On 2012-11-30 15:52, Tomas Forsman wrote:
On 30 November, 2012 - Albert Shih sent me these 0,8K bytes:

Hi all,

I would like to knwon if with ZFS it's possible to do something like that :


Removing a disk - no, one still can not reduce the amount of devices
in a zfs pool nor change raidzN redundancy levels (you can change
single disks to mirrors and back), nor reduce disk size.

As Tomas wrote, you can increase the disk size by replacing smaller
ones with bigger ones.

With sufficiently small starting disks and big new disks (i.e. moving
up from 1-2Tb to 4Tb) you can "cheat" by putting several partitions
on one drive and giving that to different pool components - if your
goal is to reduce the amount of hardware disks in the pool.

However, note that:

1) A single HDD becomes a SPOF, so you should put pieces of different
raidz sets onto particular disks - if a HDD dies, it does not bring
down a critical amount of pool components and does not kill the pool.

2) The disk mechanics will be "torn" between many requests to your
pool's top-level VDEVs, probably greatly reducing achievable IOPS
(since the TLVDEVs are accessed in parallel).

So while possible, this cheat is useful as a temporary measure -
i.e. while you migrate data and don't have enough drive bays to
hold the old and new disks, and want to be on the safe side by not
*removing* a good disk in order to replace it with a bigger one.
With this "cheat" you have all data safely redundantly stored on
disks at all time during migration. In the end this disk can be
the last piece of the puzzle in your migration.

meaning :

I have a zpool with 48 disks with 4 raidz2 (12 disk). Inside those 48 disk
I've 36x 3T and 12 x 2T.
Can I buy new 12x4 To disk put in the server, add in the zpool, ask zpool
to migrate all data on those 12 old disk on the new and remove those old
disk ?

You pull out one 2T, put in a 4T, wait for resilver (possibly tell it to
replace, if you don't have autoreplace on)
Repeat until done.
If you have the physical space, you can first put in a new disk, tell it
to replace and then remove the old.

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