ZFS: How to enable cache and logs.

[Jeremy Chadwick]

On Wed, May 11, 2011 at 09:51:58PM -0500, Bob Friesenhahn wrote:
> On Thu, 12 May 2011, Danny Carroll wrote:
> >
> >Replying to myself in order to summarise the recommendations (when using
> >v28):
> >- Don't use SSD for the Log device.  Write speed tends to be a problem.
> 
> DO use SSD for the log device.  The log device is only used for
> synchronous writes.  Except for certain usages (E.g. database and
> NFS server) most writes will be asynchronous and never be written to
> the log.  Huge synchronous writes will also bypass the SSD log
> device. The log device is for reducing latency on small synchronous
> writes.

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).

I imagine a very high-end SSD (FusionIO, etc. -- the things that cost
$900 and higher) would have extremely high write performance and would
work perfectly for this role.  Or a battery-backed DDR RAM device.

What's amusing (to me anyway) is that when ZFS was originally presented,
engineers from Sun folks kept focusing on how "you can buy cheap,
generic disks and accomplish goals!" yet if the above statement of mine
is accurate, that goes against the original principle.

Danny might also find this URL useful:

http://constantin.glez.de/blog/2011/02/frequently-asked-questions-about-flash-memory-ssds-and-zfs

> >- Is there any advantage to using a spare partition on a SCSI or SATA
> >drive as L2Arc?  Assuming it was in the machine already but doing nothing?
> 
> The L2ARC is intended to reduce read latency and is random accessed.
> It is unlikely that rotating media will work well for that.

Agreed -- this is why I tell folks that an SSD would work very well for
L2ARC, but my opinion is just to buy more RAM for the ARC ("layer 1").

-- 
| Jeremy Chadwick                                   jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.               PGP 4BD6C0CB |