7/25/2019 How to backup partition table

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Updated: August 26, 2015

For those of you new to Dedoimedo articles, you have a serious backlog of reading ahead of you, before you can confidently navigate the perilous waters of the Nerdy Bay, where wordslike partition and table are used with wild abandon.

But if you are in the groove, then you shall like this article. Today, we will talk about partitiontable backup and restore, why and when you should undertake this seemingly dubious step,and why it's useful. After me.

Note: Courtesy of FreeImages/Marcin Barlowski.

Foreword

Sort of. So, if you happen to have a spare hard disk, with multiple partitions and/or frequentuse that involves formatting, partitioning, moving and resizing of existing partitions, datacopy and such, you may want to know that the integrity of those bits you don't want deleted

is preserved when you go about changing those things that you do.

Indeed, whenever you fiddle with disks and partitions, there's a chance something might gowrong. You could get momentarily distracted or confused and delete the wrong device, orworse, format it, with all its precious data. We talked about data recovery in the past, nowwe will talk about something else.

What do you do if you suddenly destroy the partition table of a device? Or delete a partition?Now, at first glance, these may appear to be highly destructive operations, but they are quiteinnocent. In fact, most of the time, deletion of data, even formatting, is relatively innocent.It's not about what is stored on disk, but what you think is stored.

Deleting a file merely means de-indexing it from the disk log, making the space previouslyused by that file available for some other, future use. Deleting a partition is the same thing.You deallocate the markers that tell the disk, and subsequently, the operating system usingthe disk which pieces of the disk space are available for use, and which are considered free.And at the very top, there's the partition table. If you get this entry deleted, then the diskwill appear as a raw, unused device, even though it may contain a perfectly sane and usablestructure of partitions, filesystems, inodes, and actual data.

Going back to our initial statement, we want to be able to quickly and easily recover ourpartitions, should they somehow get lost. Which is why I'm going to show you how you can

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How to backup and restore the partition table

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backup the partition table, both on MBR and GPT disks. We will then recover a seeminglydead disk. And finally, discuss the added value of this little geeky game.

Backup

We will start with regular disks, i.e. MBR-ed ones, with the standard ms-dos type partitiontable. In other words, you can have up to four primary partitions, one of which can be anextended partition containing logical partitions. In this case:

sfdisk -d /dev/<disk> > <disk>.layout

Here, we use the sdfisk command, with the dump (-d) option. The output is a human-readable file, listing the partitions, including the start sector, size and filesystem type. Quiteuseful if you ever need to recover something on your device. Of course, you can always gofor automated recovery tools, but why not be sensible, and have a backup?

# partition table of /dev/sdc

unit: sectors

/dev/sdc1 : start= 4094, size=1248886786, Id= f

/dev/sdc2 : start= 0, size= 0, Id= 0

/dev/sdc3 : start= 0, size= 0, Id= 0

/dev/sdc4 : start= 0, size= 0, Id= 0

/dev/sdc5 : start= 4096, size=122880860, Id=83

/dev/sdc6 : start=122886144, size=122880000, Id=83

/dev/sdc7 : start=245768192, size=122880000, Id= 7/dev/sdc8 : start=368650240, size=122880000, Id=83

/dev/sdc9 : start=491540868, size=122865057, Id= 7

/dev/sdc10: start=614414336, size=634476544, Id=83

If your partitions are not aligned, you may see this warning:

sfdisk -d /dev/sdc > /root/sdc.layout

sfdisk: Warning: extended partition does not start at a cylinder

boundary. DOS and Linux will interpret the contents differently.

For GPT, you should use sgdisk, which is readily available in all Linux distributions. Fdisk andsfdisk are not suitable for use with GPT devices, so remember that. Indeed, before doing anybackups, you should execute the fdisk or sfdisk tools in a console windows to see whether

they will display meaningful information in the running shell. If you encounter any sort of errors with sfdisk, you ought to try sgdisk next.

Using the dd command

Another way to accomplish the backup is manually, using the dd command. With MBR, yousimply need to copy the first 512 bytes of the disk to a file, and later use it for restore.Indeed, with our /dev/sdc disk as a guinea pig:

# fdisk -l

Disk /dev/sda: 320.1 GB, 320072933376 bytes, 625142448 sectors

Units = sectors of 1 * 512 = 512 bytes

Sector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytesDisk label type: dos

Disk identifier: 0x000086a1

Device Boot Start End Blocks Id System

/dev/sda1 4094 625141035 312568471 5 Extended

/dev/sda5 4096 8392703 4194304 82 Linux swap ...

