[OpenIndiana-discuss] Good SLOG devices?
ijanmaat at xs4all.nl
Wed Mar 2 07:52:21 UTC 2011
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
I think that makes the Vertex a bit more interesting. Or should I go for
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.
> 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
> 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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