[RFC PATCH] raid5: Add R5_ReadNoMerge flag which prevent bio from merging at block layer

[RFC PATCH] raid5: Add R5_ReadNoMerge flag which prevent bio from merging at block layer

[majianpeng]

Because bios will merge at block-layer,so bios-error may caused by other
bio which be merged into to the same request.
Using this flag,it will find exactly error-sector and not do redundant
operation like re-write and re-read.

Signed-off-by: majianpeng <majianpeng@gmail.com>
---
 block/blk-core.c   |    8 ++++++++
 drivers/md/raid5.c |   16 +++++++++++++---
 drivers/md/raid5.h |    1 +
 3 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 3c923a7..ee04bfc 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1401,6 +1401,9 @@ void init_request_from_bio(struct request *req, struct bio *bio)
 	if (bio->bi_rw & REQ_RAHEAD)
 		req->cmd_flags |= REQ_FAILFAST_MASK;
 
+	if (unlikely(bio->bi_rw & REQ_NOMERGE))
+		req->cmd_flags |= REQ_NOMERGE;
+
 	req->errors = 0;
 	req->__sector = bio->bi_sector;
 	req->ioprio = bio_prio(bio);
@@ -1428,6 +1431,11 @@ void blk_queue_bio(struct request_queue *q, struct bio *bio)
 		goto get_rq;
 	}
 
+	if (unlikely(bio->bi_rw & REQ_NOMERGE)) {
+		spin_lock_irq(q->queue_lock);
+		where = ELEVATOR_INSERT_BACK;
+		goto get_rq;
+	}
 	/*
 	 * Check if we can merge with the plugged list before grabbing
 	 * any locks.
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index d267672..04f78d2 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -632,6 +632,9 @@ static void ops_run_io(struct stripe_head *sh, struct stripe_head_state *s)
 			else
 				bi->bi_sector = (sh->sector
 						 + rdev->data_offset);
+			if (test_bit(R5_ReadNoMerge, &sh->dev[i].flags))
+				bi->bi_rw |= REQ_NOMERGE;
+
 			bi->bi_flags = 1 << BIO_UPTODATE;
 			bi->bi_idx = 0;
 			bi->bi_io_vec[0].bv_len = STRIPE_SIZE;
@@ -1731,7 +1734,9 @@ static void raid5_end_read_request(struct bio * bi, int error)
 			atomic_add(STRIPE_SECTORS, &rdev->corrected_errors);
 			clear_bit(R5_ReadError, &sh->dev[i].flags);
 			clear_bit(R5_ReWrite, &sh->dev[i].flags);
-		}
+		} else if (test_bit(R5_ReadNoMerge, &sh->dev[i].flags))
+			clear_bit(R5_ReadNoMerge, &sh->dev[i].flags);
+
 		if (atomic_read(&rdev->read_errors))
 			atomic_set(&rdev->read_errors, 0);
 	} else {
@@ -1773,7 +1778,11 @@ static void raid5_end_read_request(struct bio * bi, int error)
 		else
 			retry = 1;
 		if (retry)
-			set_bit(R5_ReadError, &sh->dev[i].flags);
+			if (test_bit(R5_ReadNoMerge, &sh->dev[i].flags)) {
+				set_bit(R5_ReadError, &sh->dev[i].flags);
+				clear_bit(R5_ReadNoMerge, &sh->dev[i].flags);
+			} else
+				set_bit(R5_ReadNoMerge, &sh->dev[i].flags);
 		else {
 			clear_bit(R5_ReadError, &sh->dev[i].flags);
 			clear_bit(R5_ReWrite, &sh->dev[i].flags);
@@ -4481,7 +4490,8 @@ static int  retry_aligned_read(struct r5conf *conf, struct bio *raid_bio)
 			conf->retry_read_aligned = raid_bio;
 			return handled;
 		}
-
+		if (likely(raid_bio->bi_size >> 9) > STRIPE_SECTORS)
+			set_bit(R5_ReadNoMerge, &sh->dev[dd_idx].flags);
 		handle_stripe(sh);
 		release_stripe(sh);
 		handled++;
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index 2164021..6767d07 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -273,6 +273,7 @@ enum r5dev_flags {
 	R5_Wantwrite,
 	R5_Overlap,	/* There is a pending overlapping request
 			 * on this block */
+	R5_ReadNoMerge, /* prevent bio from merging in block-layer */
 	R5_ReadError,	/* seen a read error here recently */
 	R5_ReWrite,	/* have tried to over-write the readerror */
 
