Transcript
STORAGE SWITZERLAND ENTERPRISE SSD REQUIRES RAS (RELIABILITY, AVAILABILITY AND SERVICEABILITY)
George Crump, Senior Analyst Today Solid State Disk (SSD) is viewed by most storage managers as a niche solution to solve specific storage
Some systems, to address the flash reliability issue, use single-level cell (SLC) technology which writes only a
performance problems. For SSD to move out of the niche
single bit of data per memory cell and typically supports
category and become a more mainstream storage consideration for most of the active data in the enterprise,
about 100,000 erase cycles. This provides the highest level of erase cycles but does so at a premium price point,
it must address the reliability, availability and serviceability (RAS) concerns that those managers have about the
the factor that has typically relegated SSD to the role of a niche performance solution. At the other end of the erase
technology. Solid-State-only storage systems, also called
cycle curve is multi-level cell (MLC) technology which
“Sustainable Storage”, like those from Nimbus Data Systems, are addressing this requirement.
writes two bits of data into each cell and is typically rated at 5,000 erase cycles. With continuing reduction in NAND
Enterprise SSD Requires Reliability
manufacturing from 3x to 2x NAND (such as 25 nm), maintaining erase cycles in MLC becomes even more challenging, reducing the longevity to 1,000 to 3,000
When a flash memory cell is written to, it must first be cleared of old data. This process includes reading the cell
cycles. While the additional bit per cell increases capacity and reduces the cost of the SSD, the significantly lower
to confirm that it actually contains old data, and then writing ‘empty data’ (0’s) to every bit in the cell. This is
erase cycles means a higher probability for failure. To provide high reliability at an acceptable cost, Enterprise
called an “erase cycle” and is the cornerstone in flash
MLC (EMLC), which is similar in capacity to MLC, has
reliability concerns as every cycle shortens the life expectancy of the memory. The solid state memory that is
emerged, providing a significantly higher rating of over 30,000 erase cycles which, especially for sustainable
installed in enterprise-class sustainable storage systems must accommodate a very high number of these erase
storage systems, may strike a perfect balance.
cycles in order to maintain reliability.
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Sustainable Storage Has Different Write Patterns
as write I/O is evenly balanced across the system. This is where the flash controller plays a vital role.
Today the most common enterprise uses of solid state
Intelligent Flash Controller
storage are in very high I/O or high transaction environments, like a cache. Caches are small, keeping
Having the right type of flash for the workload is important,
costs down, but typically have a very high turnover rate, but having a controller to exploit the advantages of that meaning that they’re constantly being emptied (erased) and flash and minimize its weaknesses is equally important. As refilled (written to). Cache is dynamic and its contents will
mentioned above, one of the key responsibilities of the
change to reflect the data being accessed at that moment in time. Its relatively small size means that the same cells
flash controller is to spread out the write pattern to the flash memory so that each cell is written to evenly. This
are written to over and over again. As a result, in order to get a reasonable life expectancy out of these devices
capability, known as “wear leveling”, makes sure that one cell does not get written to over and over causing it to wear
requires the extreme endurance of SLC flash memory.
out prematurely. The more available memory cells per flash
Sustainable storage on the other hand has all the data on
controller, which is the case with sustainable storage, the more distributed that writing can be and the longer the life
solid state storage, a storage area which is comparatively large and consistent. Since the sustainable storage system
expectancy of each individual cell will be. In addition, in the case of a cell failure, the excess cell capacity, known to
is where the data lives, as opposed to being a temporary
insiders as “overprovisioning”, makes it easy to find an
area like cache, the data stored is not constantly erased and rewritten. As a result, an intelligent controller can
alternate location to write the data. Sustainable storage systems, like the Nimbus S class, have 28%
spread those writes out across more flash memory modules, decreasing the amount of erase cycles
overprovisioned spare capacity to accommodate this failover built into the system. Think of it as a hot swap drive
consumed per cell. Therefore while enterprise solid state
without having to replace the drive. In sustainable storage
storage needs higher levels of reliability than MLC it does not need the extreme reliability of SLC. EMLC modules like
systems, this extra capacity is transparent and does not take away from the advertised storage capacity of the
those used in Nimbus’ S-Class sustainable storage systems provide the perfect balance of affordability and
system.
