Re: BTRFS as image store for KVM?

[Jim Salter <jim@jrs-s.net>]

Pretty much bog-standard, as ZFS goes.  Nothing different than what's 
recommended for any generic ZFS use.

* set blocksize to match hardware blocksize - 4K drives get 4K 
blocksize, 8K drives get 8K blocksize (Samsung SSDs)
* LZO compression is a win.  But it's not like anything sucks without 
it.  No real impact on performance for most use, + or -. Just saves space.
* > 4GB allocated to the ARC.  General rule of thumb: half the RAM 
belongs to the host (which is mostly ARC), half belongs to the guests.

I strongly prefer pool-of-mirrors topology, but nothing crazy happens if 
you use striped-with-parity instead.  I use to use RAIDZ1 (the rough 
equivalent of RAID5) quite frequently, and there wasn't anything 
amazingly sucky about it; it performed at least as well as you'd expect 
ext4 on mdraid5 to perform.

ZFS might or might not do a better job of managing fragmentation; I 
really don't know.  I strongly suspect the design difference between the 
kernel's simple FIFO page cache and ZFS' weighted cache makes a really, 
really big difference.



On 09/25/2015 09:04 AM, Austin S Hemmelgarn wrote:
 > you really need to give specifics on how you have ZFS set up in that 
case.