ZFS: How to enable cache and logs.

[Bob Friesenhahn]

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.

It is worth mentioning that the ZIL is a write-only device which is 
only read when the system boots or a pool is imported.  The writes are 
usually "write and forget" since zfs uses them to improve its ability 
to cache larger transaction groups.

Bob
-- 
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/