五、代码结构(3) I/O写流程

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上一篇我们来介绍了dm dedup的空间管理
这一篇我们介绍核心流程I/O写流程

要看的特别清楚这部分的内容，需要结合我之前写过的《device-mapper 块级重删(dm dedup) <2>设计》请添加链接描述一起学习。

在块级重删 设计那一篇已经描述了这一系列的过程。
上一篇代码结构已经对kvs_hash和kvs_lbn的lookup和insert有了分析。
接下来我们来看看lookup和insert在写流程中的使用。

首先我们先看一下alloc_pbn_block给lbn的函数，后面都会用用到。

            【
                            /*因为现在还没有pbn来放置lbn，所以先申请一个pbn_new*/
                    static int alloc_pbnblk_and_insert_lbn_pbn(struct dedup_config *dc,
                                                     u64 *pbn_new,
                                                     struct bio *bio, uint64_t lbn)
                        {
                            int r = 0;
                            struct lbn_pbn_value lbnpbn_value;

                            r = allocate_block(dc, pbn_new);/*找到一个新的pbn*/

                            lbnpbn_value.pbn = *pbn_new;
                            do_io(dc, bio, *pbn_new); /*将bio的数据放置到pbn_new的位置*/

                            r = dc->kvs_lbn_pbn->kvs_insert(dc->kvs_lbn_pbn, (void *)&lbn,
                                            sizeof(lbn), (void *)&lbnpbn_value,
                                            sizeof(lbnpbn_value));
                            /*将新的lbn_pbn的关系记录到kvs_lbn_pbn中*/
                            return r;
                    }
             】

1、no hash && no lbn
即：① compute_hash_bio(dc->desc_table, bio, hash)； //对bio的data进行hash，获得到hash_pbn
-> ② dc->kvs_hash_pbn->kvs_lookup; //通过hash值，查找对应的hash_pbn_value，也就是pbn。
没有找到pbn ->③ handle_write_no_hash.[dc->kvs_lbn_pbn->kvs_lookup] //寻找bio的lbn是否存在
没有找到lbn -> ④ handle_write_no_hash.[__handle_no_lbn_pbn] //这里到了最终处理函数
这里为了减少篇章，去掉了资源申请错误处理和资源访问错误处理

static int __handle_no_lbn_pbn(struct dedup_config *dc,
                   struct bio *bio, uint64_t lbn, u8 *hash)
{
    int r, ret;
    u64 pbn_new;
    struct hash_pbn_value hashpbn_value;

    /* Create a new lbn-pbn mapping for given lbn，注意这里是pbn_new */
    r = alloc_pbnblk_and_insert_lbn_pbn(dc, &pbn_new, bio, lbn);

    /* Inserts new hash-pbn mapping for given hash.
    由于dm_io已经将bio数据放置到pbn中，然后hash_pbn记录*/

    hashpbn_value.pbn = pbn_new;
    r = dc->kvs_hash_pbn->kvs_insert(dc->kvs_hash_pbn, (void *)hash,
                     dc->crypto_key_size,
                     (void *)&hashpbn_value,
                     sizeof(hashpbn_value));

    /* Increments refcount for new pbn entry created. 增加一个引用*/
    r = dc->mdops->inc_refcount(dc->bmd, pbn_new);

    /* On all successful steps increment new write count. */
    dc->newwrites++;/* 在dc中记录下newwrites增加，为了更好的统计和分析行为*/
    goto out;

/* Error handling code path */
inc_refcount_err:
    /* Undo actions taken in hash-pbn kvs insert. */
    ret = dc->kvs_hash_pbn->kvs_delete(dc->kvs_hash_pbn,
                       (void *)hash, dc->crypto_key_size);

kvs_insert_err:
    /* Undo actions taken in alloc_pbnblk_and_insert_lbn_pbn. */
    ret = dc->kvs_lbn_pbn->kvs_delete(dc->kvs_lbn_pbn,
                      (void *)&lbn, sizeof(lbn));

    ret = dc->mdops->dec_refcount(dc->bmd, pbn_new);

