getting to 4K disk blocks in ZFS

[Aristedes Maniatis]

Thanks Stefan and Peter for the highly informative posts.

On 10/09/2014 5:48pm, Stefan Esser wrote:
> ZFS uses variable block sizes by breaking down large blocks to smaller
> fragments as suitable for the data to be stored. The largest block to
> be used is configurable (128 KByte by default) and the smallest fragment
> is the sector size (i.e. 512 or 4096 bytes), as configured by "ashift".

So this means that the ZFS developers would need to effectively (re)fragment the entire pool if they wanted to develop a way to increase the ashift size. This sounds like something that isn't going to be solved in the near future (less than three years) if it is a similar technical problem to inserting another disk into an existing vdev.

And that means that as it becomes harder to buy older 512 byte disks, everyone with a ZFS pool is going to be stuck with managing quite a lot of downtime as they upgrade. And even more pain if they boot off that pool.


On 10/09/2014 4:51pm, Peter Wemm wrote:
> For what its worth, in the freebsd.org cluster we automatically align 
> everything to a minimum of 4k, no matter what the actual drive is.
> 
> We set:  sysctl vfs.zfs.min_auto_ashift=12
> (this saves a lot of messing around with gnop etc)
> 
> and ensure all the gpt slices are 4k or better aligned.

Should the FreeBSD project change this minimum in the next release? There seems to be no downside and a huge amount of pain for people who stumble along with the defaults not knowing what a mess they are creating to solve later.


Cheers
Ari


-- 
-------------------------->
Aristedes Maniatis
ish
http://www.ish.com.au
Level 1, 30 Wilson Street Newtown 2042 Australia
phone +61 2 9550 5001   fax +61 2 9550 4001
GPG fingerprint CBFB 84B4 738D 4E87 5E5C  5EFA EF6A 7D2E 3E49 102A