[OpenIndiana-discuss] zfs question - when can _rewriting_ a block of a file fail on out-of-space?

[Kees Nuyt]

On Sat, 2 Jun 2012 01:45:05 -0400, you wrote:

> In a non-COW filesystem, one would expect that rewriting 
> an already allocated block would never fail for 
> out-of-space (ENOSPC).
>
> But I would expect that it could on ZFS - definitely if 
> there was a snapshot around, as it would create a 
> divergence from that snapshot (because both blocks would 
> be kept). Or if deduplication was in effect, and the new 
> block contents were unique when the old contents hadn't 
> been unique.

Not only then. Every block written obeys COW, the previous version is
always retained. The new version of the block is always a new
allocation. The previous version is released over time, when nothing
refers to it anymore.

> Could rewriting a block _ever_ fail with ENOSPC if there 
> _wasn't_ a snapshot present, 

Yes.

> or is the replace old block 
> with new somehow guaranteed to succeed, so as to avoid 
> introducing unexpected semantics? (say maybe there's a 
> reserved amount of free space just for rewrites to avoid 
> that sort of problem, or some other magic)

No. You have to monitor the used capacity of the pool, with alert
thresholds of, for example, 80% (warning), 95% (critical) and 98%
(fatal).
 
It might make sense to limit the size per filesystem using quota to
avoid the pool from filling up completely, protecting one zfs against
another zfs.

> I would think DBMS developers allowing databases to be 
> stored on ZFS, as well as folks using mmap(), might 
> particularly want to be aware of the cases in which an 
> errno not anticipated from experience with other 
> filesystems might arise.

http://assets.en.oreilly.com/1/event/21/Optimizing%20MySQL%20Performance%20with%20ZFS%20Presentation.pdf

-- 
Regards,

Kees Nuyt