When it comes to choosing a storage device for backup, reliability is a key consideration. The two main options are solid state drives (SSD) and hard disk drives (HDD). So which one is more reliable for backup purposes? There are pros and cons to both options.
What is an SSD?
An SSD (solid state drive) is a type of storage device that uses flash memory to store data. Unlike an HDD, an SSD has no moving mechanical parts. Data is stored in microchips rather than on spinning platters like in an HDD.
Some key advantages of SSDs:
- Faster read/write speeds – SSDs can access data much faster than HDDs.
- More shock/vibration resistant – With no moving parts, SSDs can withstand bumps and shakes better.
- Lower latency and faster access times – SSDs can access data almost instantly.
- Lower power consumption – SSDs are more energy efficient than HDDs.
- Lighter weight – SSDs weigh less than HDDs of the same storage capacity.
What is an HDD?
An HDD (hard disk drive) is a traditional spinning storage device that uses platters coated with magnetic material to store data. A read/write head moves over the platters to access data.
Some key advantages of HDDs:
- Lower cost per GB – HDDs are cheaper per gigabyte compared to SSDs.
- Higher maximum capacity – HDDs are available in larger capacities than SSDs.
SSD vs. HDD Reliability Factors
There are several factors that can affect the reliability of SSDs and HDDs:
Mechanical components
SSDs have no moving parts while HDDs have many mechanical components like actuator arms, motors and spinning platters. This gives SSDs an advantage for reliability.
Connectors
Both SSDs and HDDs use connectors that can wear out over time. For SSDs, it’s the interface like SATA or PCIe. For HDDs, it’s the SATA interface plus all the internal cabling. The minimal cabling is an advantage for SSD reliability.
Temperature
HDDs are sensitive to high temperatures which can cause platters and motors to fail. SSDs can better withstand high temps as they don’t rely on moving parts.
Shock and vibration
The shock resistance advantage clearly goes to SSDs. HDDs can fail due to shocks during operation or transport while SSDs are far more shock tolerant.
Airborne contamination
HDDs must be assembled in cleanrooms because microscopic dust particles can damage platters. SSDs have no such concerns about airborne contamination.
Wear leveling algorithms
SSDs use wear leveling to spread writes across all cells evenly so no single cell wears out too fast. This helps prolong the life of the drive.
Bad block management
Another technique SSDs use is bad block management to isolate bad NAND cells and prevent data loss. This gives SSDs an advantage in reliability.
SSD Reliability Advantages
When looking at the main reliability factors, SSDs have some clear advantages over HDDs:
- No moving parts – less mechanical failure
- Better shock/vibration tolerance
- More resistant to high temps and contamination
- Wear leveling and bad block management
The lack of moving parts in SSDs eliminates a large source of potential failures. HDDs have many mechanical parts that are subject to gradual wear and tear over time. The platters, head, motors, and seals all slowly deteriorate.
Vibration and shocks are especially problematic for HDDs. A bump at the wrong time can permanently damage the platters or head alignment leading to failure. SSDs aren’t worried about alignment issues or moving parts coming loose.
High temperatures can cause HDD motors and platters to expand resulting in failure. Contamination can scratch platters or gum up motors. SSDs don’t have these failure points.
Perhaps most importantly, wear leveling and bad block management give SSDs self-healing properties. As cells start to wear out, SSD controllers can remap data to healthy cells. This mitigates the slow deterioration over time that HDDs experience.
HDD Reliability Advantages
HDDs do have some reliability advantages over SSDs in certain areas:
- Proven long term reliability
- Recovery fromdropped writes possible
HDD technology has been around for decades and has proven reliable over the long haul. While SSDs are gaining popularity, HDDs still have a longer track record to look at.
For HDDs, a failed write operation doesn’t destroy data on the platter. The data may become inaccessible but specialized data recovery firms can recover dropped writes by examining the magnetic data on the platters directly.
SSDs store data electronically rather than magnetically, meaning a failed write can permanently wipe out data. The lack of physical recoverability can be a disadvantage.
Lifespan and Endurance
The functional lifespan of storage devices is an important reliability consideration. Lifespans are measured both in time and in write endurance.
Time limited lifespan
All storage devices have a limited useful lifespan measured in years. Lifespans are affected by things like operating temperatures and the number of on/off cycles.
Enterprise HDDs are typically rated for 5 years of operation. SSDs may be rated for as much as 10 years of life. For backup purposes, time limited lifespan is often not a major factor provided the drives are replaced periodically.
Write endurance
The total amount of data that can be written to a device over its lifetime is known as write endurance. It’s measured in terabytes written (TBW).
Enterprise HDDs can typically endure 550TBW. Enterprise SSDs are rated for much higher write endurance from 1,000TBW to 10,000TBW or even more. For read-mostly backup applications, SSDs have a big advantage in write endurance.
Failure Rate Statistics
Industry studies on actual failure rates in the field provide real evidence on SSD and HDD reliability:
Backblaze HDD failure rates
Backblaze provides cloud backup solutions and publishes quarterly HDD failure rate statistics from their data centers. Over 4 years reporting on over 100,000 HDDs, they found an average annual failure rate of around 1.5% for HDDs.
| Year | Annual HDD Failure Rate |
|---|---|
| 2013 | 3.01% |
| 2014 | 2.47% |
| 2015 | 1.28% |
| 2016 | 1.03% |
Intel SSD failure rate study
Intel published a study on SSD failure rates examining over 1 million SSDs. They found SSDs are generally more reliable than HDDs with failure rates under 2% per year:
| Time Period | Annual SSD Failure Rate |
|---|---|
| First year of service | 1.4% |
| After first year | 0.4-0.9% typical |
These real-world studies show both HDDs and SSDs have low annual failure rates, often under 2%. SSDs seem to have a slight reliability advantage with failure rates generally under 1% after the first year of service.
Which is more reliable for backup?
So taking all factors into account, is SSD or HDD more reliable for backup storage? Here is a summary of the key considerations:
SSD advantages
- No moving parts
- Better shock/vibration tolerance
- More resistant to high temps and contamination
- Wear leveling and bad block management
- Higher write endurance rating
- Slightly lower annual failure rates
HDD advantages
- Proven track record over decades
- Recovery possible with failed writes
- Lower cost per GB
For backup applications, SSDs seem to have an overall reliability advantage due to their lack of moving parts, algorithms to prolong life, and shock/vibration resistance. The higher write endurance rating also suits backup usage well.
However, HDDs can’t be counted out. Their long-term track record is proven and failed writes can potentially be recovered. And HDDs continue to offer a lower $/GB ratio.
Recommendations
So for building a reliable backup storage solution, several best practices are recommended:
- Use enterprise class drives for superior reliability over consumer models
- Use SSDs for better shock protection and write endurance
- Mirror drives or use parity protection (RAID-1/5/6) for redundancy
- Replace drives proactively as they near rated lifespan
- Use HDDs for cold storage or archival backups if cost is the priority
SSDs generally are the preferred choice for primary backup storage. But larger capacity HDDs can supplement for cold storage or archives. Combining both technologies provides a balance of reliability, capacity, and cost.
Conclusion
In summary, SSDs have key advantages for backup purposes due to lack of moving parts, shock/vibration tolerance, and algorithms that mitigate wear. But HDDs still play a role thanks to proven reliability history and lower cost. For building a robust backup solution, enterprise-class drives, redundancy, and proactive replacement are recommended best practices.