-- 
1.7.5.4

Re: [RFC PATCH] raid5: Add R5_ReadNoMerge flag which prevent bio from merging at block layer

[NeilBrown]

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On Tue, 3 Jul 2012 14:23:41 +0800 majianpeng <majianpeng@gmail.com> wrote:

> Because bios will merge at block-layer,so bios-error may caused by other
> bio which be merged into to the same request.
> Using this flag,it will find exactly error-sector and not do redundant
> operation like re-write and re-read.
> 
> Signed-off-by: majianpeng <majianpeng@gmail.com>

Hi,
 I think this patch needs a more detailed explanation.

What exactly is the situation that causes a problem, and what exactly is the
problem that it causes?

Pretend that I don't know anything about what happens below the md level..

Thanks,
NeilBrown


> ---
>  block/blk-core.c   |    8 ++++++++
>  drivers/md/raid5.c |   16 +++++++++++++---
>  drivers/md/raid5.h |    1 +
>  3 files changed, 22 insertions(+), 3 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 3c923a7..ee04bfc 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1401,6 +1401,9 @@ void init_request_from_bio(struct request *req, struct bio *bio)
>  	if (bio->bi_rw & REQ_RAHEAD)
>  		req->cmd_flags |= REQ_FAILFAST_MASK;
>  
> +	if (unlikely(bio->bi_rw & REQ_NOMERGE))
> +		req->cmd_flags |= REQ_NOMERGE;
> +
>  	req->errors = 0;
>  	req->__sector = bio->bi_sector;
>  	req->ioprio = bio_prio(bio);
> @@ -1428,6 +1431,11 @@ void blk_queue_bio(struct request_queue *q, struct bio *bio)
>  		goto get_rq;
>  	}
>  
> +	if (unlikely(bio->bi_rw & REQ_NOMERGE)) {
> +		spin_lock_irq(q->queue_lock);
> +		where = ELEVATOR_INSERT_BACK;
> +		goto get_rq;
> +	}
>  	/*
>  	 * Check if we can merge with the plugged list before grabbing
>  	 * any locks.
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index d267672..04f78d2 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -632,6 +632,9 @@ static void ops_run_io(struct stripe_head *sh, struct stripe_head_state *s)
>  			else
>  				bi->bi_sector = (sh->sector
>  						 + rdev->data_offset);
> +			if (test_bit(R5_ReadNoMerge, &sh->dev[i].flags))
> +				bi->bi_rw |= REQ_NOMERGE;
> +
>  			bi->bi_flags = 1 << BIO_UPTODATE;
>  			bi->bi_idx = 0;
>  			bi->bi_io_vec[0].bv_len = STRIPE_SIZE;
> @@ -1731,7 +1734,9 @@ static void raid5_end_read_request(struct bio * bi, int error)
>  			atomic_add(STRIPE_SECTORS, &rdev->corrected_errors);
>  			clear_bit(R5_ReadError, &sh->dev[i].flags);
>  			clear_bit(R5_ReWrite, &sh->dev[i].flags);
> -		}
> +		} else if (test_bit(R5_ReadNoMerge, &sh->dev[i].flags))
> +			clear_bit(R5_ReadNoMerge, &sh->dev[i].flags);
> +
>  		if (atomic_read(&rdev->read_errors))
>  			atomic_set(&rdev->read_errors, 0);
>  	} else {
> @@ -1773,7 +1778,11 @@ static void raid5_end_read_request(struct bio * bi, int error)
>  		else
>  			retry = 1;
>  		if (retry)
> -			set_bit(R5_ReadError, &sh->dev[i].flags);
> +			if (test_bit(R5_ReadNoMerge, &sh->dev[i].flags)) {
> +				set_bit(R5_ReadError, &sh->dev[i].flags);
> +				clear_bit(R5_ReadNoMerge, &sh->dev[i].flags);
> +			} else
> +				set_bit(R5_ReadNoMerge, &sh->dev[i].flags);
>  		else {
>  			clear_bit(R5_ReadError, &sh->dev[i].flags);
>  			clear_bit(R5_ReWrite, &sh->dev[i].flags);
> @@ -4481,7 +4490,8 @@ static int  retry_aligned_read(struct r5conf *conf, struct bio *raid_bio)
>  			conf->retry_read_aligned = raid_bio;
>  			return handled;
>  		}
> -
> +		if (likely(raid_bio->bi_size >> 9) > STRIPE_SECTORS)
> +			set_bit(R5_ReadNoMerge, &sh->dev[dd_idx].flags);
>  		handle_stripe(sh);
>  		release_stripe(sh);
>  		handled++;
> diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
> index 2164021..6767d07 100644
> --- a/drivers/md/raid5.h
> +++ b/drivers/md/raid5.h
> @@ -273,6 +273,7 @@ enum r5dev_flags {
>  	R5_Wantwrite,
>  	R5_Overlap,	/* There is a pending overlapping request
>  			 * on this block */
> +	R5_ReadNoMerge, /* prevent bio from merging in block-layer */
>  	R5_ReadError,	/* seen a read error here recently */
>  	R5_ReWrite,	/* have tried to over-write the readerror */
>  


