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King David
Asked: 2016-09-16 04:09:06 +0800 CST

linux + adding swap space

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I performed the following procedure in order to add 400m to swap,

I took the 400m from the sdb disk for the swap increasing

Everything was ok but before the last command , I performed 

mkswap /dev/rootvg/swap

And I get the following warning messages from mkswap

mkswap: /dev/rootvg/swap: warning: don't erase bootbits sectors
    on whole disk. Use -f to force.
Setting up swapspace version 1, size = 2490364 KiB
no label, UUID=2f42fa37-987d-40a5-8cf6-5707e1d8f077

Finally swap was extend to 2.4G as expected

But I little worry about the warning message from mkswap command

Please advise what is meaning of the warning:

mkswap: /dev/rootvg/swap: warning: don't erase bootbits sectors
on whole disk. Use -f to force.

And if this is OK?

My procedure on the linux redhat machine - version 6

# swapon -s
Filename                                Type            Size    Used      Priority
/dev/dm-1                               partition       2064380 0       -1

# free
        total       used       free     shared    buffers     cached
Mem:       5991892     539036    5452856        212      70444     270664
-/+ buffers/cache:     197928    5793964
 Swap:      2064380          0    2064380


# pvs
PV         VG      Fmt  Attr PSize  PFree
 /dev/sdb   rootvg  lvm2 a--u  7.97g 416.00m
 /dev/sdc   orahome lvm2 a--u 30.00g  15.00g

# lsblk /dev/sdb
NAME                  MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sdb                     8:16   0    8G  0 disk
|-rootvg-swap (dm-1)  253:1    0    2G  0 lvm  [SWAP]
|-rootvg-slash (dm-2) 253:2    0  576M  0 lvm  /
|-rootvg-usr (dm-3)   253:3    0  1.4G  0 lvm  /usr
|-rootvg-home (dm-4)  253:4    0  256M  0 lvm  /home
|-rootvg-opt (dm-5)   253:5    0  416M  0 lvm  /opt
`-rootvg-var (dm-6)   253:6    0    3G  0 lvm  /var

# swapon -s
Filename                                Type            Size    Used     Priority
/dev/dm-1                               partition       2064380 0       -1

# swapoff /dev/rootvg/swap

# lvresize /dev/rootvg/swap -L +400m
 Rounding size to boundary between physical extents: 416.00 MiB.
 Size of logical volume rootvg/swap changed from 1.97 GiB (63 extents) to  2.38 GiB (76 extents).
 Logical volume swap successfully resized.

# mkswap /dev/rootvg/swap
mkswap: /dev/rootvg/swap: warning: don't erase bootbits sectors
    on whole disk. Use -f to force.
Setting up swapspace version 1, size = 2490364 KiB
no label, UUID=2f42fa37-987d-40a5-8cf6-5707e1d8f077

# swapon /dev/rootvg/swap

# free
         total       used       free     shared    buffers     cached
 Mem:       5991892     539680    5452212        212      70536     270892
 -/+ buffers/cache:     198252    5793640
 Swap:      2490364          0    2490364
linux redhat lvm swap

1 Answers

  1. Best Answer

    Looks like a safety precaution to prevent you from destroying a boot sector, etc... when using a whole disk when you may have meant to use a partition.

    This warning message not longer present in CentOS 6.5 nor Ubuntu 16.04.

    This older (2006) mkswap.c defines bootbits vaguely: http://code.metager.de/source/xref/busybox/util-linux/mkswap.c

    /*  char     bootbits[1024];    Space for disklabel etc. */

    From the CentOS 6.5 mkswap.c:

            if (is_whole_disk_fd(fd, devname)) {
                /* don't zap bootbits on whole disk -- we know nothing
                 * about bootloaders on the device */
                whole = 1;
                zap = 0;

    I'm not amazing with C, but the new code appears more intelligent and perhaps preserves boot data. I'm not delving that far.

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