Fusion Drive: Apple jumps on the SSD cache bandwagon | Hacker News Reader

Fusion Drive: Apple jumps on the SSD cache bandwagon

extremetech.com

36 pointsevo_940 comments

"Update: After poking at this more, it’s possible that Apple is using a simplified tier structure in which entire files are moved to the SSD and backed up the HDD, rather than a more sophisticated caching mechanism."

Sensational blog post. Extremetech couldn't be troubled to follow through with due diligence, it apparently doesn't have any journalistic integrity, or editorial review. They literally falsely accuse a company (right in the blog title) of releasing an overpriced commonly available product. Then, at the end of the article excuse their horrible research, by starting with the sentance "it’s possible that Apple". Shouldn't Extremetech know what Apple is doing since you just finished writing a scathing blog post about it??

In fact, they are not caching at all. Extremetech completely blew it on this article, either revealing their lack of professionalism, bias, and/or, both.

Apple's charging $250 for a $99 SSD plus some software. It's overpriced.

Welcome to Apple's SSD pricing. Want to upgrade from 128GB SSD on a laptop to 256? $300 for that please. How about from 128GB to 512GB? $800. All the way to an astounding $1,300 for a 768GB drive. It's insanity, but since you can't open up the laptops to add one of your own it's insanity that you have to pay for.

I really hope you don't have to pay $1,300 to get a 768GB drive in your system. I ran into a situation a couple years ago when the 320GB drive in my MBP was running out of space. I didn't want to buy a whole new system, so I ordered a 640GB drive online, took it to a local certified Apple repair shop, and let them put the drive in. The cost of the work? $100.

This was before the unibody models, but I'm sure the process will be the same for the new systems. Oh, and the best part of going to an Apple repair shop was that my warranty remained intact. I never had any issues getting the Apple Store to work on my laptop after the hard drive was replaced.

If Apple prevents people from doing this with the new hardware, then this will be a disappointment. I haven't kept up with the specifics of their new laptops--I know that the RAM is soldered in, but I hope the hard drive will be replaceable.

AFAIK on the latest Apple models with SSDs, the SSDs are a proprietary form factor but not soldered on.

OWC sells an ssd upgrade for Retina MBP:

http://eshop.macsales.com/shop/SSD/OWC/Aura_Pro_Retina_2012

Apple charging more than the stand-alone market price for storage upgrades... just like they have done forever?

It's called bundling and price discrimination.

That does seem high, but if you factor in the (non-trivial) installation process, and the fact that you won't be voiding your warranty with a DIY installation, it may be an acceptable tradeoff. Apple is probably counting on your reaching this conclusion as well.

Yes, but to be fair no they shouldn't have known. Apple is infamous for not revealing any technical details (they won't even say how much memory their iOS devices have!) and the units have not shipped.

Ars was able to figure out from Schiller's description that it was probably a tiered setup, not a caching system, by 1:25PM Pacific yesterday.

http://arstechnica.com/information-technology/2012/10/apple-...

Completely agree. I would also be surprised if this weren't done at the block level, as ZFS does SSD acceleration, rather than at the file level as the article claims. It's just so much more efficient than caching at the file layer, for not that much more RAM cost (for indexing).

Apple apparently abandoned ZFS to adopt specific subsets of the technology. Someone else mentioned 4 GB of the SSD is for write cache (log device). Guessing the other XGB is the traditional cache device.

Should really improve daily usage performance. Makes sense their benchmarks only go to 4GB.

This is no excuse for Extremetech's folly, but in addition to the tiering Anandtech says that there's 4GB of the SSD reserved as a write cache.

My guess it's using some tech similar to ZFS. Creating a 4GB log drive for writing, and the rest as a cache drive. Sounds like they abandoned the adaption of ZFS to make their own interpretation.

Given the patent-trolling that NetApp was waging against Sun for ZFS, perhaps Apple was wise to stay away (let's leave aside how depressing that is).

I wanted ZFS as much as anybody, but NetApp did push the state of the art in some areas of filesystem design and the first thing everybody who used NetApp said when they heard about ZFS was "cool, it's like a NetApp". Sometimes it's not trolling; it's just enforcing your rightful patents.

It doesn't sound like a cache at all. Think of it like their old "hotfiles" relocation to the faster part of the disk, only now they're relocating the blocks from entire files to the faster volume in the corestorage group.

Edit: Alright, they updated the blogpost saying this.

But it's not an SSD 'cache' right? It's two separate disks/units merged by the OS. Not like Intel, OCZ, etc which is an HDD with some SSD chips "before the bus", Right?

http://s2.gizmologia.com/files/2011/06/2011-05-31-ocz.jpg

Or I am missing out?

It's two separate disks/units merged by the OS.

That's my understanding, and in fact I expect that would make it work better in some ways.

But if that's so - if it's two commodity pieces of hardware managed by some driver-level software - then it would make sense for it to be cheaper than the custom-hardware solutions from other sources. Or at least, we might expect the per-unit cost to Apple to be lower than the custom hardware systems.

So that just makes the alleged overpricing worse.

It would only be cheaper if the components were the same. My Seagate hybrid drive has somwthing like 4-8GB of flash. Doesn't the Apple hybrid have 128GB?

