Does FreeBSD's ZFS reserve 1/64 of the capacity for COW?

[Warner Losh]

On Dec 9, 2013, at 12:15 AM, Trond Endrestøl wrote:

> On Fri, 6 Dec 2013 11:23-0600, Mark Felder wrote:
> 
>> 
>> 
>> On Fri, Dec 6, 2013, at 6:46, Trond Endrestøl wrote:
>>> Hi,
>>> 
>>> Today I came across "Becoming a ZFS Ninja Part 1" 
>>> http://www.youtube.com/watch?v=tPsV_8k-aVU featuring Ben Rockwood who 
>>> stated that ZFS reserves 1/64 of the capacity for COW safe-guarding. 
>>> Fast forward to 00:49:44. Thus, one should take this fact into account 
>>> when planning the storage capacity of your (larger) zpools.
>>> 
>>> Does FreeBSD's ZFS also reserve 1/64 of the capacity for COW?
>>> 
>>> If it's true, then I guess this should be noted in zpool(8) and in the 
>>> Handbook.
>>> 
>> 
>> Well, the rule-of-thumb / urban legend tends to be "don't let your zpool
>> go over ~80% or you'll have severe performance issues" so I'm not sure
>> that statement is accurate or COW wouldn't be so painful when your pool
>> is over ~80%. I could be wrong though; I often am :-)
> 
> As it turns out, you're in good company, I'm also wrong far too often.

I have a zpool that I share between development (/usr/src) and videos (tivo backups). When it gets over 90% full, I start to see buildworld or svn operations slow way down...

Warner