What are the differences bewteen using ISOs vs extracting distributions in multiboot LiveUSBs?

Question

I've created a multiboot pendrive with a couple of Linux distributions by using the YUMI tool from pendrivelinux, and by default it extracts the content of the chosen operating system's ISO on a dedicated folder in the destination pendrive; it can also direclty load ISOs stored on the pendrive. Quoting from the site:

Contrary to MultiBootISO's which used grub to boot ISO files directly from USB, YUMI uses syslinux to boot extracted distributions stored on the USB device, and reverts to using grub to Boot Multiple ISO files from USB, if necessary.

From the practical point of view, what are the differences between these two methods? Choosing one over another affects performance or has some other advantages?
The statement above seems to suggest that an extracted distro is a better choice than one run directly from an ISO file (since the latter is used only "if necessary"), but why? What are the benefits?

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I'm more interested in system performance or differences after it has booted, rather than the capabilities of Syslinux Vs Grub. In other words, how it is different a system booted from an ISO to another booted from a directory where it was extracted?

Solution 1

I first thought that YUMI extracts the files to the hard disk, but that's not the case. My understanding now is that YUMI can store on the pen-drive either :

1. The ISO as-is, or
2. The extracted files from the ISO as a file system within a partition

This is the reason that for booting the extracted files it can use Syslinux, but it needs GRUB to boot from ISO.

I believe that the idea here is to improve performance by avoiding the overhead caused by using the ISO file-system. Apparently, the file-system used for the extracted files is more efficient than the one built into the ISO, which is after-all that of a CD.

There is also probably an additional overhead involved in having a two-level access : USB disk to access the ISO, then accessing the ISO file-system. This might involve virtualization-type overhead by GRUB to make Linux believe that the ISO is in effect a CD.

I have no information about the relative performance of both architectures, extracted vs. ISO, and the improvement (if any) probably heavily depends on the type of work that you do.

Solution 2

Syslinux is usually a faster boot process than GRUB and is much more basic. As far as I understand Syslinux can't handle more than one ISO so GRUB must be used when that situation arises.

Solution 3

There is a single difference between the two systems. That is the fact that an ISO is just that. A file. It is an image of a disk/filesystem that has been zipped up into a nice format for easy use.

When GRUB boots from an ISO, it will create a loopback interface to the ISO so that the actual operating system running off that image can access the files on it like a real drive. Similar to how CDs are mounted. This does cause problems as some OS cannot use this method.

When SYSLINUX boots from an extracted filesystem, there is no need for any special mounting or interfaces, because it is exactly like booting from a hard drive. This works for almost any OS that supports a different boot directory.

There is also a seperate method involving ISO images. The image can be extracted and loaded into RAM exactly like a LiveCD that extracts into RAM.

Comments

[Sekhemty]

Thank you for your reply, I'm more interested on system differences rather than bootloader ones; this wasn't very clear from my question, so I've edited it.

[Sekhemty]

I think that YUMI only uses the USB pendrive, not the internal hard disk. It can manage both original ISOs and their extracted versions (which it extracts itself when you setup your bootable pendrive), but they are both located on the pendrive, without using the hard disk at all.

[harrymc]

@Sekhemty: You are right! I rewrote my answer.