The SSD Relapse: Understanding and Choosing the Best SSD
by Anand Lal Shimpi on August 30, 2009 12:00 AM EST- Posted in
- Storage
A Wear Leveling Refresher: How Long Will My SSD Last?
As if everything I’ve talked about thus far wasn’t enough to deal with, there’s one more major issue that directly impacts the performance of these drives: wear leveling.
Each MLC NAND cell can be erased ~10,000 times before it stops reliably holding charge. You can switch to SLC flash and up that figure to 100,000, but your cost just went up 2x. For these drives to succeed in the consumer space and do it quickly, it must be using MLC flash.
SLC (left) vs. MLC (right) flash
Ten thousand erase/write cycles isn’t much, yet SSD makers are guaranteeing their drives for anywhere from 1 - 10 years. On top of that, SSD makers across the board are calling their drives more reliable than conventional hard drives.
The only way any of this is possible is by some clever algorithms and banking on the fact that desktop users don’t do a whole lot of writing to their drives.
Think about your primary hard drive. How often do you fill it to capacity, erase and start over again? Intel estimates that even if you wrote 20GB of data to your drive per day, its X25-M would be able to last you at least 5 years. Realistically, that’s a value far higher than you’ll use consistently.
My personal desktop saw about 100GB worth of writes (whether from the OS or elsewhere) to my SSD and my data drive over the past 14 days. That’s a bit over 7GB per day of writes. Let’s do some basic math:
| My SSD | |
| NAND Flash Capacity | 256 GB |
| Formatted Capacity in the OS | 238.15 GB |
| Available Space After OS and Apps | 185.55 GB |
| Spare Area | 17.85 GB |
If I never install another application and just go about my business, my drive has 203.4GB of space to spread out those 7GB of writes per day. That means in roughly 29 days my SSD, if it wear levels perfectly, I will have written to every single available flash block on my drive. Tack on another 7 days if the drive is smart enough to move my static data around to wear level even more properly. So we’re at approximately 36 days before I exhaust one out of my ~10,000 write cycles. Multiply that out and it would take 360,000 days of using my machine the way I have been for the past two weeks for all of my NAND to wear out; once again, assuming perfect wear leveling. That’s 986 years. Your NAND flash cells will actually lose their charge well before that time comes, in about 10 years.
This assumes a perfectly wear leveled drive, but as you can already guess - that’s not exactly possible.
Write amplification ensures that while my OS may be writing 7GB per day to my drive, the drive itself is writing more than 7GB to its flash. Remember, writing to a full block will require a read-modify-write. Worst case scenario, I go to write 4KB and my SSD controller has to read 512KB, modify 4KB, write 512KB and erase a whole block. While I should’ve only taken up one write cycle for 2048 MLC NAND flash cells, I will have instead knocked off a single write cycle for 262,144 cells.
You can optimize strictly for wear leveling, but that comes at the expense of performance.
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IPL - Monday, August 31, 2009 - link
I first started reading anandtech when I got seriously interested in SSDs and honestly, you write the best SSD articles around! Thank you for all the help you gave me in deciding which SSD to buy.I ordered online the new G2 last week and should be getting it in a few days. I live in Greece and the re-launched G2 has been available here for about a week now.
I am planning on replacing the HDD on my Feb 08 Macbook Pro (last refresh pre-unibody) as soon as I get it. I am just a consumer with a little bit of knowledge on tech but not a pro at all. I just thought of asking all a few questions that I have pre-drive swapping.
1. Will TRIM be supported on macs? Any news if and when?
2. When then new TRIM firmware is out, do I have to just install the firmware or will I need to format everything and start from fresh in order to get it to work?
3. I have bought a 2,5'' SATA USB enclosure in order to put my G2 in there first, connect it to the laptop via the USB and install Snow Leopard from there. After I am done, I will remove the G2 from the enclosure, swap the drives and hopefully, everything will be working. Does this sound logical? I am worried about the h/w drivers to be honest.