/dev/sda6 8394752 417994751 204800000 83 Linux

/dev/sda7 417996800 520396799 51200000 83 Linux

/dev/sda8 * 520398848 625141035 52371094 83 Linux

Disk /dev/sdc: 639.4 GB, 639432130560 bytes, 1248890880 sectors

Units = sectors of 1 * 512 = 512 bytes

Sector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytes

Disk label type: dosDisk identifier: 0x00071ad3

Device Boot Start End Blocks Id System

/dev/sdc1 4094 1248890879 624443393 f W95 Ext'd ...

/dev/sdc5 4096 122884955 61440430 83 Linux

/dev/sdc6 122886144 245766143 61440000 83 Linux

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/dev/sdc7 245768192 368648191 61440000 7 HPFS ... /dev/sdc8 368650240 491530239 61440000 83 Linux

/dev/sdc9 491540868 614405924 61432528+ 7 HPFS ... /dev/sdc10 614414336 1248890879 317238272 83 Linux

And then, some wicked dd magic:

dd if=/dev/sdc of=/root/sdc.header bs=512 count=1

1+0 records in

1+0 records out

512 bytes copied, 0.00104001 s, 3.9 MB/s

If you are not sure about sizes, then you should consult the header formats to figure out theexact values. For instance, Wikipedia entries on MBR and GPT tell us all we need to know. Ingeneral, you should use 512 and 16,384 bytes, respectively.

Restore

Let's mess up our device. Instead of using it as a source, we will use it as a destination, andwrite a bunch of zeros to it. This will simulate a scenario where the partition table gotaccidentally deleted or damaged, preventing the use of the hard disk. In reality, the actualdamage might be bigger or more complex or completely irreversible, but in our case, let'sfocus on the partition table.

Again, we will use the dd command. BTW, should I remind you NOT to play with productiondevices, without first making sure you have ample, proven backups elsewhere? This is a veryrisky activity, and don't try it unless you know what you're doing.

dd if=/dev/zero of=/dev/sdc bs=512 count=1

1+0 records in

1+0 records out

512 bytes copied, 0.00175504 s, 2.3 MB/s

If you remove the disk and reconnect it, and then run fdisk again, it will appear as if has nopartitions. The output shown below isn't very useful, but that's what you'll see, with theexception of disk notation and size.

Disk /dev/sdc: 639.4 GB, 639432130560 bytes, 1248890880 sectors

Units = sectors of 1 * 512 = 512 bytesSector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytes

Disk label type: dos

Disk identifier: 0x00071ad3

To restore the disk to a functional state, use dd to copy the saved header back to the device.Make sure you correctly specify the block size value and the count, because otherwise, youmay cause more damage.

dd if=/location-of-saved-header of=/dev/sdc bs=512 count=1

Why would you want to do this?

All right, so there, a good question. Why should any sane person, i.e. non-Linux geek everbother with this? Truth to be told, if you have a static setup, then you definitely need not toworry about your partition table. The chance of damage that isn't a complete failure of thedisk is remote. And in that case, you will have bigger issues than just a messed-up partitiontable. This is a good opportunity to remind you of proper imaging and data backups as aphilosophy and strategy in your daily computing.

Now, if you happen to be a somewhat more tech-savvy person and you do a lot of softwaretesting, including Linux distro installations, partitioning and such, it might be prudent to keepa copy of the disk header, just in case something goes (lightly) wrong.

Disclaimer

Now, this kind of rudimentary backup & restore is meaningless if you overwrite your realdata. For instance, if you zero the disk, then nothing will help really. Furthermore, you mustmake sure not to perform any destructive actions before and during the restore operation, asyou could accidentally delete important stuff.

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Conclusion

There you go. This little tutorial won't make your salary any higher, but it may just boost youwith the extra dose of confidence when it comes to working with hard disks and partitionsand making changes. If you remember that all and any device is just a bunch of zeros andones, then there's nothing sinister about them. You can backup portions of those long stringsof binary code, you can overwrite them, you can restore them.

And so, the ability to backup and restore the partition table is a rather valuable addition inone's testing and tweaking arsenal, especially for people with an ever-changing setup of operating systems. If you do a lot of formatting and partitioning, sooner or later, you mayend with the need to restore one of your devices. When that happens, it's always useful tohave a proper backup. At the very least, you will save time. I hope you liked this.

Cheers.

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