SAN (Storage Area Network) storage refers to a dedicated high-speed network which connects multiple servers and storage devices. SAN storage allows shared access to storage resources and provides high performance, scalability and reliability for enterprise data storage needs. But an important decision in designing a SAN is whether to use Solid State Drives (SSDs) or Hard Disk Drives (HDDs) for the storage media.
SSD vs HDD Overview
SSDs and HDDs are the two main types of storage media used in SAN storage. SSDs use flash memory with no moving parts while HDDs use spinning magnetic disks to store data. Some key differences between SSDs and HDDs are:
- Performance – SSDs provide much faster read/write speeds, lower latency and higher IOPS than HDDs.
- Durability – SSDs are more durable with higher tolerance to vibration, heat and magnetism. HDDs can fail due to disk crashes.
- Cost – HDDs are significantly cheaper in terms of cost per GB compared to SSDs.
Therefore SSDs are better suited for performance-critical applications while HDDs offer more storage capacity for the cost. Most SAN storage systems use a combination of both SSD and HDD media to optimize for cost and performance.
Benefits of SSD Storage in SAN
Using SSDs for SAN storage provides several key advantages:
Faster Performance
SSD drives offer much faster read and write speeds compared to HDDs. Below is a comparison of average read/write speeds:
| Drive Type | Read Speed | Write Speed |
| SSD SATA | 500 MB/s | 350 MB/s |
| SSD SAS | 900 MB/s | 800 MB/s |
| HDD SATA | 150 MB/s | 150 MB/s |
| HDD SAS | 300 MB/s | 250 MB/s |
The faster IO speeds of SSDs translate to lower application response times and ability to support more transactions per second.
Lower Latency
SSDs have much lower access latency compared to HDDs. Average latency for SSDs is around 0.1 ms while HDD latency is 3-10 ms. This allows SSDs to service IO requests much faster.
Higher IOPS
Due to faster speeds and lower latency, SSD drives deliver much higher Input/Output Operations per Second (IOPS) which is a key measure of storage performance. A typical SATA SSD can provide up to 100,000 IOPS while a SATA HDD peaks at around 200 IOPS.
Better Reliability
SSDs have no moving parts so they are more tolerant to shock, vibration, heat, magnetism etc. HDDs are mechanical devices so more prone to failure due to disk crashes, bad sectors, wear and tear.
Supports more VMs
The high performance of SSD storage allows deploying more virtual machines per storage system compared to HDDs. More VMs means increased application consolidation and utilization of compute resources.
Benefits of HDD Storage in SAN
While SSDs are better for performance, using HDDs in SAN storage provides the following key advantages:
Lower Cost per GB
HDDs are significantly cheaper in terms of cost per gigabyte of storage capacity. Average $/GB for HDD is around $0.03 while for SSD it is around $0.30. This makes HDDs more cost-effective for large storage capacity requirements.
| Drive Type | Average Cost per GB |
| HDD SATA | $0.03 |
| HDD SAS | $0.04 |
| SSD SATA | $0.30 |
| SSD SAS | $0.50 |
Higher Storage Density
HDDs support much higher total storage capacity in a given storage enclosure compared to SSDs. For example, a 4U enclosure can house 400-500 TB of HDD storage while only 150-200 TB of SSD storage.
Mature Technology
HDD technology has been around since 1950s and is well proven. SSD technology, especially flash memory, is relatively new and still evolving so has more uncertainty in long-term reliability.
Better Sequential Read/Write
For large sequential IO workloads, HDDs perform better than SSDs as they can continuously read/write large chunks of adjacent data on disk efficiently. SSD performance drops during sustained sequential IO.
SSD vs HDD – SAN Storage Usage
Given their complementary strengths, SSD and HDD drives are used together in SAN storage systems with SSDs geared towards performance and HDDs for capacity:
- SSDs are deployed for top performance tier, used for caching, high transaction databases etc.
- HDDs used for bulk storage tier for backups, archives, large files etc.
- Critical VMs and databases are placed on SSD storage pools.
- Less critical data is stored on HDD storage pools.
- Tiered storage combines SSD and HDD media transparently.
Below table summarizes typical usage scenarios suited for SSD versus HDD SAN storage:
| Usage Scenario | Recommended SAN disk type |
|---|---|
| Virtual desktop infrastructure (VDI) | SSD |
| Online transaction processing (OLTP) | SSD |
| High performance databases | SSD |
| Web servers and caching tiers | SSD |
| High frequency trading systems | SSD |
| Big data analytics | HDD or SSD |
| Video surveillance storage | HDD |
| Backup and archival | HDD |
| Bulk file storage and sharing | HDD |
SSD Form Factors for SAN
SSDs are available in various form factors for enterprise SAN storage such as:
2.5″ SSD
- Used in All-Flash-Arrays (AFA) and hybrid storage arrays.
