[CentOS] Curious fdisk report on large disk

Wed Apr 27 18:15:52 UTC 2011
Pasi Kärkkäinen <pasik at iki.fi>

On Wed, Apr 27, 2011 at 10:18:28AM -0500, Robert Nichols wrote:
> On 04/27/2011 07:26 AM, Timothy Murphy wrote:
> > James Pearson wrote:
> >
> >>> Is there a safe way of recovering the partition table?
> >>> I have a vague idea that copies are kept at various places on the disk?
> >>
> >> AFAIK, there is only one copy at the start of the disk - however what
> >> does /proc/partitions contain?
> >>
> >> This may well have the details of the partitions and sizes when the
> >> machine was booted - it this is the case, take a copy of this info -
> >> which you can then use to manually re-create the partition table using
> >> fdisk
> >
> > Thanks very much for that;
> > /proc/partitions does indeed seem to contain correct information,
> > so all will not be lost if there is a power outage tomorrow:
> It most certainly _will_ be lost.  The "files" you see in /proc are just
> windows into various kernel data structures.  The /proc file system does
> not exist anywhere on disk.
> Make a paper copy of those numbers, and don't lose it.  You will need to
> be very careful to re-create the partitions exactly as they were or risk
> losing data.  Each logical partition within the extended partition begins
> with a secondary partition table, and if those get written in the wrong
> places they could overwrite something important.  When you create the
> partitions with fdisk, you will have to play around with numbers until
> you get a listing with block counts that exactly match those numbers
> (except for the size of sdb4, the extended partition, which the kernel
> always reports as "1").  Only then can you safely write out the new table.
> It would make life so much easier if fdisk would simply accept those same
> numbers as Kilobytes, but alas it keeps trying to round up to the next
> "cylinder" boundary, so you have to fiddle a bit to get it right.  Yes,
> fdisk is a very old, crufty, and slightly buggy program.  Newer programs
> are either much too "user-friendly" (e.g., cfdisk "enter partition size
> in decimal magabytes"), or way too dangerous (e.g., parted, which writes
> each change out to disk immediately without giving you a chance to
> verify what it just did).

sfdisk has "dump" mode, and it can also import old dump to new disk.

-- Pasi