1.3 SSD 與快閃記憶體的特性#

# 01.3 範例：SSD FTL 與垃圾回收模擬
class FlashBlock:
    def __init__(self, num_pages: int):
        self.pages = [None] * num_pages   # None 表示完全空白（已抹除）
        self.valid = 0                    # 有效（未被更新取代）的頁數

class SimpleFTL:
    def __init__(self, blocks=64, pages_per_block=8):
        self.blocks = [FlashBlock(pages_per_block) for _ in range(blocks)]
        self.map = {}                     # 邏輯頁 -> (block, page)
        self.write_cursor = 0             # 循序寫入游標
        self.stats = {"write": 0, "erase": 0, "gc_copy": 0, "amp_bytes": 0}

    def _new_block(self):
        # 簡化示範：循環使用區塊，需 GC 時，把還 valid 的頁搬到新區塊
        free_idx = next((i for i, b in enumerate(self.blocks) if not b.valid
                         and b.pages[0] is None), None)
        if free_idx is not None:
            return free_idx
        # 找有效頁最少的區塊做 GC
        victim = min(range(len(self.blocks)), key=lambda i: self.blocks[i].valid)
        v = self.blocks[victim]
        self.stats["gc_copy"] += v.valid
        self.stats["amp_bytes"] += v.valid * 4096
        dst = self.blocks[next(i for i, b in enumerate(self.blocks)
                                if b is not v)]
        di = 0
        for pi, d in enumerate(v.pages):
            if d is not None:
                dst.pages[di] = d
                di += 1
        v.pages, v.valid = [None] * len(v.pages), 0
        self.stats["erase"] += 1         # 抹除 victim
        self.write_cursor = victim       # 送到空出的區塊
        return victim

    def write_page(self, lpid: int, data: bytes = b""):
        self.stats["write"] += 1
        self.stats["amp_bytes"] += 4096
        if lpid in self.map:              # 舊頁失效（copy-on-write）
            b, p = self.map[lpid]
            self.blocks[b].valid -= 1
        bi = self._new_block()
        fb = self.blocks[bi]
        pi = next(i for i, d in enumerate(fb.pages) if d is None)
        fb.pages[pi] = lpid
        fb.valid += 1
        self.map[lpid] = (bi, pi)

# 展示：反覆原地更新同一頁時，寫入放大有多嚴重
ftl = SimpleFTL()
for i in range(40):
    ftl.write_page(i % 8)                 # 反覆寫同一 8 頁的邏輯位址
s = ftl.stats
print(f"主機寫入 40 頁, 內部 GC 搬移 {s['gc_copy']} 頁, 抹除 {s['erase']} 個區塊")
print(f"寫入放大 = {(s['gc_copy'] * 4096) / (s['write'] * 4096):.2f}x")