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Adaptive LPT Journal Size to Improve Performance and Endurance of NAND Flash Storage Devices

IP.com Disclosure Number: IPCOM000243139D
Publication Date: 2015-Sep-17
Document File: 3 page(s) / 50K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a scheme in which the Logical-to-Physical (LPT) journal size is adaptive to the amount of unused space in the device, which improves performance and endurance of negative-AND (NAND) flash storage devices.

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Adaptive LPT Journal Size to Improve Performance and Endurance of NAND Flash Storage Devices

The Logical-to-Physical (LPT) table maintains the mapping between the user-visible address space and the (physical) device address space. With the continuing increase of the logical (user-visible) device size (i.e. the increasing capacity of flash storage devices), the LPT table size grows. In order to guarantee high performance of flash cards/Solid State Drives (SSDs), it is important to maintain these LPT tables in Dynamic Random Access Memory (DRAM), or at least a large portion of it. However, if LPT tables are maintained in DRAM, then said tables must be backed-up to permanent storage, typically negative-AND (NAND) flash storage, upon power loss or normal shutdown, and then restored from permanent storage upon device startup.

For configurations in which extremely large DRAM areas are dedicated to LPT tables, it becomes increasingly difficult to perform the backup of the LPT tables to permanent storage in time upon a power loss, because the backup from DRAM to permanent storage may take a lot of time and energy. As a solution, all-flash arrays often employ a journaling

technique of the LPT tables. In this technique, the permanent storage holds a snapshot of the LPT table and a journal of all modifications to the LPT table since the last snapshot. Upon power loss, only the journal needs to be closed (which is

a very quick operation) and then upon device startup first the snapshot of the LPT table is restored from the permanent storage, and then the journal of the LPT modifications is replayed.

The journal size determines the frequency at which the LPT snapshots are made. Each time a journal is full, a new snapshot of the current LPT tables has to be written to permanent storage. An old snapshot can be released (i.e., deleted to reclaim space) when the writing of the new snapshot is completed. Similarly, an old journal can be released when the new snapshot is completed. The journal size is determined based on the following factors:


• How often the organization can afford to make the LPT snapshots. The smaller the journal, the more frequently the snapshot needs to be made hence increasing the number of writes the storage device will see, negatively affecting the overall device endurance


• How long the organization can afford to replay a journal. Extremely large journals may take a long time to replay, and may increase device startup time; however, because DRAM access is typically fast, this factor is rarely limiting.


• How much space the organization can dedicate to the journal. LPT snapshots plus journals might consume a significant percentage of the total device capacity. While on one hand larger journals help to reduce the number of

writes thereby enhancing overall endurance, on the other hand too large journals can reduce the effective size of the space reserved for overprovisioning thereby drastically increasing write amplifica...