For a write request, today's flash storage cannot distinguish the logical object it comes from ( e.g. , SSTables in RocksDB). In such object-oblivious flash devices, concurrent writes from different objects are simply packed in their arrival order to flash memory blocks; hence data pages from multiple objects with different lifetimes are multiplexed onto the same flash blocks. This multiplexing incurs write amplification, worsening the performance. Tackling the multiplexing problem, we propose a novel interface for flash storage, FlashAlloc. It is used to pass the logical address ranges of objects to the underlying flash device and thus to enlighten the device to stream writes by objects. The object-aware flash storage can now de-multiplex concurrent writes from multiple objects with distinct deathtimes into per-object dedicated flash blocks. In essence, the interface enables the per-object fine-grained write streaming. Given that popular data stores tend to separate writes by logical objects, we can achieve, compared to the existing solutions, transparent streaming just by calling FlashAlloc upon object creation. Also, FlashAlloc is adaptive to workload changes, and liberates the stream conflicts in the multi-tenant environment. Our experimental results using an open-source SSD prototype demonstrate that FlashAlloc can reduce the device-level write amplification factor (WAF) under RocksDB, F2FS, and MySQL by 1.5, 2.5, and 0.3, respectively and improve their throughput by 2.7x, 1.8x, and 1.2x, respectively. Also, FlashAlloc can mitigate the WAF interference among tenants: when running RocksDB and MySQL together on the same SSD, FlashAlloc reduced WAF from 2.5 to 1.6 and doubled their throughputs.

When replacing a dirty victim page upon page miss, the conventional buffer managers flush the dirty victim first to the storage before reading the missing page. This read-after-write (RAW) protocol, unfortunately, causes the read stall problem on flash storage; because of the asymmetric I/O speed and parallelism in flash storage, the clean frames are quickly consumed, so the read for the missing page often has to wait for the slow write to complete and for the frame to be clean due to the resource conflict for the same buffer frame. RAW will thus make the performance-critical synchronous reads often blocked by writes, severely worsening transaction throughput and latency. In addition, its strict I/O ordering will make flash storage with abundant parallelism under-utilized. To avoid read stalls in the DBMS buffer, we propose RW ( fused read and write ) as a new storage interface. Using RW on read stall, the buffer manager can issue both read and write requests at once to the storage. Then, once the dirty page is copied to the storage buffer, it can immediately serve the read. In addition, to resolve read stalls in the flash storage buffer, we propose R-Buf, where the read buffer is separated from the write buffer so that reads can proceed at no stall. RW and R-Buf, working at different layers, complement each other when used together. We prototype RW and R-Buf on a real Cosmos+ OpenSSD board. Evaluation results show that RW alone improves TPC-C throughput over RAW by 3.2x and, combined with R-Buf, does by 3.9x. In addition, we demonstrate that R-Buf effectively mitigates the I/O interference in multi-tenancy.

Solid-state disks (SSDs), which are composed of multiple NAND flash chips, are replacing hard disk drives (HDDs) in the mass storage market. The performances of SSDs are increasing due to the exploitation of parallel I/O architectures. However, reliability remains as a critical issue when designing a large-scale flash storage. For both high performance and reliability, Redundant Arrays of Inexpensive Disks (RAID) storage architecture is essential to flash memory SSD. However, the parity handling overhead for reliable storage is significant. We propose a novel RAID technique for flash memory SSD for reducing the parity updating cost. To reduce the number of write operations for the parity updates, the proposed scheme delays the parity update which must accompany each data write in the original RAID technique. In addition, by exploiting the characteristics of flash memory, the proposed scheme uses the partial parity technique to reduce the number of read operations required to calculate a parity. We evaluated the performance improvements using a RAID-5 SSD simulator. The proposed techniques improved the performance of the RAID-5 SSD by 47 percent and 38 percent on average in comparison to the original RAID-5 technique and the previous delayed parity updating technique, respectively.

The I/O performances of flash memory solid-state disks (SSDs) are increasing by exploiting parallel I/O architectures. However, the reliability problem is a critical issue in building a large-scale flash storage. We propose a novel Redundant Arrays of Inexpensive Disks (RAID) architecture which uses the delayed parity update and partial parity caching techniques for reliable and high-performance flash memory SSDs. The proposed techniques improve the performance of the RAID-5 SSD by 38% and 30% on average in comparison to the original RAID-5 technique and the previous delayed parity update technique, respectively.