# Amphishea: A Novel Persistent Buffer Management Strategy to Improve SMR Disk Performance
## 1 Introduction  
The advent of shingled magnetic recording (SMR) disks significantly improves the situation where conventional magnetic recording (CMR) disks are unable to meet the growing storage capacity demands of modern big data applications [1; 2; 3]. SMR technology successfully broke the area density barrier by widening the write heads and overlapping the tracks like shingles. However, the consequence of this is that non-sequential writes can destroy valid data on adjacent tracks. To prevent data corruption, the valid data of the adjacent tracks must be read into memory and merged with the new data before being written back to the native zone, the time-consuming operation that degrades system performance, known as the read-modify-write (RMW) [4; 5]. To reduce the frequency of RMW operations, approximately 1% to 10% of the over-provisioned space within the SMR is configured as an internal persistent buffer to absorb non-sequential writes [6; 7]. Incoming data is appended in the persistent buffer in a circular log manner, and the original logical block address (LBA) is recorded by a dynamic mapping table. To ensure that there is enough space in the persistent buffer to accommodate incoming new data, the buffer data is periodically migrated to the native zone. However, cleanup activities inevitably block user requests, especially under write-intensive workloads, and the inefficient persistent buffer management mechanism invokes cleanup activities frequently, thus degrading disk performance [6; 7; 8]. Currently, from the storage medium perspective, the internal persistent buffer can be divided into two categories: flash-based persistent buffers and magnetic recording-based persistent buffers. Given the excellent read/write performance of flash, [PERSON] et al. [9; 10] proposed embedding it inside SMR disks as an internal persistence buffer to reduce the duration of the cleanup activities. However, flash as the first-level buffer of SMR needs to take a large amount of write traffic, and its limited programs and erases (called P/E cycles) not only affect the efficiency of cleanup activities but also increase the cost of SMR disks. In contrast, the magnetic recording-based persistent buffer is widely configured in shipping SMR disks due to its low cost, high capacity, and durability. In general, magnetic recording-based persistent buffers can be constructed by SMR zones or CMR zones. In the case of the SMR-based persistent buffer, it follows the SMR philosophy of maximizing space utilization. However, SMR-based persistent buffers are subject to sequential write constraints, resulting in inflexible space allocation and frequent garbage collection (GC). As a result, many existing studies have attempted to reduce cleanup costs by improving the efficiency of space management in the persistent buffer [11; 12; 13]. Another case is a CMR-based persistent buffer without sequential write constraints, which means that its in-place update feature can reduce the cleanup frequency to some extent and is also the type of interest in this paper. Several existing works have proposed approaches to optimize the management of CMR-based persistent buffers. For example, dm-zoned [14] divides the CMR buffer into segments of the same size as the SMR zone to buffer update data. [PERSON] et al. [15] further determine the optimal size of the segments based on the data layout scheme of dm-zoned. Differently, the latter proposes a spatial locality-based promotion operation to reduce the cleanup overhead. Recently, [PERSON] et al. [16] proposed a loopback-based persistent buffer management strategy to avoid the write-back of hot (frequently updated) blocks. However, these studies did not achieve the expected performance gains, mainly due to the following reasons. Firstly, the lack of sufficient knowledge about the locality characteristics of the workload prevents frequently updated blocks from being accurately captured. These blocks move back and forth between the persistent buffer and the SMR zones, increasing the frequency of buffer cleanup.