Technology considerations in selecting a direct attached storage solution for HP ProLiant Gen8 servers

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Table 3: Characteristics of HP solid state drive categories
Enterprise Value Enterprise Mainstream Enterprise Performance
Interface(s) 3 Gb/s SATA
6 Gb/s SATA
3 Gb/s SATA
6 Gb/s SAS
6 Gb/s SAS
General
description
SFF and LFF hot plug SFF and LFF hot plug SFF hot plug
Capacities 60 – 480 GB 200/400/800 GB 200/400 GB
NAND
technology
MLC SLC and MLC SLC
Workload High read/low write Balanced read/write
workload
High read/write workloads
Reliability/
endurance
3 year service life with
highly constrained write
workloads
3 year service life with
constrained write
workloads
3 to 5 year service life with
unconstrained workloads
Data
retention*
3 months minimum < 3 months < 3 months
Usage
environment
Boot devices,
applications high in
reads, few or no writes,
or data is transient
High IO/s applications
with equal read/write
workloads
Mission-critical/unrestricted
workload, high IO/s
applications
* With no power and drive near end of life
SSD longevity
Generally speaking, we expect electronics with no moving parts such as an SSD to have longer
lifetimes than mechanical devices such as a HDD, which has moving parts that can fail. The lifespan
of NAND memory in SSDs depends on the type of cell structure and the number of write/erase cycles
it experiences. Multi-level cell (MLC) memory typically has less endurance than single-level cell (SLC)
memory. SLC is less dense and therefore more expensive than MLC for the same capacity. We use
both cell types, depending on capacity and endurance goals.
HP SSDs include technologies to increase endurance to meet the requirements of enterprise
environments. Several factors affect the endurance of SSDs:
• SSDs write NAND pages, not sectors.
• SSD writes generate a read/modify/write cycle.
• SSDs must erase a NAND block (~1 MB) before writing a page (~4 KB) in that block.
• For write performance, SSDs maintain a pool of erased blocks.
• SSDs can move data from sparsely populated blocks so those blocks can be erased (known as
garbage collection).
Over provisioning
HP drives contain extra blocks, as many as 25% more blocks than the drive’s stated capacity. SSD
drives use this extra area to distribute erases and writes across a larger number of blocks over time.
Erasing blocks increases write performance and lowers write amplification. Write amplification is the
ratio of the number of NAND writes needed to accomplish host writes. A lower write amplification
increases SSD longevity.