Flash memory does most of what current RAM-based Solid State Disks (SSD) do, and it does it without requiring battery-backup, big power supplies or noisy fans. Plus it is cheaper. So, I started to wonder, with the advent of Samsung’s 32GB SSD are we seeing the beginning of the end of the always marginal RAM disk niche?
SSDs have one overwhelming advantage: speed. Data I/O rates of many thousands of random IOPS because access times are measured in microseconds (millionths) instead of milliseconds (thousandths). They leave short stroked 15k FC drives in the dust.
Repeat 5 times quickly: RAM NOR NAND
Yep, sounds like something the Coneheads would say. My first encounter with RAM disks was at DEC, where the engineers came up with a clever design that used low-quality binned DRAM and disk-like error correcting codes (ECC) to create a lower-cost, higher-margin SCSI RAM disk. Which sold about as well as most SSDs, which is to say, not very well at all. The problem: even though performance is terrific, the price is staggering on a per GB basis. Take this pricing from the StorageMojo.com Price Lists for a Texas Memory SSD:
RS-320-FC2-64 Texas Memory Systems Hardware
RS 320 64GB 2Gigabit Fibre-Channel solid-state disc w/2 FC2 ports, upgrade-able to 8 FC2 ports and 64GB. Has 3 UPS’s and 3 backup disc drives.
At over $1k per GB these SSDs are strictly for the Gucci alligator-skin pocket protector crowd.
Enter the Dragon
Not all flash is created equal. There are two main types, NOR and NAND. Here is a handy table that scopes out the differences between the two:
|Chip Multi-level Capacity||256Mb||4Gb|
So what is that Multi-level capacity? Glad you asked. Both NOR and NAND are available in Single Level (SL) and Multi-level (ML). SL stores one bit per cell, while ML stores two – and I’m hearing, maybe even four RSN. ML is cheaper for a given capacity, but not that much cheaper: only about 15 – 20% less. The really big difference is that ML is only good for about 10,000 read/write (RW) cycles, which is plenty in a camera, but not so great for a disk drive.
SL though is rated for 100,000 RW cycles, which means that each bit of storage is cheaper than ML on a total lifecycle basis.
100,000 bottles of beer on the wall. . .
So, I know what you are thinking: Robin, how could you ever replace a RAM SSD with a flash SSD – the thing would wear out in a heartbeat. And you’d be almost right.
All flash drives contain wear-leveling algorithms to ensure that all cells get similar usage. So the way to think about flash drive usage is to look at your average I/O size, and figure out how many many times that I/O will fit in that size drive times the number of RW cycles.
99,999 bottles of beer on the wall. . .
Take the new Samsung 32 GB SL flash drive. Even though it is being spec’d for the notebook market, it makes a wonderful server drive because it is so fast. But how long would it last?
Let’s say you want to use it for a log file running 2k I/Os (question: do systems still do 2k I/Os? readers please help). So a 32 GB drive has 16,384,000 2k locations, which multiplied by 100,000 equals 1.64 trillion 2k I/Os. So if your server is updating the log file 500 times per second, which would be a reasonably busy server, you’d be doing 1,800,000 RW cycles per hour. So your 32 GB flash drive would last 910,222 hours or almost 104 years of 24 hour a day operation.
At 1,000 IOPS, then 52 years. 1,000 8k IOPS, then 12 years and change. 10,000 8k IOPS then 14 months. All for, I estimate, based on chip prices for about $1k per drive, or about 1/40th the price of a standard RAM-based SSD. So call me crazy, but I say flash is set to conquer the esoteric world of high-performance SSDs.
As ever, comments welcome. Moderation is turned on to defeat comment spam, but no registration required. And please, someone, check my arithmetic. I ran it several times through a calculator and can hardly believe it myself.