The Case for HDDs: Faster is Not Always Better

Nearly every article you come across mentions how Solid-State Drives (SSDs) are better than Hard-Disk Drives (HDDs). This is often illustrated by comparing the access time, random I/O speed, reliability, energy savings, etc. However, just like every technology, you have to compare the use cases and what problem is helps you solve. 

Many data centers and servers still use HDDs over SSDs. So, what gives? If SSDs are so great and fast, why still use HDDs? I'll explain why. 

The first reason is that HDDs are less expensive than SSDs and offer more storage capacity. If you were to go to any online store, like Newegg, this difference in costs is astronomical. 

• A Samsung 870 EVO Series 2 TB SSD will cost $99.99. 
• A Seagate Barracuda 8 TB HDD will cost $139.50. 

So, for x4 more the storage, you pay an extra $40 with an HDD than an SSD. 

This is important because when we are dealing with servers and data centers, we are primarily interested in the most data storage capacity for the lowest cost. 

One specific example of this is Facebook. Their data centers require 100,000 TB drives per data center, and they have 7 data centers. So, that is a total of 700,000 TBs. It is more cost efficient to invest in HDDs than it is for SSDs. Also, let's remember, you are interested in permanent data storage, not access time, or random I/O speed.

The second reason is that data recovery is easier with HDDs than SSDs. One feature of SSDs that isn't talked about is TRIM. 

TRIM is a command for the ATA interface and many SSDs have this enabled by default or automatically. Essentially, every time a file is deleted, a rewrite of the block to 0 is followed. SSDs cannot just overwrite data in a cell like HDD. It needs to erase the cell first, before writing to it again. However, with HDD's, the pointer to that data is deleted and marked for overwriting, but it is still there. This makes data recovery so much easier with HDDs than SSDs. 

Another aspect of this is knowing when your drive will fail. Since, HDDs are mechanical and have moving parts, there are signs that indicate they are starting to fail and need to be replaced. This sign will be a clicking sound. However, with SSDs, there are no moving parts, so you don't know when it will fail, until it happens. 

The third reason is that SSDs degrade over time, the more write operations you do. This means that the lifespan of an SSD is limited. However, with HDDs that is not the case. Their lifespan is directly related to the wear and tear of the mechanical parts.