Hi Michael,
just to avoid misunderstandings regarding '*Flash* based NVRAM' (see below) - and to be picky - ... ;-)
Most 'NVRAM' is actually NVMEM - battery backed DIMMs on the mainboard (22x0, 32x0 not to mention some of the older models). So AFAIK negligible speed difference.
Exception being the NVRAM8 (62x0), where we have a PCIe card with DIMMs and destaging to Flash ("persistent write log"). To quote the FAQ: "The persistent write log feature on the FAS/V6200 destages NVRAM contents to flash memory in the event of a dirty shutdown. Since the NVLOG contents are stored in flash memory, they are protected permanently regardless of how long the power outage lasts. During initial bootup after power is restored the destaged NVLOG will be replayed, enabling the file system contents to include any acknowledged writes since the last consistency checkpoint (CP) to disk." "In the event of a dirty shutdown, NVRAM8 uses its battery to keep the NVLOG DIMMs refreshed while it destages their contents to flash memory. The operation takes about a minute and afterward the NVRAM8 card turns itself off."
So in conclusion: no Flash (except in case of NVRAM8 destaging), just DIMMs with regular DIMM speeds.
Sebastian
On 1/24/2014 2:15 PM, Michael Bergman wrote:
Sebastian Goetze wrote:
I usually describe NVRAM as a transaction log...
Being really really picky now [sorry]... is it..?
The NVRAM holds a meticulously designed copy (super high integrity) of the transaction log for WAFL, while at the same time limiting the size of the transaction log which is in effect in RAM in the controller.
So saying to ppl that the NVRAM actually *is* the transaction log, is not strictly correct either. Which is why I personally prefer not to say that. Frankly avoiding to mention the NVRAM at all (including the size of it) is the best thing most often, and then when ppl (happens quite often) say things like: "ha ha, a NetApp has sooo little write cache EMC VNX is much better" then you take out the big arsenal and teach them... ;-)
It is true that the speed of the Flash based NVRAM isn't zero effect on things inside ONTAP when writing to disk. But it's small, very small. Tiny, insignificant compared to other factors.
It's also true that the size of the transaction log, as limited by the size of the NVRAM, can affect things w.r.t. performance in various ways. It's good to a certain extent to have a bigger transaction log, but the bigger the NVRAM the worse it gets in a HA failover situation. It slows things down, that's the trade-off. There's no problem per se to make the NVRAM (= the WAFL log) much bigger, the HW guys at NetApp could easily do that but the SW teams responsible for the HA cluster won't allow it, that's basically how it plays out
Cheers, /M
On 1/20/2014 11:33 PM, Jeff Mohler wrote:
PS: I think we just agreed. :)
But its good to make the hard point, that NVRAM is not part of a write. Its common to see it as such, but it's systemically more correct to never mention NVRAM when talking about writes..cuz it doesnt matter. Its just a protection.
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