ZFS: How to enable cache and logs.

[krad]

On 11 May 2011 23:38, Jason Hellenthal <jhell at dataix.net> wrote:

>
> Jeremy,
>
> On Wed, May 11, 2011 at 05:08:30AM -0700, Jeremy Chadwick wrote:
> > On Wed, May 11, 2011 at 02:17:42PM +0300, Daniel Kalchev wrote:
> > > On 11.05.11 13:51, Jeremy Chadwick wrote:
> > > >Furthermore, TRIM support doesn't exist with ZFS on FreeBSD, so folks
> > > >should also keep that in mind when putting an SSD into use in this
> > > >fashion.
> > >
> > > By the way, what would be the use of TRIM for SLOG and L2ARC devices?
> > > I see absolutely no benefit from TRIM for the L2ARC, because it is
> > > written slowly (on purpose).  Any current, or 1-2 generations back SSD
> > > would handle that write load without TRIM and without any performance
> > > degradation.
> > >
> > > Perhaps TRIM helps with the SLOG. But then, it is wise to use SLC
> > > SSD for the SLOG, for many reasons. The write regions on the SLC
> > > NAND should be smaller (my wild guess, current practice may differ)
> > > and the need for rewriting will be small. If you don't need to
> > > rewrite already written data, TRIM does not help. Also, as far as I
> > > understand, most "serious" SSDs (typical for SLC I guess) would have
> > > twice or more the advertised size and always write to fresh cells,
> > > scheduling an background erase of the 'overwritten' cell.
> >
> > AFAIK, drive manufacturers do not disclose just how much reallocation
> > space they keep available on an SSD.  I'd rather not speculate as to how
> > much, as I'm certain it varies per vendor.
> >
>
> Lets not forget here: The size of the separate log device may be quite
> small. A rule of thumb is that you should size the separate log to be able
> to handle 10 seconds of your expected synchronous write workload. It would
> be rare to need more than 100 MB in a separate log device, but the
> separate log must be at least 64 MB.
>
> http://www.solarisinternals.com/wiki/index.php/ZFS_Evil_Tuning_Guide
>
>
> So in other words how much is TRIM really even effective give the above ?
>
> Even with a high database write load on the disks at full compacity of the
> incoming link I would find it hard to believe that anyone could get the
> ZIL to even come close to 512MB.
>
>
> Given most SSD's come at a size greater than 32GB I hope this comes as a
> early reminder that the ZIL you are buying that disk for is only going to
> be using a small percent of that disk and I hope you justify cost over its
> actual use. If you do happen to justify creating a ZIL for your pool then
> I hope that you partition it wisely to make use of the rest of the space
> that is untouched.
>
> For all other cases I would reccomend if you still want to have a ZIL that
> you take some sort of PCI->SD CARD or USB stick into account with
> mirroring.
>
> --
>
>  Regards, (jhell)
>  Jason Hellenthal
>
>
> You have just spotted a gap in the market I suspect. Maybe SSD
manufacturers need to produce a sata based ssd of 1 or 2 gb of the fastest
write speed available flash on the market. Produce it for < £50 and you
should have a big market