1 作用
从字面意思理解，replacement即是替换。我们知道硬盘都有一定的使用寿命，可以在硬盘失效之前通过该功能将就盘的数据迁移至新盘。因为replacement的流程是从旧盘中读出数据直接写入新盘，因此比重构少很多读和校验值计算的操作，效率更高。
另外在raid2.0中，由于硬盘切片的使用方式，当系统只添加一块新盘时无法直接给raid扩容，需要先进行资源均衡，使得各盘空闲空间一致后再扩容，所以replacement同样适用于均衡场景中切片回收替换的逻辑。
2 代码解析
2.1 需替换设置
通过命令 echo want_replacement > /sys/block/md0/md/dev-sdb/state 设置硬盘标记为"需替换"状态，该sys命令会执行如下代码：
state_store()
/* Replacement标记表明成员磁盘新盘，不能被设置为需替换 */
\_ if (rdev->raid_disk >= 0 && !test_bit(Replacement, &rdev->flags))
/* 给旧盘设置标记表明该成员磁盘是需要替换的 */
set_bit(WantReplacement, &rdev->flags);
/* 设置md为不要需要重构状态 */
\_ set_bit(MD_RECOVERY_NEEDED, &rdev->mddev->recovery);
/* 唤醒raid5d */
\_ md_wakeup_thread(rdev->mddev->thread);
|- raid5d()
/* 检查是否需要同步
* 此时在sys接口的调用栈中，try_lock失败直接退出未创建同步线程
*/
\_ md_check_recovery()
2.2 加入新盘
通过命令 mdadm --manage -a /dev/md0 /dev/sde 给块设备加入新盘，新盘加入后自动开始同步。
函数调用关系如下：
md_ioctl()
\_ add_new_disk()
raid5d()
\_ md_check_recovery()
\_ remove_and_add_spares()
\_ raid5_add_disk()
\_ md_register_thread()
\_ md_wakeup_thread(mddev->sync_thread)
md_do_sync()
这里关键函数为 raid5_add_disk() ，在函数内设置了相关rdev的各项标记，这里只说明该函数的相关逻辑，如下：
static int raid5_add_disk(struct mddev *mddev, struct md_rdev *rdev)
{
struct r5conf *conf = mddev->private;
struct disk_info *p;
int first = 0;
int last = conf->raid_disk - 1;
/* 遍历所有磁盘 */
for (disk = first; disk <= last; disk++) {
p = conf->disk + disk;
/* 如果设置了需替换标记且尚未指定新盘 */
if (test_bit(WantReplacement, &rdev->flags) &&
p->replacement == NULL) {
/* 设置磁盘状态为未同步 */
clear_bit(In_sync, &rdev->flags);
/* 设置新盘在md中的磁盘索引 */
rdev->raid_disk = disk;
/* 设置md需要全盘同步 */
config->fullsync = 1;
/* 给replacement指针赋值使其指向新盘 */
rcu_assign_pointer(p->replacement, rdev);
break;
}
}
}
加入新盘后调用 md_do_sync() 会发起同步
2.3 条带处理
在同步函数中，循环调用 sync_request() ，该函数主要逻辑如下：
static inline sector_t sync_request(struct mddev *mddev, sector_t sector_nr, int *skipped, int go_faster)
{
/* 获取一个空闲条带 */
sh = get_active_stripe(conf, sector_nr, 0, 1, 0);
if (sh == NULL) {
sh = get_active_stripe(conf, sector_nr, 0, 0, 0);
schedule_timeout_uninterruptible(1);
}
/* 设置同步标记 */
set_bit(STRIPE_SYNC_REQUESTED, &sh->state);
/* 将条带推入条带状态机处理 */
handle_stripe(sh);
release_stripe(sh);
return STRIPE_SECTORS;
}
2.3.1 下发读请求
函数调用关系：
handle_stripe()
\_ analyse_stripe()
\_ handle_stripe_fill()
\_ fetch_block()
\_ ops_run_io()
代码逻辑如下：
static void handle_stripe(struct stripe_head *sh)
{
/* 在sync_request中设置了该标记 */
if (test_bit(STRIPE_SYNC_REQUESTED, &sh->state)) {
spin_lock(&sh->stripe_lock);
/* 此时条带不是处理DISCARD请求 */
if (!test_bit(STRIPE_DISCARD, &sh->state) &&
/* 清掉STRIPE_SYNC_REQUESTED标记 */
test_and_clear_bit(STRIPE_SYNC_REQUESTED, &sh->state)) {
/* 设置条带同步中标记 */
set_bit(STRIPE_SYNCING, &sh->state);
/* 清除条带一致状态的标记 */
clear_bit(STRIPE_INSYNC, &sh->state);
}
spin_unlock(&sh->stripe_lock);
}
clear_bit(STRIPE_DELAYED, &sh->state);
/* 解析条带状态 */
analyse_stripe(sh, &s);
/* s.replacing为真进入handle_stripe_fill */
if (s.to_read || s.non_overwrite
