Re: debian-installer: how to exactly specify partition sizes?
Andy Smith composed on 2025-07-10 22:52 (UTC):
> Let's say I have an existing machine that was installed using the text
> mode trixie RC1 debian-installer, and I want to exactly copy the
> partition layout of this machine on the next machine.
> Probably the best long term way would be to record the partitioning
> details in a preseed file and load that.
> But let's say I want to do it by hand. How?
> The manual partitioner accepts B, MB, GB and the binary equivalents as
> units. However, there is not any value I can specify that replicates an
> existing disk layout.
> For example, given this:
> $ head -4 /proc/partitions
> major minor #blocks name
> 259 1 7501465944 nvme1n1
> 259 2 487424 nvme1n1p1
> You'd expect that specifying "499122176B" (that's 487424 * 1024) would
> result in a partition of exactly 487424 1024-byte blocks being created,
> right? Nope, it creates one 498073600 bytes long which is exactly 1 MiB
> (1048576 bytes) less than what I asked for.
> Also trying to use MB or MiB never does result in anything that exactly
> matches the other existing partition layout.
> I can make the first partition match by manually adding 1048576 bytes
> and asking for that. But when I try the same trick with the next
> partition, again it gives me something slightly off.
> Why does it alter the value I specify? The other machine was installed
> using debian-installer so if there is some alignment thing going on,
> well, the original one was acceptable to debian-installer so it
> shouldn't need to alter the values I specify.
> Presumably I could replicate it by remembering the value I typed in when
> installing the first one, since hopefully the changes it makes are
> deterministic. But I don't remember.
> Any ideas? Perhaps I can get sfdisk into the installer environment then
> I could paste the output of sfdisk -d into sfdisk at a shell prompt.
> It feels wrong that the installer doesn't give me n amount of bytes when
> I ask for it with "nB".
I can't help with what you actually asked for, because I never use any OS
installer's partitioner to add, remove, resize, or anything else to do with
partition table writes that isn't merely about UUIDs, LABELs or filesystem
formats. I use OS installers for nothing more than formatting or naming, if
required, or assigning existing partitions to mount points. All partitioning I do
I have been doing the same way since last century, using text mode partitioner
from http://www.dfsee.com/ that writes fully compatible tables whether run from
its binaries for DOS, OS/2, Windows, Linux, or (as I hear it) MacOS (where I boot
Linux to partition). Using DFSee gives me exactly what I desire on a consistent
and reliable basis, and I get logs in the process whose selected sections serve as
catalogs of what I have installed where on the hundred or more HDDs and SSD
scattered about the premises.
--
Evolution as taught in public schools is, like religion,
based on faith, not based on science.
Team OS/2 ** Reg. Linux User #211409 ** a11y rocks!
Felix Miata
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