back to article DRAM! Speedy software upstart PernixData's caching up fast

VMware IO cacher PernixData has upped the quick access ante by caching virtual machine disk IO in DRAM — much faster for data access than flash. The startup’s FVP software runs in the ESXi hypervisor, and it's also added user-defined fault domains and adaptive network compression to its clustering technology. FVP accelerates …

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No sh!t, DRAM to the rescue!

Wow, once again we find out that massive DRAM caches speed things up. Who would have thought?

These days a 60MHz 32bit processor is smaller than your thumbnail, costs $5, and people are still amazed that massive DRAM caches improve I/O. Back when IDE drives had just been introduced, I bought a smart controller and populated it with 16Mb of cache. Wow, builds could fly! And when I shutdown the system, the writes continued for a minute. Same thing here, different day, same premise.

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Um...

How is this different to what OSs already do with file I/O? I don't what other OSs do, but Linux uses all available RAM that hasn't been specifically allocated for anything else as cache and buffer space. What am I missing?

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Re: Um...

Also windows since vista. There are basically two reasons to write to disk.

1. A requirement to not lose the data when shutdown or other system crash (including the VM host).

2. Overflow storage where there is insufficient RAM to work in memory (very large datasets).

This solution doesn't solve 1 and if you are using it for 2 then why not just give it that RAM as RAM?

RAM backed SSD can make sense to boost longevity by avoiding overwriting the same block (discussed here) but this use case smells weird to me.

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Re: Um...

Windows has been setting aside RAM for I/O since NT and I've been adjusting the parameters by machine purposes since Windows 2000. And that doesn't even touch using custom software by device to shape what/where/when on more operating systems than I can count, readily, going back to big iron.

It's another knob to tweak, if you can get good performance data and (!) have a good safety net. I also wonder why it isn't a core feature. EMC related? Supporting other vendors? The VMware ecosystem has been good for a rather long (internet) time.

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Business model?

What prevents vmware to add this feature to its hypervisor?. Seems like the simplest way to boost performance. Or just buy them up.

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Re: Business model?

Yeah, I wish VMware would buy Atlantis or any of these other SDS companies. Would really change the game for vSAN in my opinion.

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More than just centralised caching

I think there are a few points that are being missed here.

There's a difference between caching on a single server compared to a hypervisor; vSphere etc generate much more random I/O, so any opportunity to eliminate I/O is good; where data is repeatedly written, that I/O can be consolidated and written to disk more periodically, without loss of resiliency, but saving backend I/O. Cache can also be de-duplicated. This has significant benefit in making the cache "go further" in a shared environment.

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Re: More than just centralised caching

Exactly. It is much better to have a unified caching layer rather than letting individual Guest OSes handle all the caching. I explain it more in detail here:

http://blog.infinio.com/more-ram-doesnt-mean-faster-vms

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