ZFS: How to enable cache and logs.

[Rick Macklem]

> On Wed, 11 May 2011, Jeremy Chadwick wrote:
> >
> > Bob, please correct me if I'm wrong, but as I understand it a log
> > device
> > (ZIL) effectively limits the overall write speed of the pool itself.
> > Consumer-level SSDs do not have extremely high write performance
> > (and it
> > gets worse without TRIM; again a 70% decrease in write speed in some
> > cases).
> 
> It is certainly a factor. However, large block writes (something like
> 128K, I don't remember exactly) bypass the dedicated log device and
> instead are written to the main store (with only a reference being
> added to the dedicated device). The reason this is done is for the
> exact reason you point out. The SSD has a very fast seek and zero
> rotational latency but being a singular resource it suffers from
> bandwidth limitations. The main store usually suffers from
> multi-millisecond seeks and rotational latency but offers linearly
> scalable and substantial write performance for larger writes.
> 
> Matt Ahrens has described this a few times on the zfs-discuss list and
> there is mention of it on slide 15 of the presentation found at
> "http://www.slideshare.net/edigit/zfs-presentation".
> 
> The large write feature of the ZIL is a reason why we should
> appreciate modern NFS's large-write capability and avoid anchient NFS.
> 
The size of a write for the new FreeBSD NFS server is limited to
MAX_BSIZE. It is currently 64K, but I would like to see it much larger.
I am going to try increasing MAX_BSIZE soon, to see what happens.

This sounds like another good reason to increase it.

However, a client chooses what size to use, up to the server`s
limit (and, again, MAX_BSIZE for the FreeBSD client).

rick