Linux里面block代表什么意思？

在2.4 的 fs/buffers.c 的函数

static int __block_prepare_write(struct inode *inode, struct page *page,

unsigned from, unsigned to, get_block_t *get_block)

中有如下代码:

if (!page->buffers)

create_empty_buffers(page, inode->i_dev, blocksize)

head = page->buffers

bbits = inode->i_sb->s_blocksize_bits

block = page->index <<(PAGE_CACHE_SHIFT - bbits)

for(bh = head, block_start = 0bh != head || !block_start

block++, block_start=block_end, bh = bh->b_this_page) {

if (!bh)

BUG()

block_end = block_start+blocksize

if (block_end <= from)

continue

if (block_start >= to)

break

if (!buffer_mapped(bh)) {

err = get_block(inode, block, bh, 1)

if (err)

goto out

if (buffer_new(bh)) {

unmap_underlying_metadata(bh)

if (Page_Uptodate(page)) {

set_bit(BH_Uptodate, &bh->b_state)

continue

}

if (block_end >to)

memset(kaddr+to, 0, block_end-to)

if (block_start <from)

memset(kaddr+block_start, 0, from-block_start)

if (block_end >to || block_start <from)

flush_dcache_page(page)

continue

}

}

对unmap_underlying_metadata 的意义不是特别明确, 注释说是释放潜在的bh. 实现是从hash中找到bh代表的设备上, bh-->blocknr 那个块的oldbh, 如果有的话把它释放.

奇怪的是,既然是新分配的,为什么在hash中能找到? 如果hash中本来就有, 需要新分配吗?

/*

* We are taking a block for data and we don't want any output from any

* buffer-cache aliases starting from return from that function and

* until the moment when something will explicitly mark the buffer

* dirty (hopefully that will not happen until we will free that block -)

* We don't even need to mark it not-uptodate - nobody can expect

* anything from a newly allocated buffer anyway. We used to used

* unmap_buffer() for such invalidation, but that was wrong. We definitely

* don't want to mark the alias unmapped, for example - it would confuse

* anyone who might pick it with bread() afterwards...

*/

static void unmap_underlying_metadata(struct buffer_head * bh)

{

struct buffer_head *old_bh

old_bh = get_hash_table(bh->b_dev, bh->b_blocknr, bh->b_size)

if (old_bh) {

mark_buffer_clean(old_bh)

wait_on_buffer(old_bh)

clear_bit(BH_Req, &old_bh->b_state)

/* Here we could run brelse or bforget. We use

bforget because it will try to put the buffer

in the freelist. */

__bforget(old_bh)

}

}

我要没记错：

block

是块，这个是系统文件系统的最小分配单位，注意是系统的，不是硬件的。

这个block

是看文件系统建立时的设置情况，类似于

Windows

下面所说的簇。

这个是在格式化系统时进行设置，具体多大看文件系统，我记得现在默认都是

4k

。至少

Ext3

默认是

4k

。而且

block

大小和磁盘最大限制有关系的，如果你用

4k

，ext3

极限最高

16T

，也就是

4k

*

2^32

，如果你用

1k

，那么就缩小到了

4T。

其他场合，block

同样也是最小分配单位的意思，但有些仅限于当作一个“计量单位”来使用。当作计量单位时，好像一般都是

1k。

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