Is "swap" partition part of the partition table of the MBR?

m4rtin · 09-10-2011, 08:37 PM

I have a Dell PowerEdge 860 server with two 1TB SATA HDD's. First HDD has a Linux installation on it and the second HDD is blank:

Code:

root@PE860:~# fdisk -l

Disk /dev/sda: 1000.2 GB, 1000204886016 bytes
255 heads, 63 sectors/track, 121601 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xc94e81b3

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1      119608   960748544   83  Linux
/dev/sda2          119608      121602    16011265    5  Extended
/dev/sda5          119608      121602    16011264   82  Linux swap / Solaris

Disk /dev/sdb: 1000.2 GB, 1000204886016 bytes
255 heads, 63 sectors/track, 121601 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xe2c527ab

Disk /dev/sdb doesn't contain a valid partition table
root@PE860:~#

Now if I copy the MBR of the /dev/sda to the /dev/sdb:

Code:

root@PE860:~# dd if=/dev/sda of=/dev/sdb count=1 bs=512
1+0 records in
1+0 records out
512 bytes (512 B) copied, 0.000343479 s, 1.5 MB/s
root@PE860:~#

..the swap partition for some reason is not copied:

Code:

root@PE860:~# fdisk -l

Disk /dev/sda: 1000.2 GB, 1000204886016 bytes
255 heads, 63 sectors/track, 121601 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xc94e81b3

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1      119608   960748544   83  Linux
/dev/sda2          119608      121602    16011265    5  Extended
/dev/sda5          119608      121602    16011264   82  Linux swap / Solaris

Disk /dev/sdb: 1000.2 GB, 1000204886016 bytes
255 heads, 63 sectors/track, 121601 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xc94e81b3

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *           1      119608   960748544   83  Linux
/dev/sdb2          119608      121602    16011265    5  Extended
root@PE860:~#

Isn't the swap actually present in the partition table of the MBR?

justboo2u · 09-10-2011, 09:10 PM

I cant swear to it, but...
you're working with terabyte drives... I doubt one sector is enough...

Try redoing it with 1024 and see if the remainder shows up.

My 2cts!

syg00 · 09-10-2011, 09:45 PM

Quote:

Originally Posted by m4rtin

Isn't the swap actually present in the partition table of the MBR?

[rant]Unless you *really* know what you are doing, copying MBR(s) is just asking for problems.[/rant]
Rebuild the second drive as you want it. Or use a tool designed to do it, like sfdisk.

Your issue has nothing (directly) to do with it being a swap partition, but the fact that it is a logical partition.

johnsfine · 09-11-2011, 07:35 AM

Quote:

Originally Posted by syg00

[rant]Unless you *really* know what you are doing, copying MBR(s) is just asking for problems.[/rant]

Nothing in my further explanation below should distract from syg00's correct answer quoted above.

The portion of the partition table in the same sector as the mbr covers only the four initial positions of the partition table that may be primary or extended. Most Linux partition naming rules reserve numbers 1-4 for those.

The first logical partition is number 5, even though you only used two of the initial four positions of the table.

The rest of the partition table is chained together in space preceding each logical partition (Notice SDA2 is one block larger than SDA5). Since you didn't copy any of that, you didn't get any of the info about your logical partition.

The first sector of your SDA2 contains the description of SDA5 as well as a linkage that would be used to indicate which sector would contain the description of SDA6 if there were one.

So after you copied the MBR, and got Linux to recognize sdb2, you could have copied the first sector of sda2 to sdb2, and similarly gotten Linux to recognize sdb5. But unless you have a really good reason for messing around on that level, you are still just asking for trouble.