Thanks in advance for your help. I will post some non-scientific time results as soon as get this done. Cant wait.
gstrickler - Monday, August 31, 2009 - link
The simplest way to swap the HD on most Mac OS machines is:1. Connect both the old and the new drive to the machine (internally or in an external USB or FireWire case).
2. Use Disk Utility (included in Mac OS X) to set the appropriate partitioning scheme (GUID for Intel based Macs, Apple Partition Scheme for PPC Macs) on the new drive.
3. Partition and format the new drive.
4. Use Carbon Copy Cloner (shareware) to clone the old drive to the new drive.
5. Try booting off the new drive. Note that PPC Macs can't boot from USB drives, but Intel based Macs can. All PPC and Intel Macs with a built-in FireWire port can boot from a FireWire drive.
6. If not already done, physically swap the drives to the desired locations, boot and set the preferred startup drive.
IPL - Tuesday, September 1, 2009 - link
Awesome, thanks for the help.I have checked Carbon Copy Cloner and it is already one of my options. Never tried it before but looked easy enough.
I havent decided yet which way I will do it (fresh install or clone existing drive) but I will make my mind up when everything is ready!
Anand Lal Shimpi - Monday, August 31, 2009 - link
Thank you for reading and saying such wonderful things, I really do appreciate it :)1) I don't believe TRIM is presently supported in Snow Leopard. I've heard that Apple may be working on it but I don't think it's there now.
2) From what I've seen, it should preserve your data. It's still worth backing up just in case something ridiculous happens.
3) What you're describing should work, although if I were you I'd just swap the drives and install. Hook your old drive up via USB and pull any data you need off of that.
Take care,
Anand
sunbear - Monday, August 31, 2009 - link
Another fantastic article. I just wanted to draw your attention to recent reports that the majority of currently available laptops (including the MacBookPro) are unable to support transfer rates greater than SATA-150 (http://www.hardmac.com/news/2009/06/16/new-macbook...">http://www.hardmac.com/news/2009/06/16/...imited-1....Since most laptops can't even use the full performance of these SSD's, do you have any recommendation regarding which one would be the best bang-for-the-buck to speed up a laptop?
Personally, I am interested in putting SSD's in a laptop not only for the speed improvements, but I'm also hoping that it reduces the amount of heat that my laptop will put out so that I can finally find a laptop that you can use comfortably on your lap!
Incidentally, it would be really great if laptop reviewers checked to see if they could comfortably work with a laptop at full load on their lap as a standard test.
Anand Lal Shimpi - Monday, August 31, 2009 - link
Even on a SATA-150 interface, you're generally only going to be limiting your sequential read speed and perhaps your sequential write speed a bit. Random read/write speeds don't really go above 60MB/s so you're fine there.They recommendations remain the same; Intel at the top end, anything Indilinx MLC to save a bit. If anything, a SATA-150 interface makes the Intel drive look a bit better since its 80MB/s sequential write limit isn't as embarrassing :)
Take care,
Anand
Dobs - Monday, August 31, 2009 - link
I hope Seagate / Western Digital etc. bring even more innovation / competition in SSD's next year... and not just Enterprise products.And one thing I don't fully understand is why there aren't more dedicated 3.5" drives. Patriot has the adapter but what about the rest??? No money in desktops anymore???
Anand Lal Shimpi - Monday, August 31, 2009 - link
OCZ is making a 3.5" Vertex drive, waiting on it for review :)Take care,
Anand
kisjoink - Monday, August 31, 2009 - link
Now that the good performing SSDs are half the price of last year, I'd really like to see a 2xSSD in RAID 0 article!mgrmgr - Monday, August 31, 2009 - link
I second the request for a 2xSSD RAID-0 article...with specific discussions about which applications it benefits (Photoshop?) and which ones it doesn't.Before October 22nd when I buy a new Win7 computer? Please. :-)