SMR Drive Skip-Bypass Writing for Non-Sequential Commands
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Solution Overview
Problem
Shingled magnetic recording (SMR) hard disk drives (HDDs) face inefficiencies in handling non-sequential write commands, leading to significant write amplification and latency due to the need to rewrite entire bands, especially in RAID scenarios, where media caches become quickly filled and performance decreases.
Innovation Solution
Implement a skip-bypass mode that involves reading data between non-sequential write commands into memory and writing them directly to a new SMR band in a single sequential operation, bypassing storage in the media cache.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If non-sequential write commands are handled using conventional SMR methods with media cache, then data can be stored temporarily, but write amplification increases significantly and latency increases when cache fills up
Solution Approach 1:
The system dynamically switches between two operational modes: cache-based mode for normal operations and direct-write mode for non-sequential writes. This dynamic adaptation allows the system to optimize performance based on the specific write pattern being encountered, resolving the contradiction between reliable data storage and maintain write performance.
Solution Approach 2:
The write handling process is segmented into two distinct pathways: sequential writes that utilize media cache for buffering, and non-sequential writes that bypass the cache and write directly to the disk. This segmentation allows each pathway to be optimized independently, preventing write amplification while maintaining storage reliability.
2Reliability
If entire SMR bands are rewritten to update small amounts of data, then data integrity is maintained, but write amplification increases and drive performance decreases
Solution Approach 1:
Instead of rewriting entire SMR bands to update small amounts of data, the system performs partial writes only to the specific tracks or sectors that need updating. This partial action approach maintains data integrity for updated portions while avoiding the energy waste of rewriting unnecessary data throughout the entire band.
Solution Approach 2:
The system changes the write operation parameter from band-level rewriting to track-level or sector-level updating. This parameter change allows the system to maintain data integrity where needed while significantly reducing the total volume of data written, thereby reducing energy consumption.
3Ease of operation
If media cache is used to buffer non-sequential writes, then write commands can be queued, but cache fills up quickly in RAID scenarios causing performance degradation
Solution Approach 1:
The system extracts non-sequential write commands from the cache queueing process and handles them through a separate direct-write pathway. This extraction prevents these commands from occupying cache space and causing flush latency, while still maintaining proper write command handling through the alternative direct-write interface.
Solution Approach 2:
The controller acts as an intermediary that detects non-sequential write patterns and redirects them to bypass the media cache entirely. This intermediary function prevents the cache from becoming a bottleneck in RAID scenarios while maintaining ease of operation through unified write command interfaces.
4Productivity
If sequential read operations are performed to retrieve data between non-sequential write commands, then data can be obtained for direct writing, but additional read operations are required
Solution Approach 1:
The system performs preliminary sequential read operations to retrieve data that will be needed for subsequent non-sequential writes. By preparing this data in advance through reads, the system enables direct writes without requiring complex real-time data retrieval operations during the write process itself.
Solution Approach 2:
The system merges the sequential read operation with the non-sequential write operation into a coordinated process. The read operations retrieve necessary data in advance, and the write operations use this pre-retrieved data directly, combining these operations efficiently to minimize overall complexity while maintaining high write throughput.
Data Source
AI summary
Non-sequential write commands in a shingled magnetic recording (SMR) drive are efficiently executed using a skip-bypass mode. When certain conditions are detected in the command history of the drive, the drive changes to the skip-bypass mode. In skip-bypass mode, data associated with the write commands are received into memory of the drive and written directly to a new SMR band in a sequential write operation, thereby bypassing storage in a media cache of the SMR drive. Prior to the sequential write operation, data associated with logical block addresses (LBAs) disposed between the LBAs referenced by the non-sequential write commands are read into the memory by a sequential read operation. The data associated with the non-sequential write commands can then be written, along with the data read in the sequential read operation, in a single sequential write operation to the new SMR band.