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Flash-based SSDs are not all the
same, despite many appearing in
a 2.5” form factor that looks like
an HDD. Beyond common ash
parts and convenient mounting
and interconnect, philosophical
dierences create two classes of
SSDs with important distinctions
in use.
2
Client SSDs are designed
primarily as replacements for
HDDs in personal computers. A
large measure of user experience
is how fast the machine boots
an operating system and loads
applications. SSDs excel, providing
very fast access to les in short
bursts of activity. RAM caching,
using a dedicated region of
PC memory, or software data
compression can be used to
improve performance.
But typical PC users are not
constantly loading or saving les,
so the SSD in a PC often sits idle.
Client SSDs with low idle power can
dramatically reduce system power
consumption. Idle time also allows
a drive to catch up, completing
queued write activity and
performing background cleanup
known as TRIM, recovering ash
blocks where data has been
deleted.
Increasing requests on a client
SSD often result in inconsistent
performance – which most PC
users tolerate, realizing they kicked
o too many things at once.
Users can also pay a price during
operating system crashes or power
failures, without protection from
losing data or corrupting les.
Data center SSDs are designed
for speed as well, but also prioritize
consistency, reliability, endurance,
and manageability. Most
applications use multiple drives
connected to a server or storage
appliance, accessed by numerous
requests from many sources. Idle
time is reduced in 24/7 operation.
Lifespan becomes a concern as
ash memory cells wear with
extended write use.
An SSD that stalls under load,
suers data errors, or worse yet
fails entirely, can put an entire
system at risk. Enterprise-class ash
controllers and rmware avoid any
dependence on host software for
performance gains; many drives
in use would consume scarce
processing resources. Consistency
means high-performance
command queue implementations,
combined with constrained
background cleanup.
To protect sensitive data, data
center SSDs implement several
strategies. Advanced rmware
uses low-density parity check
(LDPC) error correction with a more
ecient algorithm taking less
space in ash, resulting in faster
writes. Surviving system power
interruptions requires power-fail
protection (PFP) with tantalum
capacitors holding power long
enough to complete pending write
operations. If encryption is required,
self-encrypting drives implement
algorithms internally in hardware.
Mean time between failure
(MTBF) was critical for HDDs, but is
mostly irrelevant for SSDs. Useful
comparisons for SSDs are two
metrics: TBW and DWPD. TBW is
total bytes written, a measure of
endurance. DWPD – device writes
per day – reects the number of
times the entire drive capacity
can be written daily, for each day
during the warranty period. The
latest V-NAND ash technology is
beginning to appear in data center
SSDs, oering up to double the
endurance of planar NAND.
Some system architects
overprovision across multiple
client SSDs to oset consistency
and endurance concerns.
Overprovisioning counts on greater
idle time, reserves more free blocks,
and uses more drives – incurring
more cost, space, and power – than
necessary, compared to using fewer
data center SSDs. Understanding
use cases and benchmarks can
help avoid this expensive practice.
DIFFERENT CLASSES: WHEN NOT JUST ANY SSD WILL DO
Consumer-Class Data Center SSDs
- VS -
Lower latency
Designed for sustained performance
Mixed workload I/O
Latency increases as workloads increase
Built for short bursts of speed
Lower mixed workload capabilities
VIEW INFOGRAPHIC
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