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Re: Re: [RFC PATCH] raid5: Add R5_ReadNoMerge flag which prevent bio from merging at block layer

[majianpeng]

On 2012-07-03 14:38 NeilBrown <neilb@suse.de> Wrote:
>On Tue, 3 Jul 2012 14:23:41 +0800 majianpeng <majianpeng@gmail.com> wrote:
>
>> Because bios will merge at block-layer,so bios-error may caused by other
>> bio which be merged into to the same request.
>> Using this flag,it will find exactly error-sector and not do redundant
>> operation like re-write and re-read.
>> 
>> Signed-off-by: majianpeng <majianpeng@gmail.com>
>
>Hi,
> I think this patch needs a more detailed explanation.
>
>What exactly is the situation that causes a problem, and what exactly is the
>problem that it causes?
>
>Pretend that I don't know anything about what happens below the md level..
>
>Thanks,
>NeilBrown
>
>
>> ---
>>  block/blk-core.c   |    8 ++++++++
>>  drivers/md/raid5.c |   16 +++++++++++++---
>>  drivers/md/raid5.h |    1 +
>>  3 files changed, 22 insertions(+), 3 deletions(-)
>> 
>> diff --git a/block/blk-core.c b/block/blk-core.c
>> index 3c923a7..ee04bfc 100644
>> --- a/block/blk-core.c
>> +++ b/block/blk-core.c
>> @@ -1401,6 +1401,9 @@ void init_request_from_bio(struct request *req, struct bio *bio)
>>  	if (bio->bi_rw & REQ_RAHEAD)
>>  		req->cmd_flags |= REQ_FAILFAST_MASK;
>>  
>> +	if (unlikely(bio->bi_rw & REQ_NOMERGE))
>> +		req->cmd_flags |= REQ_NOMERGE;
>> +
>>  	req->errors = 0;
>>  	req->__sector = bio->bi_sector;
>>  	req->ioprio = bio_prio(bio);
>> @@ -1428,6 +1431,11 @@ void blk_queue_bio(struct request_queue *q, struct bio *bio)
>>  		goto get_rq;
>>  	}
>>  
>> +	if (unlikely(bio->bi_rw & REQ_NOMERGE)) {
>> +		spin_lock_irq(q->queue_lock);
>> +		where = ELEVATOR_INSERT_BACK;
>> +		goto get_rq;
>> +	}
>>  	/*
>>  	 * Check if we can merge with the plugged list before grabbing
>>  	 * any locks.
>> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
>> index d267672..04f78d2 100644
>> --- a/drivers/md/raid5.c
>> +++ b/drivers/md/raid5.c
>> @@ -632,6 +632,9 @@ static void ops_run_io(struct stripe_head *sh, struct stripe_head_state *s)
>>  			else
>>  				bi->bi_sector = (sh->sector
>>  						 + rdev->data_offset);
>> +			if (test_bit(R5_ReadNoMerge, &sh->dev[i].flags))
>> +				bi->bi_rw |= REQ_NOMERGE;
>> +
>>  			bi->bi_flags = 1 << BIO_UPTODATE;
>>  			bi->bi_idx = 0;
>>  			bi->bi_io_vec[0].bv_len = STRIPE_SIZE;
>> @@ -1731,7 +1734,9 @@ static void raid5_end_read_request(struct bio * bi, int error)
>>  			atomic_add(STRIPE_SECTORS, &rdev->corrected_errors);
>>  			clear_bit(R5_ReadError, &sh->dev[i].flags);
>>  			clear_bit(R5_ReWrite, &sh->dev[i].flags);
>> -		}
>> +		} else if (test_bit(R5_ReadNoMerge, &sh->dev[i].flags))
