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The IP.com Prior Art Database
Undisclosed
English (United States)
Avoiding Flash’s Read-Disturb in Storage System READ Disturb error in Flash drives:
Read disturb is a well-known effect in NAND flash world wherein the reading data from one flash cell can cause the threshold voltages of other (unread) cells that are present in the same block to shift to a higher value. Even if the single time voltage application to the block is very low, but this may cause serious problem when the frequency of the occurrence increases with multiple reads and generates read disturb errors in the drives.
Simply, in the block storage drives, the reading of some blocks in the Flash drive may affect the neighboring blocks in the page because of read-disturbs experienced by these adjacent blocks. When the heavy READs are observed by a block in the storage system, associated Flash drive will also experience the more READ commands, hence certain blocks in the flash drive will be accessed multiple times via providing the read reference voltage. Because of this hot reading operations on certain set of blocks, the blocks that are near to this extremely READ hot block may experience read disturb errors. These errors are often handled at flash controller and at FTL level wherein IO counters are maintained at flash level to match the threshold and the blocks experiencing read-disturb are re-written to some other location for refreshing the contents of the flash cells. This mechanism induces write amplification inside the flash drive along with the increment in I/O latency during internal data movement. Where is the problem:
In the modern storage virtualization systems, multiple disks are attached to the virtualization appliance and that are virtualized to create a single consolidated storage space on which the volumes are carved out for the applications and host attachments. These storage virtualization engines use Flash Drives as a tier-1 storage location for heavily accessed data to serve the hot data with low latency. This extent placement is performed based on the access heat temperature monitoring by tiering demons. Because of this, the extents experiencing heavy I/O are moved to flash tier hence underlying flash drives will experience more I/O commands. There are advanced features like replication, mirroring and RAID are implemented at virtualization level that maintains multiple copies of same data at multiple different drives at the backend disk subsystems. These additional features allow high data availability as they are stored at more than one location. The access policy of the storage controller has no view of underlying read-disturb errors, it keeps accessing the same location even if the read- disturb threshold about to reach for the block which is maintained at disk level. Once the disk drive's FTL detects that the read-disturb threshold is reached, then it internally moves the data to another location by writing complete page and FTL level LBA mapping table is updated. This increases the write amplification in the drive. A...
