# Balloon: An Elastic Data Management Strategy for Interlaced Magnetic Recording
## 2 Background and Related Work
### Related Work  
In terms of system implications and algorithms, previous studies have primarily focused on ways to minimize the occurrence of time-consuming RMW operations through data placement and data shuffling strategies. Data placement policies play a key role in allocating tracks to accommodate incoming data, with the goal of avoiding unnecessary track rewrites. Typically, data placement policies include two approaches: static track allocation based on space usage and dynamic track allocation based on track hotness. Classic static placement policies include the two-phase track write management method and the three-phase write management method [12; 13]. In both policies, the bottom track is sequentially allocated when space utilization is below 50%. During this phase, all writes to the bottom track do not trigger RMW operations. When space utilization exceeds 50% (when top track allocation begins), the two-phase write management policy continues sequential allocation until the top tracks are exhausted, while the three-phase write management policy uses a jump allocation strategy. For example, after top track 1 is allocated, the next allocation is to top track 5 instead of top track 3. Consequently, the three-phase write management method reduces rewrite overhead in the 50% to 75% space utilization range, because at most one top track is involved in each rewrite. After space utilization exceeds 75%, the three-phase write management method allocates the remaining top tracks sequentially. Building on the three-phase write management method, [PERSON] [14] uses zigzag allocation instead of radial allocation to improve the spatial locality of logical data blocks by reversing the allocation direction during the second phase. However, the frequency of track rewrites is not only determined by space utilization; the hotness of adjacent tracks is also a critical factor. Therefore, [PERSON] et al. [10] proposed a bottom-up track allocation policy to strike a balance between logical data block sequentiality and track rewrite overhead. On the one hand, the logical data block is placed on the track adjacent to its physical location, which reduces the seek overhead of sequential I/O. On the other hand, each track is assigned a track update frequency counter that helps the disk place incoming data as close as possible to the top track with the lowest rewrite probability. Figure 2: Illustration of the track rewrite handling. Data shuffling aims to achieve the desirable hot and cold data layout by migrating hot data to the top tracks where it can be freely written. To reduce the mapping overhead of redirected data, [PERSON] et al. [15] proposed three techniques for data shuffling at the track level, namely selective track caching, track flipping, and dynamic track mapping. The core idea of these techniques is to reduce the introduction of track rewrites overhead by migrating the hot bottom track to the freely written top tracks or reserved region within the disk. However, these techniques either make poor use of the space available for freely written tracks or ignore the hotness of the adjacent bottom track, which limits their performance gain for IMR. In addition, to minimize the introduction of extra I/Os, MOM [11] suggests moving top tracks affected by write interference to other free top tracks with low rewrite probability instead of the typical RMW operation. However, this method retriggers the RMW operation when the disk is full because there are not enough free tracks to accommodate the moving data. As a typical representative of block-based data shuffling, [PERSON] et al. [16] proposed Top-Buffer and Block-Swap, which can not only use unallocated tracks to buffer write-interfering data blocks but also actively swap eligible hot blocks to the top region, effectively improving the efficiency of data shuffling. However, with Top-Buffer, the buffer track contains data blocks from multiple bottom tracks, resulting in an unpredictable duration of data migration.