Whole disk ZFS or -a4k partition

[Fleuriot Damien]

On Jul 3, 2013, at 11:22 AM, Alexandr <shuriku at shurik.kiev.ua> wrote:

> Hello, community!
> 
> I have a laptop with 2 disks - mSATA 32Gb SSD and 500Gb HDD. I plan to
> use SSD as whole disk ZFS pool with root and /usr partitions and 500Gb
> SATA as ZFS pool for /home and /var. My questions are:
> 
> 1. Do I need to create a 4k aligned partition on HDD for zfs pool or
> best to use a whole disk?
> 2. Where the best place for swap - ssd or hdd? SSD is much faster, but
> with limited write life cycle.
> 


I advise against using the whole disk for the following reason:

DISK1, manufacturer 1: 3tb (give or take)
DISK2, manufacturer 2: 3tb (give or take MINUS 10 BYTES)

Et voila, you can't use DISK1 with DISK2 because DISK2 is 10 bytes smaller and won't fit.


When building RAID arrays, it's recommended to:
- use disks from different manufacturers
- use disks ordered at different times (for example half at T1, half at T2)
- shave a few MBs off your disks, so that they all present the same size


Here's an example from my nas at home:

% gpart show /dev/ada6
=>        34  5860533101  ada6  GPT  (2.7T)
          34           6        - free -  (3.0k)
          40      102400     1  freebsd-ufs  (50M)
      102440  5860430688     2  freebsd-zfs  (2.7T)
  5860533128           7        - free -  (3.5k)


Notice how the disk presents 5860533101 sectors of 512 bytes.
Also notice how I've shaved off 50mbytes in the first, unused partition.

This way, if I replace a disk later on and the new disk only has, say 5860533095 512-byte sectors, I can still use it in my pool.