|| (conf->level == 6 && s.to_write && s.failed)
|| (s.syncing && (s.uptodate + s.compute < disks))
|| s.replacing
|| s.expanding)
handle_stripe_fill(sh, &s, disks);
/* 此时 s.locked == 0 条件不成立不会进入该if分支 */
if (s.replacing && s.locked == 0
&& !test_bit(STRIPE_INSYNC, &sh->state)) {
/* Write out to replacement devices where possible */
for (i = 0; i < conf->raid_disks; i++)
if (test_bit(R5_UPTODATE, &sh->dev[i].flags) &&
test_bit(R5_NeedReplace, &sh->dev[i].flags)) {
set_bit(R5_WantReplace, &sh->dev[i].flags);
set_bit(R5_LOCKED, &sh->dev[i].flags);
s.locked++;
}
set_bit(STRIPE_INSYNC, &sh->state);
}
/* 此时 s.locked == 0 条件不成立不会进入该if分支 */
if ((s.syncing || s.replacing) && s.locked == 0 &&
test_bit(STRIPE_INSYNC, &sh->state)) {
md_done_sync(conf->mddev, STRIPE_SECTORS, 1);
clear_bit(STRIPE_SYNCING, &sh->state);
if (test_and_clear_bit(R5_Overlap, &sh->dev[sh->pd_idx].flags))
wake_up(&conf->wait_for_overlap);
}
/* 下发读请求 */
ops_run_io(sh, &s);
}
static void analyse_stripe(struct stripe_head *sh, struct stripe_head_state *s)
{
int do_recovery = 0;
/* 遍历所有条带/设备 */
rcu_read_lock();
for (i=disks; i--; ) {
/* 加入新盘的成员磁盘replacement存在但不满足
* rdev->recovery_offset >= sh->sector + STRIPE_SECTORS（同步时同步进度小于sh->sector）
* 走到else分支
*/
rdev = rcu_dereference(conf->disks[i].replacement);
if (rdev && !test_bit(Faulty, &rdev->flags) &&
rdev->recovery_offset >= sh->sector + STRIPE_SECTORS &&
!is_badblock(rdev, sh->sector, STRIPE_SECTORS,
&first_bad, &bad_sectors))
set_bit(R5_ReadRepl, &dev->flags);
else {
if (rdev)
/* 设置R5_NeedReplace标记 */
set_bit(R5_NeedReplace, &dev->flags);
rdev = rcu_dereference(conf->disks[i].rdev);
clear_bit(R5_ReadRepl, &dev->flags);
}
/* 在replacement处理中所有硬盘都是正常的，do_recovery为0，s->failed也为0 */
if (!test_bit(R5_Insync, &dev->flags)) {
if (s->failed < 2)
s->failed_num[s->failed] = i;
s->failed++;
if (rdev && !test_bit(Faulty, &rdev->flags))
do_recovery = 1;
}
}
/* 在handle_stripe中设置了该标记 */
if (test_bit(STRIPE_SYNCING, &sh->state)) {
/* 条件都未成立走else分支 */
if (do_recovery ||
sh->sector >= conf->mddev->recovery_cp ||
test_bit(MD_RECOVERY_REQUESTED, &(conf->mddev->recovery)))
s->syncing = 1;
else
s->replacing = 1;
}
rcu_read_unlock();
}
static void handle_stripe_fill(struct stripe_head *sh,
struct stripe_head_state *s,
int disks)
{
int i;
/* 未设置条带状态进入fetch_block */
if (!test_bit(STRIPE_COMPUTE_RUN, &sh->state) && !sh->check_state &&
!sh->reconstruct_state)
for (i = disks; i--; )
if (fetch_block(sh, s, i, disks))
break;
set_bit(STRIPE_HANDLE, &sh->state);
}
static int fetch_block(struct stripe_head *sh, struct stripe_head_state *s,
int disk_idx, int disks)
{
struct r5dev *dev = &sh->dev[disk_idx];
struct r5dev *fdev[2] = { &sh->dev[s->failed_num[0]],
&sh->dev[s->failed_num[1]] };
/* 此时所有条带/设备都未发起请求且未包含最新数据 */
if (!test_bit(R5_LOCKED, &dev->flags) &&
!test_bit(R5_UPTODATE, &dev->flags) &&
(dev->toread ||