reliability. Observed another way, assume a 10TB
Intelligent flash controllers can reduce the time required for
Sustainable Storage System, such as the Nimbus S-Class. With 30,000 erase cycle NAND throughout the system, one
the above erase cycle to take place. During this process the controller will pre-scan the flash memory to remove old
could write 10TB of data 30,000 times, or 300,000 TB, before the NAND cells would wear out. That’s 165 TB of
data so that when a write occurs, half of the erase cycle process will have already been performed. If this process,
data written per day, 365 days per year, for 5 years. Most IT often called “garbage collection”, can stay ahead of managers would agree that this level of reliability is ample in an enterprise applicable. However, with 25 nm MLC
inbound writes, then performance will be consistent as an application writes data to the storage. If not, the application
rated at 3,000 erase cycles, this write load would burn out all the NAND memory in a mere 6 months. Thus, unless
can experience performance slowdowns and may actually appear to pause. Many traditional SSDs without
customers are expecting writing more than 16x the
overprovisioned capacity suffer noticeable performance
capacity of their storage array in fresh data per year, for over 5 years, EMLC provides adequate protection, as long
degradation as they fill up with data, and in the enterprise, predictable performance is essential, making overprovisioning an essential must-have feature.
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The amount of contiguous memory available also impacts how quickly the garbage collection process can complete.
provide protection from a component failure. This means multiple paths to the storage network infrastructure,
Similar to how disk defragmentation takes longer as a disk
redundant power, cooling and redundant controllers. It also
fills up, garbage collection takes longer when there is less flash memory available to erase and reuse. The advantage
should provide redundant paths to flash modules themselves so that data access can continue despite a
of sustainable storage, since it is 100% solid state, is that there is typically more addressable memory per flash
drive path failure.
controller than in conventional hybrid systems.
The Reliability of Power Consumption
Flash RAID
One of the well-known benefits of a solid state storage
Even with the reliability of the NAND components and the
system is its power efficiency. This is where the sustainable storage moniker is derived. Efficient power consumption
intelligence of the flash controller, there is still a slight
however is often thought of only in terms of dollars saved
chance that a component may fail. If that occurs, enterprise-grade solid state storage systems should be
through lower electric bills. While that is certainly a positive, increased power efficiency has another benefit as well.
protected by RAID technology similar to that used in mechanical hard drive-based systems. A solid-state-only
According to a 2007 study by Google, who ranks among the largest consumers of storage in the world, heat and
system, as it does with other storage challenges, makes
vibration are leading causes of drive failure in mechanical
RAID better. RAID algorithms are struggling to keep pace with the ever increasing capacity of mechanical hard
systems. Sustainable storage systems run significantly cooler and have no moving parts other than the fans, so
drives. Today when a drive fails it can take double-digit hours, if not days, for a rebuild to complete. A comparable
there is no drive vibration to reduce drive life expectancy. This means the two biggest culprits in mechanical drive
sustainable storage system, based on a multitude of flash
failure simply don’t exist in a solid-state-only storage
blades, typically rebuilds in less than an hour and is less impacted by capacity like legacy HDDs are.
system.
Enterprise IT managers expect their storage systems to Flash RAID also provides system administrators flexibility in provide near 100% uptime. A concern about implementing RAID protection levels because performance between the
solid state storage in those environments has traditionally
different RAID levels is virtually identical. This allows them to choose the greater redundancy of double or even triple
been its reliability. A solid-state-only storage system, like the Nimbus S-Class, not only addresses the concerns
parity and see practically zero performance impact.
specific to solid state storage it can actually address some of the traditional reliability issues that plague mechanical
System Redundancy
drives, such as RAID rebuild times and heat and vibration
All of this redundancy to provide data protection is useless
related failures. Solid-state-only storage should be considered not only because of its obvious performance
if the system itself fails. As is the case with conventional enterprise storage systems sustainable storage needs to
advantages but also for its ability to improve reliability, availability and serviceability.
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