noexec,nosuid,gid=5,mode=620)
> > fusectl on /sys/fs/fuse/connections type fusectl (rw)
> > /dev/sda1 on /media/Preload type fuseblk
> (ro,nosuid,nodev,allow_other,blksize=4096)
> > /dev/sda5 on /media/PMagic1 type fuseblk
> (rw,nosuid,nodev,allow_other,blksize=4096)
> > /media/Preload on /c type none (rw,bind)
> > /media/PMagic1 on /e type none (rw,bind)
> > /media/PMagic1/tlroche/backups/t_drive on /t type none (rw,bind)
> 
> I'm assuming `mount` is just presenting the information in
> /proc/mounts. (Am I missing something?) But I'm wondering,
> 
> * why does /proc/mounts show the mount by UUID but not `mount`?
> 
> * why does neither show the swap partition as mounted? 
> 
> TIA, Tom Roche <Tom_Roche at pobox.com>
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