# Efficient Key-Value Data Placement for ZNS SSD
## 6 Experiments
### Analysis of Space Utilization  
To evaluate how effectively the proposed method uses space of the ZNS SSD, we checked how much key range was available for the \"_fillseq + overwrite_\" workload with four different implementations. The results of the evaluation are shown in Figure 13. It was confirmed that the conventional method could be used for up to 4 million key ranges. The \"original + GC\" (just the garbage collection (GC) logic added to the conventional method) and \"hot/cold\" (which does not have a garbage collection logic but allocates a zone based on the lifetime) used up to 6 million key ranges. It was confirmed that \"hot/cold + GC\", which had all the features of the proposed technique, could be used in up to 7 million key ranges, a 75% increase compared to the conventional method. This means that an 8 GiB ZNS SSD with a 16-byte key and an 800-byte value, cannot freely overwrite if the number of unique keys exceeds 4 million (approximately 3.2 GB). However, it can freely overwrite up to 6 million (about 4.9 GB) unique keys by adopting one of the garbage collections or the zone allocation method based on lifetime. Using both of these, up to 7 million (about 5.7 GB) unique keys can be freely overwritten. The above evaluation results show that the proposed methods alleviated the delayed physical deletion due to zonefiles with different lifetimes mixed into one zone. As a result, zones could be reclaimed faster and more empty zones could be secured. The proposed zone allocation method considering the lifetime of data is a solution that minimizes the physical deletion delay problem by separating short-lifetime data from long-lifetime data. On the other hand, garbage collection is a post-solution method that reduces the physical deletion delay by extracting long-lifetime valid data from the zone containing invalidated data. Figure 14 shows how the lifetime-based zone allocation method minimized physical deletion delays caused by interference problems. The figure shows the pending time taken from logical deletion time to physical deletion time when \"_fillseq + overwrite_\" was executed for 4 million keys. The ratio of the amount of deletion that occurred for each lifetime is 1:1.5:1, in the order of SHORT, MEDIUM and EXTREME, respectively. Figure 13: The key range that can be used for each evaluation target. It can be confirmed that SHORT, which has a short lifetime by the conventional method, was delayed in deletion by MEDIUM and EXTREME, which have a relatively long lifetime. However, in the lifetime-based zone allocation method, data with a short lifetime are aggregated into separated zones, so that physical deletion is performed virtually, immediately after logical deletion. Otherwise, physical deletion of long-lifetime data would be further delayed, because data with a long lifetime interfere with each other. However, it is negligible, because the benefit of removing physical deletion delay of short-lifetime data is much greater. For example, let us suppose that a zone with invalid data applies the percentage of deletions by lifetime to cause the number of SHORT data to be two, of MEDIUM data to be three and of EXTREME data to be two physical deletion delays. With the conventional method, it can be predicted that a delay of 118 s would occur when calculated with the average deletion delay time values of 45 s, 4 s and 8 s for SHORT, MEDIUM and EXTREME, respectively. In contrast, the zone allocation method based on lifetime needs 58 s when calculated with the average delay times of 0.08 s, 12s and 11 s. This means that the lifetime-based zone allocation method reduces the overall physical deletion delay time by dramatically reducing the physical deletion delay time for data with a short lifetime, even though the deletion delay for zones with relatively long-lifetime data increases. Figure 15 shows that the method of combining garbage collection and the zone allocation method by lifetime was useful for efficient space utilization.