ZFS: How to enable cache and logs.

[Daniel Kalchev]

On 12.05.11 06:36, Jeremy Chadwick wrote:
> 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.
>
Perhaps I misstated it in my first post, but there is nothing wrong with 
using SSD for the SLOG.

You can of course create usage/benchmark scenario, where an (cheap) SSD 
based SLOG will be worse than an (fast) HDD based SLOG, especially if 
you are not concerned about latency. The SLOG resolves two issues, it 
increases the pool throughput (primary storage) by removing small 
synchronous writes from it, that will unnecessarily introduce head 
movement and more IOPS and it provided low latency for small synchronous 
writes.

The later is only valid if the SSD is sufficiently write-optimized. Most 
consumer SSDs end up saturated by writes. Sequential write IOPS is what 
matters here.

About TRIM. As it was already mentioned, you will use only small portion 
of an (for example) 32GB SSD for the SLOG. If you do not allocate the 
entire SSD, then wear leveling will be able to play well and it is very 
likely you will not suffer any performance degradation.

By the way, I do not believe Windows benchmark has any significance in 
our ZFS usage for the SSDs. How is TRIM implemented in Windows? How does 
it relate to SSD usage as SLOG and L2ARC?

How can ever TRIM support influence reading from the drive?!

TRIM is an slow operation. How often are these issued? What is the 
impact of issuing TRIM to an otherwise loaded SSD?

Daniel