# Amphishea: A Novel Persistent Buffer Management Strategy to Improve SMR Disk Performance
## 2 Background and Motivation
### Characteristics of SMR Disks  
SMR is a promising technology for breaking through the bottleneck of perpendicular magnetic recording to further increase the area density of disks. Because SMR disks are an evolution of the existing CMR disk architecture, they can significantly reduce production costs and be easily integrated into modern storage systems [18; 19]. The rationale behind the ability of SMR disks to increase in storage density is twofold. First, the tracks are overlapped like shingles to improve space utilization, and second, the write head is enlarged to 3-8 times the size of the read head (also known as R/W_Head), which allows the magnetic field generated by the write heads to concentrate the same amount of data in a smaller area [5]. Note that non-sequential writes must be handled carefully, as the enlarged write head can overwrite valid data on adjacent tracks. As shown in Figure 1, the width of W_Head is three times the width of R_Head. If the data on track A needs to be updated, the disk must read the valid blocks within the write head coverage (i.e., from track B and track C) into memory, merge them with the updated blocks, and then write them back to the native zone. This time-consuming update process is known as RMW. To avoid the domino effect caused by data updates, the entire disk space is divided into many bands/zones, each consisting of a group of tracks, and guard regions are placed between them. In this way, the data involved in the RMW operation is confined to one band/zone.  
Currently, SMR disks are classified into three types, drive-managed SMR (DM-SMR), host-managed SMR (HM-SMR), and host-aware SMR (HA-SMR) [7]. In particular, both DM-SMR and HA-SMR use a persistent buffer consisting of SMR zones (approximately 1% to 10%) to reduce the frequency of RMW operations. DM-SMR uses an internal Shingle  
Figure 1: Track layout difference between CMR and SMR disk.  
Translation Layer (STL) [2; 3] to hide data management details, allowing it to operate like a CMR disk. Unlike DM-SMR, HM-SMR and HA-SMR can provide real-time information to host applications about the location of the write pointer and the number of active zones, which can help the host make data placement decisions. However, the performance of DM/HA-SMR disks is degraded by frequent buffer cleanup activities. To achieve predictable performance, the persistent buffer is no longer configured on HM-SMR disks, and all non-sequential write requests return error messages to the host. Typically, HM-SMR disks reserve a fixed number of CMR zones that can be used to store metadata or build a custom persistent buffer [1; 14; 15; 16] to meet the performance needs of the storage system in different scenarios. Recently, the industry has been developing a hybrid SMR to further achieve the balance between capacity and performance [16; 20]. Intuitively, the principles of HM-SMR and hybrid SMR are very similar in that both consist of CMR and SMR zones, and the data management strategies can be implemented on the host system. The difference is that the latter can dynamically adjust the ratio of CMR and SMR zones at runtime. Therefore, this type of disk is orthogonal to our strategy, i.e., our strategy can be integrated into it.