Host Cache Flush During RAID Rebuild to Skip Cached LBAs
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Solution Overview
Problem
RAID-based data storage systems face significant rebuild time delays and processing bandwidth consumption when transitioning to a DEGRADED state due to unrecoverable errors, as conventional rebuild processes access and recalculate data from all drives, including cached and uncached areas.
Innovation Solution
Implementing rebuild-flush operations that differentiate between valid-modified and valid-unmodified cache data, writing dirty data to both the virtual disk and hot spare drive, while writing clean data only to the hot spare, and maintaining hash tables to skip over cached LBAs, thereby reducing rebuild time by leveraging the storage cache.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional rebuild processes are used to restore degraded virtual RAID disks, then data redundancy is restored, but rebuild time is excessive and processing bandwidth is significantly consumed
Solution Approach 1:
The patent performs preliminary actions by identifying and flushing valid-modified cache data to the hot spare drive before the rebuild process begins. This pre-positioning of data that will be needed during rebuild reduces the amount of data that must be retrieved and processed during the actual rebuild operation, thereby reducing overall rebuild time while maintaining data redundancy.
Solution Approach 2:
The patent extracts and separates valid-modified cache data from the overall data set and handles it differently from valid-unmodified data. By identifying and extracting this specific subset of data that requires flushing to both the virtual disk and hot spare, the system optimizes the rebuild process by avoiding redundant operations on this data portion.
2Reliability
If conventional rebuild processes access all drives including cached areas, then complete data restoration is achieved, but processing bandwidth consumption increases
Solution Approach 1:
The patent applies local quality by treating different portions of cached data differently based on their validity and modification status. Valid-modified cache data is flushed to both the virtual disk and hot spare, while valid-unmodified data is handled differently. This localized differentiation optimizes processing bandwidth by avoiding redundant operations on data that doesn't require the same treatment.
Solution Approach 2:
The patent performs partial action by selectively flushing only the valid-modified cache data to the hot spare drive, rather than flushing all cached data. This partial approach ensures that the necessary data is restored while avoiding unnecessary processing bandwidth consumption that would result from handling all cached data uniformly.
3Loss of time
If the host storage cache is leveraged to skip cached LBAs during rebuild, then rebuild time is reduced, but system complexity increases
Solution Approach 1:
The patent uses feedback by checking whether logical block addresses (LBAs) are present in the host storage cache before performing rebuild operations. This feedback mechanism allows the system to identify cached LBAs and skip redundant operations, thereby reducing rebuild time. The complexity introduced by this feedback check is offset by the significant time savings achieved during the rebuild process.
Data Source
AI summary
A storage management method includes receiving a degrade signal indicating a degraded state of a virtual disk associated with a host system. Rebuild-flush operations may be performed. The operations may include writing, to the virtual disk and also to a hot spare drive (HSP) associated with the virtual disk, valid-modified data, stored in a host storage cache, associated with the virtual disk. In contrast, valid-unmodified storage cache data associated with the virtual disk, may be written to the HSP only. After the rebuild-flush completes, the virtual disk may be rebuilt. During rebuild, however, any cached-LBA may be skipped where traditional RAID rebuild operations are performed for un-cached LBAs only.