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Five questions about SSD's

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magellan

Member
Joined
Jul 20, 2002
I'm not even sure if all these questions are even applicable to SSD's or even make sense, but here goes:

1. What sector size do SSD's use (512 byte? 2k?)?

2. Are there equivalent SMART stats for SSD's that can be used to predict
SSD failure?

3. Is the wearing out factor of an SSD due completely to writing or erasing
cells?

4. Do SSD's use a lot less power than HDD's?

5. Are current gen SSD's more reliable than HDD's (since they don't have any electromechanical components to fail)?
 
2. Sometimes they just stop working. Very little if any warning signs.
3. I think it's both writing and erasing.
4. Yep, they do.
5. Kinda. IMO, yes, and at the same time, they simply won't last as long due to the write limits.
 
1. What sector size do SSD's use (512 byte? 2k?)?
SSDs don't really have sectors. They report a sector size of 512 bytes for compatibility purposes. What they do have is a page size, which is the smallest unit that can be read or written. The page size depends on the NAND used but 16k is common nowadays. There is also the block size, which is the smallest unit that can be deleted. It is usually a multiple of the page size, like 256x.

2. Are there equivalent SMART stats for SSD's that can be used to predict
SSD failure?
Yes. At the minimum they should report a OK/ not OK SMART result depending on whether the drive is healthy. Most manufacturers report additional SMART information as well. Interpreting that is drive specific.

3. Is the wearing out factor of an SSD due completely to writing or erasing
cells?
Mostly, technically reading does wear out the cells as well, but writing and erasing has a much bigger effect and is what most worry about.

4. Do SSD's use a lot less power than HDD's?
Generally yes, although there are details that complicate matters. There is a big difference in how much power hard drives use. The low power, ultra-mobile hard drive uses a lot less power than a high-speed server hard drive. SSDs use the most power when writing. During a write a SSD can use more power than a low power hard drive. The big win for SSDs is that they can go into devsleep where they use a lot less power then a low power hard drive. Combine this with them being so fast reading/writing and you end up with them spending most of their time asleep, at least in mobile devices. In servers where you write to the SSD 24/7 you wont see such differences.

5. Are current gen SSD's more reliable than HDD's (since they don't have any electromechanical components to fail)?
Mostly yes. The catch is that most SSD failuers seem to be controller/firmware related so SMART doesnt do anything to warn you of such failures. Overall things have gotten better on this front and, on average, the good quality SSDs are more reliable than hard drives.
 
Doesn't a 16K page size result in a lot of wasted slack space?
 
Doesn't a 16K page size result in a lot of wasted slack space?
Not really. SSDs rely on parallelism to get high write speeds, writing in parallel to multiple NAND dies. In order to achieve this with lots of small writes it does stores those small writes in a cache until it has a big clump and then splits it up into appropriate sized chunks to write in parallel to multiple dies.

The data on the SSD doesn't sit in place either. There is a lot of GC and wear-leveling going on. This gives it additional opportunities to gather up small pieces into whole pages.
 
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