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

Cosmos OpenSSD架构分析--FSC

FSC控制器结构如下:

Cosmos OpenSSD架构分析--FSC

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
Cosmos OpenSSD架构分析--FSC

下面列出一个example:

Cosmos OpenSSD架构分析--FSC

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:

Cosmos OpenSSD架构分析--FSC

读过程:

Cosmos OpenSSD架构分析--FSC

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)

Cosmos OpenSSD架构分析--FSC

way controller

Cosmos OpenSSD架构分析--FSC

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
Cosmos OpenSSD架构分析--FSC

②Page read, page program, block erase commands are executed in 2-3 stages
Cosmos OpenSSD架构分析--FSC

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