Shingled Drives for Re-Roofing Your Storage?
In the last several years, there have been an increasing number of storage options. Initially we had just magnetic hard drives with a single rotational speed. Then they started to come in several varieties. Now we have a range of drive speeds starting at 15,000 rpm at the top end, followed by 10,000 rpm drives, then the ubiquitous 7,200 drives, and slower drives with speeds such as 5,900, 5,400, 4,500 and even variable speed drives.
The rotational speed of the disk drive is strong indicator of performance, price, capacity and power usage. Typically the higher the speed, the more expensive the drive. And usually high-speed drive has a smaller capacity, better performance and higher power consumption. As the drive speed comes down, the drive price decreases, the capacity increases, the performance decreases, and the power usage decreases.
There are other sources of drive variation, for example, drive cache size and physical drive size (2.5" and 3.5"). There is also the drive communication protocol such as SATA, SAS or Fiber Channel. There are also protocol speed differences such as 6 Gigabits per second (Gbps), 3 Gbps and slower (although these are older drives).
Now there is a new type of drive that is coming out. These drives are referred to as Shingled Magnetic Recording (SMR) drives.
How SMR Drives Operate
A conventional hard drive has two heads. One of them is for writing, and one is for reading. The write head is larger than the read head, which means that the tracks on a hard drive platter have to be wider than necessary from a read perspective. In addition, there is guard space between tracks so that the write head doesn't disturb the data on neighboring tracks. This layout is illustrated below in Figure 1.
Figure 1: Conventional hard drive track layout
The read head is on the left and is marked with an "R." The write head is marked with a "W." Notice that it is much larger than the read head. The green is the width of the track necessary for writing. The blue is the width of the track necessary for reading. Separating the tracks is a Guard Space.
What SMR drives do is to reduce the guard space between tracks and overlap the tracks. Figure 2 below is a diagram that illustrates this configuration.
Figure 2: SMR hard drive track layout
The tracks are overlapped so they look like roof shingles, hence the name "Shingled Magnetic Recording." The diagram shows that the write head writes the data across the entire track (the blue and green stripes in a specific track). But the drive "trims" the data to the width the read head actually needs to read the data (this is the blue area of each track). This allows the areal density, the number of bits in a specific unit area, to be increased, giving us more drive capacity.
Let's explore what happens when the drive reads and writes data to the shingled tracks. Figure 3 below shows what happens when the data is starting to be written to the drive.
Figure 3: SMR hard drive track layout
The tracks are overlapped and are in green. The write drive head writes the data to the full track. Notice that the data is written both to Track N and Track N+1. Then the drive "trims" the data behind the write head to the width of the read head. The trimmed data is the blue.
The drive head continues writing to the track leaving behind the trimmed data as shown in the Figure 4.
Figure 4: SMR hard drive track layout
Only a single track is shown so that you can see how the write head writes to the entire width of the track but is trimmed to the width of the read head (the blue section of the track).