Yes, 128GB for $250. But excellent 128GB SSDs can be had at retail for as little as ~$90 on sale. What does that 128GB drive - purchased in huge quantities directly from the manufacturer - cost Apple?

Your Seagate hybrid isn't the only system of it's kind, it's not the comparison I had in mind.

That was my understanding. I had thought they had automated the process of having an SSD for often-used applications plus an HDD for files and less-used applications that can be loaded slowly.

It's not a new concept, I do it now on my Win7 machine, but the automated part seems handy.

Isn't this exactly what intel RST does?

Sorry, had to look that up. From my brief glance this is not the same thing as I understand it. Intel's RST seems to be just their version of RAID. The Fusion Drive doesn't seem like a RAID to me. RAID provides speed benefits when data is split across multiple HDDs that can be accessed simultaneously. This doesn't seem to be what Fusion Drive does.

For example, on my Win7 machine I have a SSD as my main drive and a HDD for storage. Since space is at a premium on my SSD I can install applications that I rarely use and files for storage on the HDD. Applications that I want to load quickly I install them on the SSD.

But I have to do all this manually.

Fusion Drive seems to be software in the OS that handles that idea for you. Anything you use constantly will be moved to the SSD for the speed benefit while everything else you rarely use goes to the HDD where speed is not as big a deal.

Sounds like ReadyBoost that Windows Vista introduced...

In practical terms last week if you wanted a Flash+HDD solution from apple it cost about $500 to add a 256GB flash drive and 1GB hard disk (I can't remember exactly, and anyway I'm more familiar with UK prices). Now you can get a fusion drive for $250.

They're not directly comparable technically, but I'm in the market for a 27" iMac. The cost of the optimum available spec for me just went down and the convenience went up, so I'm a happy camper.

It sounds more like tiered storage than cache to me, or perhaps it's both. But why let that get in the way of a sensational headline.

It sounds like a huge improvement over my current workflow of having my OS and Apps on the SSD and then having symlinks to my old larger hard drive for all my photos and videos.

Has this sort of thing really existed on Windows for years or are people just seeing the words "flash" and "cache" and oversimplifying it?

Intel's Z68 SRT has existed some time with windows-only drivers, but it's not clear how Apple's fusion drive compares in functionality, portability or performance with an equivalently specced SRT-drive (ie, a 128GB SSD used as the cache drive in a windows box).

A detailed review + teardown will able to answer these questions.

Well you cant call it caching. By definition, that means data is written to both drives at the same time. In the case of Fusion Drive data is always read from and written to the SSD. It is basically a software which moves files around.

Data is always written to the SSD, but _not_ always read from the SSD, just from whatever device it's stored on.

Well you cant call it caching. By definition, that means data is written to both drives at the same time.

Writing data to both the cache and the main store is not a fundamental property of caches. Consider, for example, the caches on the microprocessor of an x86 chip. The only time both the cache and main memory will contain the same data is if the trait is set to 'WT', which is very rare.

It's true that (coherent) caches behave in such a way that you can't tell that the data isn't duplicated at each level, but that's achieved by writing back the data when, for example, an external device requests some particular cached data.

If the cache is persistent, and external devices can access the cache as easily as the main store, there's no particular reason to write data back, other than evictions [1], i.e., you should expect the cache to basically be "a software which moves files around"

[1] This isn't strictly true -- depending on the access patterns and the characteristics of the cache and the bus, you might want to opportunistically write data back to avoid having to write back on an eviction. Disclaimer: I've only worked on microprocessor caches and not disk caches, so I'm not intimately familiar with performance traces for disk caches. However, based on simple back-of-the envelope reasoning, it seems to make sense to make disk caches the type of cache you consider not to be a cache at all.

That's the definition of a write-through cache, not caching in general. http://en.wikipedia.org/wiki/Cache_(computing)#Writing_polic...

Also, is it really true that data is always read from the SSD? So if a block is requested that only exists on the hard drive, the data must be copied from the hard drive to the SSD, then read from the SSD--rather than simply reading the data from the hard drive?

Which chipset does the new imac use?

Intel has SSD caching support built into the Z68 chipset now, it sounds eerily similar, they could simply be using that. Both reads and writes are cached.

http://www.anandtech.com/show/4329/intel-z68-chipset-smart-r...

There's actually nothing built into the chipset; Intel RST is a Windows driver (it just refuses to run on any chipset that it doesn't like). RST behaves fairly differently from descriptions of Fusion Drive.

Alright, I was under the impression it was a partial hardware implementation. I knew that it was artificially restricted to the Z68 chipset only, but still thought it was sort of a winmodem type solution.

IE: there's still some special hardware there but the real smarts was in the driver.

I stand corrected. Regardless, this is only a stopgap solution while SSD prices fall. I expect to see it MIA in 2-3 generations.

I just want to know, can I override its behavior so as to favor applications or directories I want better throughput with.

Nope; no ricing allowed.

I'd love it if they would introduce support for it outside of the BTO option. I'd like to rip out my optical drive and put in an SSD, but the pain of having to transition everything over has so far kept me from doing so.

This idea originated from technology that has existed in advanced storage arrays for awhile, and in fact ZFS is optimized to take advantage of it.

Apple has consumerized it now.

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