SSD Page Allocation #

Page allocation happens at write, while read directly lookup in FTL. A request is parsed into page-sized request for address translation. Allocation is divided into two: plane allocation (PLAlloc) and block allocation (BLAlloc):

Plane allocation #

Goal: determine physical (channel, way, die, plane, block, page)

Write requests are striped to various units to increase parallelism, following a certain allocation order:

• Static Allocation: assign pages to free blocks based on certain fixed order
• Dynamic Allocation: further take into considering: wear leveling, busy/idle channels. For example, if target chip is busy, writes can be re-directed to another chip. The drawback is maintenance of another mapping table below FTL ¹. Tavakkol et al. ² conclude the superiority of dynamic allocation in terms of degree of parallelism
• OCSSD 2.0 defines a sequential page allocation scheme at chunk level: it maintains a write pointer and requires sequential allocation to advance the pointer until the chunk is fully consumed.

Block allocation #

Goal: determine physical block:

• Either wear-aware or first-fit allocation
• Inside a block, pages are programmed sequentially, so the page ID is determined by the write frontier

Static Allocation #

There are 24 possible combinations of allocation schemes:

³

The optimal allocation scheme is PFCD: Plane → Flash-chip → Channel → Die, concluded by Myoungsoo Jung et al. with detailed analysis ⁴.