[OpenIndiana-discuss] Good SLOG devices?

Ivar Janmaat ijanmaat at xs4all.nl
Wed Mar 2 07:52:21 UTC 2011

Thank you!

I read in the DDRdrive paper that X-25E does not have  a power cell 
onboard to complete operations if power fails.
The Vertex 2 Pro does have some capacitors to complete operations after 
power failure.
I think that makes the Vertex a bit more interesting. Or should I go for 
a UPS?
I have already done some statistics with zilstate. I will try to see if 
I can also do some wear calculations.
The problem is however that I first need to build the system so it can 
support lots of VMs.
The proper ZIL statistics will be only then available when the system 
reaches full load.

Kind regards,


wessels schreef:
> forget the acard for the x4150
> there aren't many datapoints (experiences, tests) about the OCZ Vertex
> 2 series, not like the intel SSD's or the DDRdrive and I've no
> experience with them or any SSD as SLOG for that matter.
> Yes the zil is checksummoned. But you need another copy of the data.
> Otherwise when the zil is read, and errors are detected, from the slog
> device you're still in trouble. Remember the zil is rarely read, it's
> mainly write-only. But when the slog devices fails during normal
> operation it's offlined and the zil is written on the pool like
> without the slog.
> The mtbf isn't correlated with the wear of the device. Every time you
> (implicit) erase a page the cells in the page wear a bit. As rule of
> thumb SLC cells can be programmed 100000x and MLC cells 10000x. So
> after these erases the cell's can't be used anymore. 10000 writes is a
> bit short but the SSD employ wear-leveling techniques to ensure that
> all cells gets evenly erased. How long your SSD last is a function of
> how much data you write to it. A smaller SSD will wear out earlier
> than a larger SSD of the same product line because of wear leveling.
> Google the calculations. And newer is not always better. The new 25nm
> generation of flash memory wears out much faster than previous 34nm
> generation.
> To determine how much the zil is used you can use the excellent tool
> from Richard Elling at
> http://www.richardelling.com/Home/scripts-and-programs-1/zilstat By
> monitoring how much your zil is used you can determine how much is
> written to it in say a day. With that number you can calculate when
> your ssd wears out.
> TRIM is not needed on good SSD. Look at early intel SSD's they didn't
> have TRIM and still performed excellent. Remember the zil is very
> small a couple of gigabytes at the most. So the rest of the drive can
> be used for wear-leveling. Maybe the drive can be helped be limiting
> it's capacity via e.g. DCO you only need 4 or 6GB for the zil. But you
> need to test this.
> All of the above is explained in much more depth in the ddrdrive paper
> I mentioned earlier.
> The DDRdrive and the Intel X25E are validated solutions all other
> drives are a gamble...
> Labeling a product "enterprise" doesn't necessary mean it's a
> validated enterprise product...
> Good luck

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