out:
    return r;
}

2、no hash && has lbn
即：① compute_hash_bio(dc->desc_table, bio, hash)； //对bio的data进行hash，获得到hash_pbn
-> ② dc->kvs_hash_pbn->kvs_lookup; //通过hash值，查找对应的hash_pbn_value，也就是pbn。
没有找到pbn ->③ handle_write_no_hash.[dc->kvs_lbn_pbn->kvs_lookup] //寻找bio的lbn是否存在
找到lbn -> ④ handle_write_no_hash.[__handle_has_lbn_pbn] //这里到了最终处理函数

static int __handle_has_lbn_pbn(struct dedup_config *dc,
                struct bio *bio, uint64_t lbn, u8 *hash,
                u64 pbn_old)
{
    int r, ret;
    u64 pbn_new;
    struct hash_pbn_value hashpbn_value;

    /* Allocates a new block for new pbn and inserts lbn-pbn lapping.注意这是覆盖 */
    r = alloc_pbnblk_and_insert_lbn_pbn(dc, &pbn_new, bio, lbn);

    /* Inserts new hash-pbn entry for given hash. */
    hashpbn_value.pbn = pbn_new;
    r = dc->kvs_hash_pbn->kvs_insert(dc->kvs_hash_pbn, (void *)hash,
                     dc->crypto_key_size,
                     (void *)&hashpbn_value,
                     sizeof(hashpbn_value));

    /* Increments refcount of new pbn. */
    r = dc->mdops->inc_refcount(dc->bmd, pbn_new);
    /*注意这里需要将pbn_new+,pbn_old-,这样有可能pbn_old减到可回收的可能*/
    /* Decrements refcount for old pbn and decrement logical block cnt. */
    r = dc->mdops->dec_refcount(dc->bmd, pbn_old);

    dc->logical_block_counter--;

    /* On all successful steps increment overwrite count. */
    dc->overwrites++;  /*这里是overwrites++,和上面的newwrites不一样*/
    goto out;

/* Error handling code path. */
dec_refcount_err:
    /* Undo actions taken while incrementing refcount of new pbn. */
    ret = dc->mdops->dec_refcount(dc->bmd, pbn_new);

inc_refcount_err:
    ret = dc->kvs_hash_pbn->kvs_delete(dc->kvs_hash_pbn, (void *)hash,
                       dc->crypto_key_size);

kvs_insert_err:
    /* Undo actions taken in alloc_pbnblk_and_insert_lbn_pbn. */
    ret = dc->kvs_lbn_pbn->kvs_delete(dc->kvs_lbn_pbn, (void *)&lbn,
                      sizeof(lbn));

    ret = dc->mdops->dec_refcount(dc->bmd, pbn_new);

out:
    return r;
}

3、hash && no lbn
即：① compute_hash_bio(dc->desc_table, bio, hash)； //对bio的data进行hash，获得到hash_pbn
-> ② dc->kvs_hash_pbn->kvs_lookup; //通过hash值，查找对应的hash_pbn_value，也就是pbn。
找到pbn ->③ handle_write_with_hash.[dc->kvs_lbn_pbn->kvs_lookup] //寻找bio的lbn是否存在
没有找到lbn -> ④ handle_write_no_hash.[__handle_no_lbn_pbn_with_hash] //这里到了最终处理函数
既然找到了hash_pbn,就是pbn可以复用的，直接将lbn将pbn关联就行

static int __handle_no_lbn_pbn_with_hash(struct dedup_config *dc,
                     struct bio *bio, uint64_t lbn,
                     u64 pbn_this,
                     struct lbn_pbn_value lbnpbn_value)
{
    int r = 0, ret;

    /* Increments refcount of this passed pbn */
    r = dc->mdops->inc_refcount(dc->bmd, pbn_this);
    if (r < 0)
        goto out;

    lbnpbn_value.pbn = pbn_this;

