Mac Musings

Tomorrow's Solid State Drives and Notebooks

Daniel Knight - 2008.09.04 -

Flash memory is pretty amazing stuff. Invented in 1980, it made digital photography possible. It's been used in PDAs for ages. And its inside those useful USB flash drives that have become so affordable and become a normal way of moving data between computers.

And now flash drives (also known as Solid State Drives or SSDs) are becoming popular in notebook computers. They're a lot more expensive than hard drives, but they have no moving parts and may reduce power consumption compared with hard drives. (Some tests find this to be true, others find the opposite.)

Flash drives are not without their drawbacks. I'm not just talking about price and capacity - prices keep dropping, and capacity keeps increasing, as is also true of hard drives and computer RAM. SSDs may never achieve price parity with hard drives, especially at high capacities, but they will become more competitive on the low end.

Shortcomings of Flash Memory

Flash drives have negligible seek time and latency compared to hard drives, as they have no moving parts. And with no moving parts, they don't generate as much heat as hard drives - or as much noise. They may also provide faster startup time and are very reliable.

Well, in the short term at least. It's believed that flash memory cells are good for 10,000 to 1,000,000 write cycles, which is a lot less than hard drives. One way to work with this is to have a file system that spreads writes throughout the drive's capacity (this is called "wear leveling"), and with this technology today's flash drives can last "up to 20 years" with typical use.

Perhaps the biggest technical drawback to flash memory is slow write speed compared to read speed. The original NOR chips, invented at Intel, had long erase and write times, and the newer NAND chips, introduced by Toshiba in 1989, have faster read and write speed along with higher storage density. Where NOR chips have true random access to every byte, NAND chips access data in blocks of hundreds or thousands of bytes.

The size of these blocks is important, as flash RAM can only be erased a block at a time. This is part of the reason SSDs have significantly slower write speed than read speed. Because today's operating systems make extensive use of virtual memory, which uses hard drive (or SSD) space as though it were system memory - albeit much slower than DRAM. This involves lots of reads and writes, so the slow write speed of flash RAM is a big drawback.

Flash Drives vs. Hard Drives

On a cost-per-gigabyte basis, hard drives have it all over flash memory, and they're also available in much higher capacities. Read and write speeds are close to each other, making hard drives much better than SSDs for virtual memory or other applications that frequently write data. The drawback is that hard drives have moving parts, generate heat and noise, may draw more power than SSDs, and are susceptible to damage when dropped.

Perhaps the biggest difference between SSDs and hard drives is buffering. Hard drives have buffers (also called caches), which may be as small as 64 KB on really old drives or as large as 32 MB on some of today's top-end drives. The drive cache is memory on the drive controlled by its microcontroller. The purpose of the cache is to store data read from the hard drive and pass it along to the computer more quickly than it can be read from the drive platters.

In the early days of Macintosh computing, SCSI hard drives had minuscule buffers. You had to set the interleave of your hard drive so it wouldn't overwhelm your Mac's SCSI bus - and that was a different setting for the Mac Plus, Mac SE, and Mac II, making it a real hassle to move a drive between Macs.

Possibly the first widely used drive to overcome this problem was the Quantum Prodrive, which had a 64 KB buffer. Interleave was no longer an issue, as the buffer could accept data as fast as the hard drive could dish it out and feed it to the Mac as fast as the SCSI bus could accept it. This unleashed hard drive performance, and Prodrives were very popular "back in the day" (circa 1990).

From that point forward, bigger buffers were one way to improve drive performance, and today it's not uncommon for a drive to have an 8 MB or 16 MB buffer. Even small hard drives, such as the 5 GB drive used in the original iPod and the 80 GB drive in the MacBook Air, have buffers (1 MB and 2 MB respectively).

Improving Flash Performance

Flash drives don't have buffers.

Why not? If a DRAM buffer can make such a difference for hard drive performance, it stands to reason that it could do the same thing for SSDs. Writes could be buffered, which should help a lot with virtual memory. Reads would be buffered, also improving performance. Virtual memory performance would be better.

Adding a buffer to SSDs would add very little to its price, and I suspect the performance advantage would give the first company to do it a real leg up on the competition.

Another way to improve SSD performance would be to design new notebooks with hardware support for RAID and use two flash drives in parallel. This could theoretically double the data rate of reads and writes, giving SSD-based notebooks much perkier performance. This would occupy more space inside the computer, but the two lower capacity flash drives might also be cheaper than one higher capacity drive, resulting in improved performance at no additional cost - or doubling capacity for those who need more than today's highest capacity SSDs offer.

Another option is to include both a flash drive and a hard drive in the same computer. The flash drive would be the working drive, containing the core OS and other frequently accessed files. The hard drive could be the storage drive, holding your iPhoto and iTunes libraries, your work documents, help files, and other items that would be accessed occasionally. This would allow use of a lower capacity, lower cost flash drive while also providing the level of hard drive storage many of us need for media files. This isn't a low cost option, but it would be a lot less costly to include a 32 GB SSD and 160 GB microdrive than a 128 GB SSD.

Now imagine Apple offering SSD RAID plus a hard drive in the MacBook Pro line, along with at least one quad-core CPU option, and Apple's already awesome pro notebooks would stand head and shoulders above the rest.

We're still in the early days of flash drives, much as we were in the early days of hard drives in the 1980s. Buffers, higher rotation speeds, increased data density, and RAID were all ways that hard drives got better. Buffers, higher throughput, increased data density, and RAID are also ways flash drives can get better.

As good as SSD is, it can get better. Who will be the first to do it?

Resources

• Flash memory, Wikipedia
• Solid-state drive, Wikipedia
• Virtual memory, Wikipedia
• Hard drive, Wikipedia
• Disk buffer, Wikipedia