Alex Watson
The hard disk is reaching the end of its life, with SSDs ready to take over, but Apple’s silence over a key issue means they’re not a worry-free buy.
There are few pastimes know-it-all geeks enjoy more than berating Apple (and loyal Apple users) for its slow uptake of technology they consider standard for the average PC.
‘What, you mean you can’t get a Mac with an HDMI port?’
‘I wonder when Apple will make a Mac with a Blu-ray drive? It’s only been on the market for, what, four years?’
And so on and so forth. The truth is that Apple is like most technology companies, in that there are times when it’s slow at introducing features and moments when it’s very aggressive. Apple laptop consumers had to wait until the middle of 2009 to get an SD card slot – a delay comparable to a nation not inventing scissors until the late 1980s. On the other hand, Apple was one of the first big computer companies to drop floppy disk drives and to start using TFT screens for its desktop computers, which, in 2002, was a bit like having a car in the 1700s.
While Apple’s computers have, at least in terms of the technology they’ve supported, been something of a mixed bag, it’s rarely put a foot wrong with its iPod line-up. In particular, I’m thinking of the bold move it made in killing the iPod mini and replacing it with the iPod nano. The nano was slimmer and lighter than the mini, thanks mainly to the fact it didn’t use a hard disk for storage – instead, it was the first real iPod (I’m discounting the shuffle) to use flash memory. Since then, the hard disk has been marginalised when it comes to iPods and it won’t be long before the sole hard disk iPod reaches the end of its life.
While Apple’s move to flash memory in 2005 was an adventurous move, these days no-one would seriously think about making a mobile audio player or smartphone with a hard disk. They’re just too fragile, use too much power and are too bulky.
Strangely though, it’s still considered completely normal to build a mobile computer using a hard disk, even though all three of those issues – robustness, power use and size – are negatives for a laptop. We’re starting to see a change though, as the era of the hard disk is coming to an end. Just as with iPods, computers are now increasingly available with flash memory bulk storage, in the shape of the solid-state disk (SSD for short). Apple now offers an SSD as an optional extra on all its MacBook Pros, and it’s even part of the priciest Air’s standard configuration.
There are some mild technical differences between what you get inside an iPhone and the SSD, but essentially they’re the same: a bank of memory chips on which to store your data. Unlike system Ram, SSDs use non-volatile flash memory, so they retain the data they’re storing even when powered off. Unlike a hard disk – which is really just a very, very refined record player, spinning a platter round and round and reading data using a read head – there are no moving parts in an SSD. It makes no noise, doesn’t vibrate and typically runs cooler than a hard disk.
It’s also potentially a lot faster. With a hard disk, when the software makes a request for a piece of information, that information needs to be located, so there’s latency while the read arm waits for the disk to spin round to the right place. With an SSD, the data is all equally accessible. Today’s fastest consumer hard disks have a random access time of around 7 milliseconds; with an SSD it’s around 0.1ms. It’s not just access times that are faster – reading data is much quicker, too. You can look forward to around 225MB/sec from a good SSD, compared with around 141MB/sec for a top-of-the-range consumer desktop hard disk.
Why are hard disks still around then? Two reasons: cost and capacity. A high-capacity desktop drive will generally cost you around 6p per gigabyte – an SSD will typically top £2 per gigabyte. Capacity-wise, they tend to top out at around 500GB, whereas hard disks have reached 2TB and have 3TB in their sights.
That said, SSD prices are falling: choosing an SSD for a 17in MacBook Pro adds only £160 to the price (although it drops you from 500GB of space to 128GB). An SSD is still a premium product, but for those buying top-of-the-range machines, an extra couple of hundred quid isn’t a deal-breaker. My desktop PC has an SSD drive and it’s not just the rapidity that I enjoy – the silence, and the lack of the chk-chk-chk when I double-click a program is blissful.
There are still issues with SSDs, though. First, as Apple only offers an SSD as an option, it still needs to design the computer chassis to accommodate a hard disk, so doesn’t reap the size advantage of using chips instead of mechanical platters. SSDs are still expensive – and limited in capacity – so you can’t just stop offering hard disks. However, the SSD-only approach would suit one MacBook: the Air. Apple offers it with a hard disk, but it’s a small-capacity one (120GB), as Air users already accept it’s a secondary computer or one that makes a lot of use of Internet-based software. Dropping provision for a hard disk might help Apple make the next Air even more ludicrously slender.
It’s not just the chassis that needs to be designed with an SSD in mind – the same is true for the operating system. One thing we’ve found in testing SSDs for bit-tech.net is that while they do offer amazing performance when new, SSDs can suffer from horrendous performance degradation.
An SSD comprises numerous chips of Nand flash memory, and these are subdivided into cells. Imagine a 20KB cell; you then save two 10KB text files, which the OS places in said cell. When you later delete one, the SSD doesn’t actually get rid of it – it simply flags the cell as having 10KB of free space. When it comes to writing in a new file, while a hard disk can just write over the old data, when SSDs have ‘dirty’ cells (such as ours, which has one 10KB file in it that we need to keep), they need to copy the data they’re keeping out to cache, wipe the whole cell clean and then write in the kept data, plus the new information. This read-modify-write process can dramatically slow down an SSD.
It’s difficult to pick one figure to sum it up (because effects vary across models and in different tests), but the sequential write speed of one, fairly typical SSD dropped from 175MB/sec to 83MB/sec when it went from brand new to used.
It’s a real problem, and the SSD manufacturers started to release their own software fixes for it, utilising a command called Trim. Most have programs you manually run that force the SSD to actually delete files instead of just flagging cells as free. With Windows 7, Microsoft has now patched this fix into the OS, so it’s triggered when you empty the recycle bin if your SSD is running compatible firmware.
So the big question is, how does junk data affect Apple Macs, and does OS X support Trim? There’s no mention at all of Trim for SSDs on the Apple website, despite the fact that if you go to Google and type in ‘Mac OS X SSD’, Trim is the first suggestion it adds. The web is filled with people posting on forums wanting clarification – and I don’t blame them. If Mac OS X doesn’t support Trim and suffers from the same levels of performance degradation we’ve seen in Windows systems (pre-Trim), then paying extra for an SSD in your MacBook just isn’t worth it.
I tried asking Apple and their PR’s response was that ‘we don’t go into that level of detail’. However, after some discussion and a few emails back and forth, they are going to send us a MacBook Air with an SSD to test – look out for the performance analysis in a forthcoming issue.
