Installing Gentoo alongside Arch

Proinsias · Posted: Sun Apr 05, 2015 2:13 am Post subject: Installing Gentoo alongside Arch

Using Gentoo in a virtual machine on my Arch desktop last year and enjoying it I decided to try it out on hardware. Not wanting to interfere too much with my current setup I took the 20GB HDD from my long dead xbox 360, stuck it in my i3 haswell desktop, and installed Gentoo. I use Arch as the default on my SSD and occasionally opt for Gentoo by going into the bios/uefi/motherboardmenu/menuihavelittlecontrolover and selecting the old 20GB HDD. My Gentoo experience seems a little counter-intuitive at the moment as I'm comparing the binary approach of Arch on a shiny new SSD to the source based approach of Gentoo on an old HDD.

I have three internal hard drives:
120GB SSD running Arch
20GB HDD running Gentoo
1TB HDD for stuff.

The 20GB Gentoo HDD is the only one over 50% capacity. Ideally I'd like to move the Gentoo installation I'm currently using from the 20G HDD to the SSD where it will live happily alongside Arch. I don't mind starting afresh, or a bit of both if need be.

To the best of my knowledge, which isn't saying much, Arch on the SSD is uefi/gpt and Gentoo on the HDD is bios/gpt. Arch is booting with gummiboot and Gentoo with grub2.

What I'm thinking is either resizing my Arch /home partition(sda3) down by ~30GB or moving the vbox partition(sda5) to my 1TB drive, to make space for Gentoo - current plan is ~20GB for / and ~10GB for /home. I've read stories of 'here be dragons' regarding multiple OS's on one disk. After doing a little reading I feel more confused than I was before as to bios/efi/uefi//gpt/bootloader/OS. I'm not sure if i need one, none or two bootloaders and which, if any OS, should be in charge.

I've got working/bootable rsync backups of my Arch install and there is nothing on the SSD that would constitute a terrible loss in the event of the inevitable, just looking for pointers on an easy life before I log in as root and start messing with low level filesystems & disk partitions which are beyond my grasp at the moment.

My current view from Gentoo. Arch is on sda, Stuff on sdb and Gentoo on sdc:

totony · n00b Joined: 03 Dec 2014 Posts: 40

Uefi (or bios) is decided by your computer, not your hard drive. If you have a 120g SSD most chances are that your firmware is UEFI
EDIT: Clarification: SSD techmology came around the same time as UEFI spreading
Multiple operating systrms is generally ok, except if one destroys the bootloader/other partitions (windows does this sometimes)

NeddySeagoon · Posted: Sun Apr 05, 2015 11:41 am Post subject:

Proinsias,

Move things around as you see fit. There should be no need to start over.

Choose a single boot loader and make it boot everything.
Put all you boot files into /boot - be careul that you don't have file name collisions.
Share swap and maybe /tmp unless you need to hibernale.

You can treat your current Gentoo install as a stage4. You will need to fix /etc/fstab to suit its new home and add GPT support to the Gentoo kernel.
If you are moving from BIOS to UEFI, you can build the kernel as an EFI program too.

-- edit --
Consider putting /usr/portage on the rotating rust too. This is your portage tree, the distfiles and any binary packages you build for yourself.
Its only used at update time, so you won't notice the slowdown due the tho HDD.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

totony · n00b Joined: 03 Dec 2014 Posts: 40

NeddySeagoon · Posted: Sun Apr 05, 2015 8:08 pm Post subject:

totony,

It will all be sucked into the kernel drive cache anyway.
You will load the cache faster from the SSD than HDD.

The portage tree on the SSD will waste a lot of space unless you put partage in squashfs. The SSD block size will be 4k.
Many portage files are very small compared to 4k. You can have a fs with a 1k block size on a drive with a physical block size of 4k but that hurts the access speed.
The HDD is big enough to not mind needing 4G for the portage tree but only actually having data in about 750Mb.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

totony · n00b Joined: 03 Dec 2014 Posts: 40

Proinsias · Posted: Mon Apr 06, 2015 10:18 pm Post subject:

Thank you.

totony,
Firmware is described as UEFI dual bios, http://www.gigabyte.com/products/product-page.aspx?pid=4489#ov it's about 18 months old and seems fairly friendly.

Neddy,
A stage 4 install puts it in better context, cheers. I will likely put /usr/portage on the 1TB HHD, so as to avoid messing around with the partions of two working OS's on sperate disks simultaneously.

NeddySeagoon · Posted: Tue Apr 07, 2015 5:50 pm Post subject:

totony,

Both SSDs and Advanced Format Hard Drives use 4k physical sectors.

When you create a filesystem with a 1k block size you rocce the underlying device to deal with unaligned accesses.
To change one filesystem block (1k) nthe drive needs to do a read/modify/write. On a rotating rust drive, that incurs a delay oy at least one platter rotation.
On the SSD, its still a read/modify/write, which is slom if its in place as flash must be erased before its written. If you are lucky, the block will be rewritten to an arleady erased block.

Reads do not have quite the same issue as reads tend to be cached, both by the drive and the kernel.

What it boils down to is that aligned accesses are fastest.

With old rotating rust diver its always a non issue as the physical sector size is 512b, so every filesystem block is multiple physical sectors.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.