(dev->towrite && !test_bit(R5_OVERWRITE, &dev->flags)) ||
s->syncing || s->expanding ||
/* want_replace()函数中会判断disk_idx对应的成员磁盘是否有replacemenet且
* 条带起始位置大于等于replacement重构位置返回1
* 在replacing过程中设置了replacement的成员磁盘进入if
*/
(s->replacing && want_replace(sh, disk_idx)) ||
(s->failed >= 1 && fdev[0]->toread) ||
(s->failed >= 2 && fdev[1]->toread) ||
(sh->raid_conf->level <= 5 && s->failed && fdev[0]->towrite &&
!test_bit(R5_OVERWRITE, &fdev[0]->flags)) ||
(sh->raid_conf->level == 6 && s->failed && s->to_write))) {
/* we would like to get this block, possibly by computing it,
* otherwise read it if the backing disk is insync
*/
BUG_ON(test_bit(R5_Wantcompute, &dev->flags));
BUG_ON(test_bit(R5_Wantread, &dev->flags));
/* 对设置了replacement的成员磁盘下发读请求 */
if (test_bit(R5_Insync, &dev->flags)) {
set_bit(R5_LOCKED, &dev->flags);
set_bit(R5_Wantread, &dev->flags);
/* 自增locked计数 */
s->locked++;
}
}
return 0;
}
static void ops_run_io(struct stripe_head *sh, struct stripe_head_state *s)
{
/* 遍历所有条带/设备 */
for (i = disks; i--; ) {
/* 对设置了读标记的下发读请求 */
if (test_and_clear_bit(R5_Wantread, &sh->dev[i].flags))
rw = READ;
/* 跳过其他不需要读的设备 */
else
continue;
if (rdev) {
bio_reset(bi);
bi->bi_bdev = rdev->bdev;
bi->bi_rw = rw;
bi->bi_end_io = raid5_end_read_request;
bi->bi_private = sh;
atomic_inc(&sh->count);
if (use_new_offset(conf, sh))
bi->bi_sector = (sh->sector + rdev->new_data_offset);
else
bi->bi_sector = (sh->sector + rdev->data_offset);
if (test_bit(R5_ReadNoMerge, &sh->dev[i].flags))
bi->bi_rw |= REQ_FLUSH;
bi->bi_vcnt = 1;
bi->bi_io_vec[0].bv_len = STRIPE_SIZE;
bi->bi_io_vec[0].bv_offset = 0;
bi->bi_size = STRIPE_SIZE;
/* 提交bio */
generic_make_request(bi);
}
}
}
2.3.2 下发写请求
函数调用关系：
handle_stripe()
\_ analyse_stripe()
\_ ops_run_io()
代码逻辑如下：
static void handle_stripe(struct stripe_head *sh)
{
/* 解析条带状态 */
analyse_stripe(sh, &s);
/* 在fetch_block中设置了replacement的条带已经是最新数据
* R5_UPTODATE条件为真，所以不会设置任何新的需要读取
*/
if (s.to_read || s.non_overwrite
|| (conf->level == 6 && s.to_write && s.failed)
|| (s.syncing && (s.uptodate + s.compute < disks))
|| s.replacing
|| s.expanding)
handle_stripe_fill(sh, &s, disks);
/* replacing在analyse_stripe设置
* 未下发新的读请求locked为真
* 条带同步状态再第一轮中清除
* 进入if
*/
if (s.replacing && s.locked == 0
&& !test_bit(STRIPE_INSYNC, &sh->state)) {
for (i = 0; i < conf->raid_disks; i++)
/* 读成功后R5_UPTODATE为真，R5_WantReplace在analyse_stripe中设置 */
if (test_bit(R5_UPTODATE, &sh->dev[i].flags) &&
test_bit(R5_NeedReplace, &sh->dev[i].flags)) {
/* 设置R5_WantReplace标记在下发写请求时判断 */
set_bit(R5_WantReplace, &sh->dev[i].flags);
/* 设置R5_LOCKED标记表明条带/设备准备调度IO */
set_bit(R5_LOCKED, &sh->dev[i].flags);
/* 自增locked计数 */
s.locked++;
}
/* 将条带设置为同步状态 */
set_bit(STRIPE_INSYNC, &sh->state);
}
/* 上个if中locked自增不进入if */
if ((s.syncing || s.replacing) && s.locked == 0 &&
test_bit(STRIPE_INSYNC, &sh->state)) {
md_done_sync(conf->mddev, STRIPE_SECTORS, 1);
clear_bit(STRIPE_SYNCING, &sh->state);
