There’s
a lot of chatter as of late about the longevity of SSDs. While the new
tech has brought with it massive performance gains, it’s still unclear
exactly what drive failure looks like in flash-based storage systems,
especially in the real world.
Thankfully, Bianca Schroeder from the
University of Toronto, along with Raghav Lagisetty and Arif Merchant
from Google, have been using flash storage in data centers for six
years, and now millions of drive days worth of diagnostic data
have revealed some new truths.
What’s
most impressive about Google’s data is just how clean it is. The flash
chips themselves are standard off-the-shelf parts from four different
manufacturers – the same chips you’d find in almost any commercial SSD –
and include MLC, SLC, and eMLC, the three most common varieties. The
other half of the SSD is the controller and code, but Google takes that
variable out of the equation by using only custom PCIe controllers. That
means any errors will either be consistent across all device, or
highlight the difference between manufacturer and flash type.
Read errors are more common than write errors
Schroeder
classifies the errors into two different categories: transparent and
non-transparent. Errors that the drive corrects and moves past without
alerting the user are transparent, and those that cause a user-facing
error are non-transparent.
Transparent
errors are either accounted for by the drive’s internal error
correction code, work after retrying the operation, or occur when the
drive fails to erase a block. These aren’t critical errors, and the
drive can get around them. The real issue are non-transparent errors.
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These
errors occur when the error is larger than the drive’s correction
firmware can handle, or when simply attempting the operation again
doesn’t do the trick. The term also applies to errors when accessing the
drive’s meta-data, or when an operation times out after three seconds.
Among
these non-transparent errors, the most common is a read error.
This occurs when a drive attempts to read data and the operation fails
even after multiple attempts. By Schroeder’s measurement, these appear
in anywhere from 20 to 63 percent of drives. There’s also a strong
correlation between final read errors and errors the drive can’t
correct, suggesting that final read errors are largely caused by
corrupted bits too long for the code to fix.
Non-transparent
write errors, on the other hand, are quite rare, only popping up in 1.5
to 2.5 percent of drives. Part of the reason for that is when a write
operation fails in one part of a drive, it can move to another and write
there. As Schroeder puts it, “A failed read might be caused by only a
few unreliable cells on the page to be read, while a final write error
indicates a larger scale hardware problem.”
But
what’s the useful potential of all that data about things going wrong?
In order to contextualize, we need to think about errors in terms of raw
bit error rates, or REBR. This rate, “defined as the number of
corrupted bits per number of total bits read” is the most common for
measuring drive failure, and allows Schroeder to evaluate drive failure
rates across several factors: wear-out from erase cycles, physical age,
workload, and previous error rate.
While
the study finds that all of these elements factor into a drive’s
failure rate, with the exception of previous errors, some of them are
more detrimental than others. Importantly, physical age has a much more
profound correlation to REBR than erase cycles, which suggests other
non-workload factors are at play. It also means that tests that
artificially increase read and write cycles to determine failure rates
may not be generating accurate results.
SSDs are more reliable than mechanical disks
Uncorrectable
errors are one thing, but how often does a drive actually fail
completely? Schroeder tracks this by measuring how often a drive was
pulled from use to be repaired. The worst drives had to be repaired
every few thousand drive days, while the best went 15,000 days without
needing maintenance. Only between five and ten percent of drives were
permanently pulled within four years of starting work, a much lower rate
than mechanical disk drives.
Which
brings us to the final section of the study, which contextualizes the
data in a more tangible way. It starts by downplaying the importance of
REBR, pointing out that while it can help indicate a more relevant sense
of hardware failure than other methods, it’s not good at predicting
when failures will occur.
That
doesn’t mean we can’t draw some conclusions from it. For one, SLC
chips, which are generally more expensive than MLC chips, aren’t
actually more reliable than cheaper options. It is true that SLC chips
are less likely to be problematic, but additional instances of MLC chip
failure do not necessarily translate to a higher rate of non-transparent
errors, or a higher rate of repair. It seems that firmware error
correction does a good job of working around these problems, thus
obscuring the difference in actual use.
Drives
based on eMLC do seem to have an edge, though. They were less likely to
have a fault, and drives based on eMLC were the least likely to need
replacement.
Flash
drives also don’t have to be replaced nearly as often as mechanical
disk drives, but the error rate of SSDs tends to be higher. Fortunately,
the firmware does a good job of preventing errors from becoming a
problem.
Some drives are better than others
The
study identifies a few key areas where more research is needed to
identify issues down the line. Previous errors on a drive are a good
indicator of uncorrectable errors down the road, a point which Schroeder
says is already being investigated to see if bad drives can be
identified early in their life.
This
is only emphasized by the fact that most drives are either quite prone
to failure, or solid for the majority of their lifespan. In other words,
a bad drive is likely to show signs of being bad early on. If
manufacturers know what to look for they may be able to weed these
drives out during quality control. Similarly, owners might be able to
identify a problem drive by examining its early performance.
While
Google did look at drives from a number of companies, the company
declined to name any names in this study, as has happened in past
examinations of mechanical drives. While we now know that SSDs are more
reliable than mechanical drives, and that the memory type used doesn’t
have a huge impact on longevity, it’s still not clear which brand is the
best for reliability.
