Cosmos OpenSSD架构分析--FSC

接口速度：

type		bw
read	75μs	1s/75μs*8k/1s=104m/s
write	1300μs	1s/1300μs*8k/1s=6m/s
erase	3.8ms	1s/3.8ms*8k*256/1s=526m/s
PCIe Gen2 x8	5Gbps	5Gbps/8*8=4.88G/s
DDR3	533MHz（32bit）	32bit*533MHZ/8=2.132GB/s
system bus- AXI-Lite (bus width: 32 bits)	Used for control Operates @ 100MHz
storage data bus- AXI (bus width: 64 bits, burst length: 16)	Used for DMA operations between flash storage controller/PCIe interface and DRAM Operates @ 100 MHz	64bit*100MHZ/8=800m/s
flash bus	Operates @ 50MHz, DDR (100MB/s data rate)	8bit*50MHZ/8=50m/s

数据传输

PCIE <--DMA--AXI--> DRAM <--DMA--AXI> FSC <--flash bus--> DIE

FSC控制器结构如下：

Way arbiter

Way arbiter是用在当有多个way controller都要请求commonflash bus 和 access page buffer，因为是公用的，所以就要分配permission权限，算法如下：

lastGrantedWay = 0
while (true)
　　if (requestArrived)
　　　　for (i = 0 to numberOfWays)
　　　　　　nextCandidate = (lastGrantedWay + i) % numberOfWays　　　　//就是从上次使用的way继续往下轮训着找
　　　　　　if (hasRequested(nextCandidate))
　　　　　　　　grantPermission(nextCandidate)

permission权限的意思是指（每次只有一个way可以使用）：

● Access page buffer (through data path)
● Use flash bus

下面列出一个example：

Buffer controller

Buffer controller controls data bus between page buffer and BCH ECC encoder/decoder

Buffer size is 2198B
● Data 2048B + 60B ECC parity + reserved (90B)
　　– Reserved space was intended to support more powerful ECC (currently not used)
● Page size of flash device is 8192B, thus, data transfer occurs 4 times

写过程：

读过程：

page结构

1.Physical page size of flash device
■ 8KB+448B (8640B)
2.There are four chunks
■ Each chunk consists of a pair of data and parity
3.Reserved 40B area is intended to store FTL metadata (currently not used)

way controller

下面来看flash controller的具体结构：

①Reset, mode change commands are directly executed at their dedicated modules

● Reset module and mode change module are responsible for the control

②Page read, page program, block erase commands are executed in 2-3 stages