if (test_and_clear_bit(R5_Overlap, &sh->dev[sh->pd_idx].flags))
wake_up(&conf->wait_for_overlap);
}
/* 下发写请求 */
ops_run_io(sh, &s);
}
/* 和上轮一轮不在赘述 */
analyse_stripe()
static void ops_run_io(struct stripe_head *sh, struct stripe_head_state *s)
{
struct r5conf *conf = sh->raid_conf;
int i, disks = sh->disks;
might_sleep();
/* 遍历所有条带/设备 */
for (i = disks; i--; ) {
if (test_and_clear_bit(R5_WantReplace, &sh->dev[i].flags)) {
rw = WRITE;
replace_only = 1;
}
bi = &sh->dev[i].req;
rbi = &sh->dev[i].rreq; /* For writing to replacement */
rcu_read_lock();
rrdev = rcu_dereference(conf->disks[i].replacement);
smp_mb(); /* Ensure that if rrdev is NULL, rdev won't be */
rdev = rcu_dereference(conf->disks[i].rdev);
if (rw & WRITE) {
/* 将rdev标空后只能使用rrdev */
if (replace_only)
rdev = NULL;
}
rcu_read_unlock();
/* 使用rbi向rrdev下发写请求 */
if (rrdev) {
set_bit(STRIPE_IO_STARTED, &sh->state);
bio_reset(rbi);
rbi->bi_bdev = rrdev->bdev;
rbi->bi_rw = rw;
BUG_ON(!(rw & WRITE));
rbi->bi_end_io = raid5_end_write_request;
rbi->bi_private = sh;
atomic_inc(&sh->count);
if (use_new_offset(conf, sh))
rbi->bi_sector = (sh->sector + rrdev->new_data_offset);
else
rbi->bi_sector = (sh->sector + rrdev->data_offset);
rbi->bi_vcnt = 1;
rbi->bi_io_vec[0].bv_len = STRIPE_SIZE;
rbi->bi_io_vec[0].bv_offset = 0;
rbi->bi_size = STRIPE_SIZE;
generic_make_request(rbi);
}
}
}
2.3.3 完成
函数调用关系：
handle_stripe()
\_ md_done_sync()
代码逻辑如下：
static void handle_stripe(struct stripe_head *sh)
{
/* 解析条带状态 */
analyse_stripe(sh, &s);
/* 在fetch_block中设置了replacement的条带已经是最新数据
* R5_UPTODATE条件为真，所以不会设置任何新的需要读取
*/
if (s.to_read || s.non_overwrite
|| (conf->level == 6 && s.to_write && s.failed)
|| (s.syncing && (s.uptodate + s.compute < disks))
|| s.replacing
|| s.expanding)
handle_stripe_fill(sh, &s, disks);
/* replacing在analyse_stripe设置
* 未下发新的读请求locked为真
* STRIPE_INSYNC在上轮处理中设置，本轮不在进入if
*/
if (s.replacing && s.locked == 0
&& !test_bit(STRIPE_INSYNC, &sh->state)) {
/* Write out to replacement devices where possible */
for (i = 0; i < conf->raid_disks; i++)
if (test_bit(R5_UPTODATE, &sh->dev[i].flags) &&
test_bit(R5_NeedReplace, &sh->dev[i].flags)) {
set_bit(R5_WantReplace, &sh->dev[i].flags);
set_bit(R5_LOCKED, &sh->dev[i].flags);
s.locked++;
}
set_bit(STRIPE_INSYNC, &sh->state);
}
/* replacing、locked、STRIPE_INSYNC条件都成立 */
if ((s.syncing || s.replacing) && s.locked == 0 &&
test_bit(STRIPE_INSYNC, &sh->state)) {
md_done_sync(conf->mddev, STRIPE_SECTORS, 1);
clear_bit(STRIPE_SYNCING, &sh->state);
}
}
void md_done_sync(struct mddev *mddev, int blocks, int ok)
{
/* 自增完成计数 */
atomic_sub(blocks, &mddev->recovery_active);
/* 通知同步前程继续进行 */
wake_up(&mddev->recovery_wait);
if (!ok) {
set_bit(MD_RECOVERY_INTR, &mddev->recovery);
set_bit(MD_RECOVERY_ERROR, &mddev->recovery);
md_wakeup_thread(mddev->thread);
}
}
至此一个条带的同步流程分析完毕。
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