    /* Insert lbn->pbn_this entry */
    r = dc->kvs_lbn_pbn->kvs_insert(dc->kvs_lbn_pbn, (void *)&lbn,
                    sizeof(lbn), (void *)&lbnpbn_value,
                    sizeof(lbnpbn_value));
    if (r < 0)
        goto kvs_insert_error;

    dc->logical_block_counter++;

    bio->bi_status = BLK_STS_OK;
    bio_endio(bio);
    /*bio完成*/
    dc->newwrites++;
    goto out;

kvs_insert_error:
    /* Undo actions taken while incrementing refcount of this pbn. */
    ret = dc->mdops->dec_refcount(dc->bmd, pbn_this);

out:
    return r;
}

4、hash && lbn
即：① compute_hash_bio(dc->desc_table, bio, hash)； //对bio的data进行hash，获得到hash_pbn
-> ② dc->kvs_hash_pbn->kvs_lookup; //通过hash值，查找对应的hash_pbn_value，也就是pbn。
找到pbn ->③ handle_write_with_hash.[dc->kvs_lbn_pbn->kvs_lookup] //寻找bio的lbn是否存在
找到lbn -> ④ handle_write_no_hash.[__handle_has_lbn_pbn_with_hash] //这里到了最终处理函数
既然找到了hash_pbn和lbn_pbn，这里存在了两种情况：
一、overwrite，也就是hash_pbn的pbn和lbn_pbn是一个。
二、No relationship，也就是这是将一个pbn的内容写到一个新的lbn位置

static int __handle_has_lbn_pbn_with_hash(struct dedup_config *dc,
                      struct bio *bio, uint64_t lbn,
                      u64 pbn_this,
                      struct lbn_pbn_value lbnpbn_value)
{
    int r = 0, ret;
    struct lbn_pbn_value this_lbnpbn_value;
    u64 pbn_old;

    pbn_old = lbnpbn_value.pbn;

    /* special case, overwrite same LBN/PBN with same data */
    if (pbn_this == pbn_old)
        goto out;

  /*如果hash_pbn和lbn_pbn不相等，1、增加pbn引用和新lbn和pbn关联*/
    /* Increments refcount of this passed pbn */
    r = dc->mdops->inc_refcount(dc->bmd, pbn_this);
    if (r < 0)
        goto out;

    this_lbnpbn_value.pbn = pbn_this;

    /* Insert lbn->pbn_this entry */
    r = dc->kvs_lbn_pbn->kvs_insert(dc->kvs_lbn_pbn, (void *)&lbn,
                    sizeof(lbn),
                    (void *)&this_lbnpbn_value,
                    sizeof(this_lbnpbn_value));
    if (r < 0)
        goto kvs_insert_err;

  /*减小lbn之前记住的pbn_old的引用，这个pbn_old将可能被回收*/
    /* Decrement refcount of old pbn */
    r = dc->mdops->dec_refcount(dc->bmd, pbn_old);
    if (r < 0)
        goto dec_refcount_err;

    goto out;   /* all OK */

dec_refcount_err:
    /* Undo actions taken while decrementing refcount of old pbn */
    /* Overwrite lbn->pbn_this entry with lbn->pbn_old entry */
    ret = dc->kvs_lbn_pbn->kvs_insert(dc->kvs_lbn_pbn, (void *)&lbn,
                          sizeof(lbn), (void *)&lbnpbn_value,
                      sizeof(lbnpbn_value));
    if (ret < 0)
        DMERR("Error in overwriting lbn->pbn_this [%llu] with"
              " lbn-pbn_old entry [%llu].", this_lbnpbn_value.pbn,
              lbnpbn_value.pbn);

kvs_insert_err:
    ret = dc->mdops->dec_refcount(dc->bmd, pbn_this);
    if (ret < 0)
        DMERR("Error in decrementing previously incremented refcount.");
out:
    if (r == 0) {
        bio->bi_status = BLK_STS_OK;
        bio_endio(bio);
        dc->overwrites++;
    }

    return r;
}

这一篇介绍了，写流程的四种情况，更加清晰了解释了dm dedup设计一文中的流程图。
希望读者看完后，能够对dm dedup这种简单逻辑的方式所吸引，从而喜欢上块重删这个技术。

--------------未完待续--------------

【本文只在51cto博客作者 “底层存储技术” https://blog.51cto.com/12580077 个人发布，公众号发布：存储之谷】，如需转载，请于本人联系，谢谢。

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