(very OT) Ideal partition schemes (history of partitioning)
mueller6722 at twc.com
Sun Aug 30 22:12:57 UTC 2020
from Polytropon (excerpt):
> Most multi-OS settings seem to work best with BIOS + MBR.
> Boot selection can either happen using a PF key at system
> startup, if it's more than one disk, or you have a separate
> software solution, like GRUB or FreeBSD's boot manager, if
> all systems are on one and the same disk.
How is that? It seems to me that GPT would be practically the only way for multi-OS.
Grub 2 can be used to select the partition to boot.
Some or many OSes can not run on a logical partition, only a primary partition, using MBR scheme.
And what if hard disk > 2 TB, or would that be 16 TB in the case of 4096-byte sectors?
> With MBR, you can have up to 4 "DOS primary partitions",
> i. e., slices. FreeBSD needs only one, other systems may
> need one or more. If 4 isn't enough, you need to remove
> one, and put an "DOS extended partition" in the free slot,
> which then can hold "logical volumes inside a DOS extended
> partition". This is typically called EBR.
> The limitations of MBR are no longer valid with GPT: Any
> new partition gets a new number, and if you want to have
> 20 partitions, that's no problem - it's natively possible.
> The problem could be systems not properly understanding
> GPT layout, and if I remember correctly, FreeBSD's boot
> manager does not support multiboot from GPT.
> On systems that do not have BIOS or "legacy mode", if I
> did understand everything correctly, MBR is not supported
> unless a EFI partition has been prepared beforehand.
> And in case of ... I won't name it to avoid trouble,
> even that won't work - you need to enable a Compatibility
> Support Module (if present!) and so on...; all this
> because you "should" use UEFI + GPT.
> It's actually quite complicated... ;-)
> You can avoid all this for FreeBSD-only systems by not
> using any partitioning at all - you just add labels (using
> the disklabel / bsdlabel command). Note that only letters
> 'a' to 'h' are possible (with exception of 'c').
> To summarize, you have the following usable (!) options:
> a) BIOS + dedicated
> b) BIOS + MBR
> c) BIOS + GPT
> d) UEFI + GPT
> In case of MBR, FreeBSD gets one slice; in case of
> GPT, FreeBSD gets as many partitions as it needs.
> The partition types are also different for GPT and MBR,
> and depending on the desired layout, the partitions you
> need can change.
> Within a slide*, you can create multiple partitions. The
> common approach today is to have one big / and some swap.
> The idea of "functional partitioning" typically suggests
> a layout like this:
slide? Did you mean slice?
> ------------ device in /dev -----------
> dedicated MBR GPT
> /boot - - ada0p1
> / ada0a ada0s1a ada0p2
> swap ada0b ada0s1b ada0p3
> /tmp ada0d ada0s1d ada0p4
> /var ada0e ada0s1e ada0p5
> /usr ada0f ada0s1f ada0p6
> /opt ada0g ada0s1g ada0p7
> /home ada0h ada0s1h ada0p8
> This is just a very simplified example.
> Also remember that most other systems fail to understand
> dedicated partitioning. The idea of "partitions within a
> slice" comes from BSD coming from non-PC systems to the PC,
> where MBR was required for multi-boot interoperability (to
> a certain, often quite limited degree). With GPT, there are
> plenty of types, so this is no longer needed.
I run several versions of FreeBSD and NetBSD, also need to save partitions for Linux and Haiku.
FreeBSD and NetBSD can't read each other's disklabel or bsdlabel-type subpartitions, and sub-partitioning a NetBSD slice with disklabel is very tricky, drives me crazy.
GPT means I never have to deal with traditional BSD disklabels any more; I don't run OpenBSD or DragonFlyBSD.
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