- Interface is SAS, SATA or NVMe connected to storage backplane.
- Fast access speeds around 100K IOPS.
- Capacities from 400GB to 15TB.
U.2 NVMe SSD
- NVMe drives in 2.5″ form factor.
- High speed PCIe Gen3 or Gen4 interface.
- Bandwidth ~3200-6400 MB/s.
- Latency as low as 30 microseconds.
- Capacities up to 60TB.
EDSFF E1.S SSD
- Enterprise and Datacenter SSD Form Factor (EDSFF).
- PCIe Gen4x4 interface.
- Up to 700K IOPS and 11GB/s throughput.
- Capacities up to 30TB in E1.S size.
M.2 NVMe SSD
- Gumstick sized M.2 form factor.
- PCIe Gen3 or Gen4 interface.
- Bandwidth up to 4000MB/s.
- Used in all-flash arrays and servers.
- Capacities from 256GB to 4TB.
The high performance and compact size makes these form factors ideal for enterprise SSD storage implementations.
HDD Form Factors for SAN
Following are common HDD form factors used in SAN storage:
3.5″ HDD
- Largest HDD form factor used in servers and storage arrays.
- SATA, SAS, FC interfaces available.
- RPM from 5400 to 15K.
- Capacities up to 16TB SATA or 2.4TB SAS.
2.5″ HDD
- Smaller performance HDD for space constrained storage.
- Primarily 12Gbps SAS interface.
- Up to 15K RPM speed.
- Capacities up to 1.2TB.
Helium HDD
- Helium filled drives rather than air.
- 7 platters allowing much higher capacity.
- Used in dense storage rack enclosures.
- SATA and SAS interfaces.
- Capacities up to 16TB.
HDDs continue to provide the most cost-effective solution for bulk and archival storage in the data center.
Storage Networking for SAN
SAN storage networks are designed to provide the high speed connectivity required by fast SSDs and low latency access to storage. Common SAN network technologies include:
Fibre Channel SAN
- Dedicated Fibre Channel switch infrastructure.
- Fibre Channel HBAs in servers connect to Fibre Channel switches.
- Storage arrays connect via Fibre Channel.
- Fast 8, 16 and 32 Gbps link speeds.
- Lossless, low latency connectivity.
- Expensive and complex to scale-out.
iSCSI SAN
- IP storage networking protocol encapsulated in Ethernet.
- Runs on converged data network switches.
- Lower cost using Ethernet components.
- 10Gbps speeds with 40/100Gbps evolving.
- Somewhat higher latency than Fibre Channel.
NVMe over Fabrics
- Transports NVMe commands over RDMA fabrics.
- Low latency access to NVMe SSDs.
- Supports Fibre Channel, TCP, iWARP and Infiniband.
- Evolving ecosystem and standards.
The network infrastructure is a critical part of deploying SSD SAN storage and enabling applications to benefit from the performance capabilities of flash storage.
SSD Storage Vendors
Leading enterprise SAN storage vendors offer all-flash and hybrid storage arrays supporting SSD drives such as:
- Dell EMC – PowerMax, XtremIO, Unity XT
- NetApp – AFF All Flash FAS
- HPE Primera, Nimble Storage
- Hitachi VSP 5000 Series
- IBM FlashSystem 9100
- Pure Storage FlashArray
These vendors utilize ultra-fast SSD drives like NVMe combined with data reduction technologies to deliver high performance and consolidation density for virtualized environments.
SSD Cost Considerations
While SSDs are dropping in cost, HDDs continue to provide significant savings for high capacity storage:
- HDD $/GB still 5-10X lower than SSDs for enterprise class drives.
- To store 5PB data, all SSD SAN would cost $15M vs $3M with all HDD (at $0.03/GB).
- Server-side flash provides cost-effective performance boost complementing SAN storage tiering.
Organizations need to analyze their capacity needs, performance requirements and application characteristics to design the optimal SAN storage configuration balancing SSD and HDD media.
Conclusion
In summary, while HDDs continue to dominate in terms of cost and density, SSDs are increasingly deployed in SAN storage to deliver faster performance and lower latency required by modern applications. Most organizations use a combination of SSD and HDD storage in a tiered architecture to get the best of both technologies. SSD is well-suited for high performance transactional applications, databases and virtualization while HDDs are used for backups, archives and bulk unstructured data. The choice depends on the specific storage performance and capacity requirements as well as cost constraints. Ongoing innovation in flash memory and storage networking continues to make SSD storage more affordable and accessible across a wider range of workloads.
