SSD vs. HDD: Performance and Reliability: Page 2 -

SSD vs. HDD: Performance and Reliability - Page 2

Now, in practice those higher percentages are not earth shattering. Most modern storage has redundant technology that minimizes data damage from a failed disk and allows hot replacements. But when you are talking about drive reliability, clearly that number is worth talking about.

Again, small blame to the HDD vendors for putting their best foot forward. No one really expects them to publish reams of data on how often their products fail… especially since the SSD vendors do the same thing. And on the whole, HDDs tend to fail more gracefully in that there may be more warning than a suddenly failing SSD. This does not negate the huge performance advantages of SSD but does give one pause.

SSD’s Reliability Failures

Some SSD failures are common to any storage environment, but they do tend to have different causes than HDD failures. Common points of failure include:

· Bit errors: Random data bits stored to cells, although it sounds much more impressive to say that the electrons leaked.

· Flying or shorn writes: Correct writes written in the wrong location, or truncated writes due to power loss.

· Unserializability: A hard-to-pronounce term that means writes are recorded in the wrong order.

· Firmware: Ah, firmware. Firmware fails, corrupts, or upgrades improperly throughout the computing universe: SSD firmware is no exception.

· Electronic failures: In spite of no moving parts, physical components like chips and transistors fail, taking the SSD down right along with it.

· Power outages: DRAM SSDs have volatile memory and will lose data if they lack a battery power supply. NAND SSDs are also subject to damaged file integrity if they are reading/writing during power interruptions.

Improving Reliability

As SSDs mature, manufacturers are improving their reliability processes. Wear leveling is a controller-run process that tracks data movement and component wear across cells, and levels writes and erases across multiple cells to extend the life of the media. Wear leveling maps logical block addresses (LBA) to physical memory addresses. It then either rewrites data to a new block each time (dynamic), or reassigns low usage segments to active writes (static) in order to avoid consistent wear to the same segment of memory. Note that writes are not the only issue: so is deletion. HDDs can write and read from the same sector, and in case of modified data can simply overwrite the sector. SSDs don’t have it this easy: they cannot overwrite but must erase blocks and write to new ones.

Data integrity checks are also crucial to data health. Error correction code (ECC) checks data reads and corrects hardware-based errors to a point.  Cyclic Redundancy Check (CRC) checks written data to be sure that it is returned intact to a read request. Address translation guards against location-based errors by verifying that a read is occurring from the correct logical address, while versioning retrieves the current version of data.

Garbage collection helps to reclaim sparsely used blocks. NAND SSD only writes to empty blocks, which will quickly fill up an SSD. The firmware can analyze the cells for partially filled blocks, merge data into new blocks, and erase the old ones to free them up for new writes.

Data redundancy is also a factor. External redundancy of course occurs outside of the SSD with backup, mirroring, replication, and so on. Internal redundancy measures include internal batteries in DRAM SSDs, and striped data parity in NAND flash memory.

So Which Wins, SDD or HDD?

SSDs are clearly faster in performance, and if an HDD vendor argues otherwise then consider the source. However, reliability is an ongoing issue outside of hostile environments. We find that SSD reliability is improving and is commensurate with, or moving slightly ahead of, HDDs. SSD warranties have stretched from 3 to 5 years with highly reliable Intel leading the way. Intel and other top NAND SSD manufacturers like Samsung (at present, the world’s largest NAND developer), Kingston and OCZ are concentrating on SSD reliability by improving controllers, firmware, and troubleshooting processes.  

The final score between NAND/DRAM SSDs and HDDs? Costs are growing commensurate. Reliability is about the same. Performance is clearly faster, and should rule the final decision between SSD and HDD. Hard drives will have their place for a long time yet in secondary storage, but I believe that they already lost their edge in high IO computing. For that, look to SSDs.

Photo courtesy of Shutterstock.

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