>> +			clear_bit(R5_ReadNoMerge, &sh->dev[i].flags);
>> +
>>  		if (atomic_read(&rdev->read_errors))
>>  			atomic_set(&rdev->read_errors, 0);
>>  	} else {
>> @@ -1773,7 +1778,11 @@ static void raid5_end_read_request(struct bio * bi, int error)
>>  		else
>>  			retry = 1;
>>  		if (retry)
>> -			set_bit(R5_ReadError, &sh->dev[i].flags);
>> +			if (test_bit(R5_ReadNoMerge, &sh->dev[i].flags)) {
>> +				set_bit(R5_ReadError, &sh->dev[i].flags);
>> +				clear_bit(R5_ReadNoMerge, &sh->dev[i].flags);
>> +			} else
>> +				set_bit(R5_ReadNoMerge, &sh->dev[i].flags);
>>  		else {
>>  			clear_bit(R5_ReadError, &sh->dev[i].flags);
>>  			clear_bit(R5_ReWrite, &sh->dev[i].flags);
>> @@ -4481,7 +4490,8 @@ static int  retry_aligned_read(struct r5conf *conf, struct bio *raid_bio)
>>  			conf->retry_read_aligned = raid_bio;
>>  			return handled;
>>  		}
>> -
>> +		if (likely(raid_bio->bi_size >> 9) > STRIPE_SECTORS)
>> +			set_bit(R5_ReadNoMerge, &sh->dev[dd_idx].flags);
>>  		handle_stripe(sh);
>>  		release_stripe(sh);
>>  		handled++;
>> diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
>> index 2164021..6767d07 100644
>> --- a/drivers/md/raid5.h
>> +++ b/drivers/md/raid5.h
>> @@ -273,6 +273,7 @@ enum r5dev_flags {
>>  	R5_Wantwrite,
>>  	R5_Overlap,	/* There is a pending overlapping request
>>  			 * on this block */
>> +	R5_ReadNoMerge, /* prevent bio from merging in block-layer */
>>  	R5_ReadError,	/* seen a read error here recently */
>>  	R5_ReWrite,	/* have tried to over-write the readerror */
>>  
>
>
How about the below explanation:

Because bio will be merged at block-layer,so bios-error may caused by other
bio which be merged into to the same request.
For example: if chunk_aligned_read failed, it will add bio to some stipe.
But because the bio-merge function,those bios at most be merged.It will like
the chunk_aligned_read and returen error.Then it will re-write and re-read.
Suppose RAID5 created by n disk and chunk-size is 512K.
If read 512k chunk_aligned and met error(sector 0 is media error), 
then add 512/4=128 stipe.If those bios merged and must be error.The rewrite operation
will read (n-1) * 128 and computer 128 stripe.But using this flag,we only exec one rewrite.
This occur in resync/repair situation and chunk-aligned-read.

May be using this flag,we can find exact bad-sector.Not recorded the whole request bios 
as bad-sector.

Re: [RFC PATCH] raid5: Add R5_ReadNoMerge flag which prevent bio from merging at block layer

[NeilBrown]

[-- Attachment #1: Type: text/plain, Size: 5714 bytes --]

On Tue, 3 Jul 2012 16:52:49 +0800 majianpeng <majianpeng@gmail.com> wrote:

> On 2012-07-03 14:38 NeilBrown <neilb@suse.de> Wrote:
> >On Tue, 3 Jul 2012 14:23:41 +0800 majianpeng <majianpeng@gmail.com> wrote:
> >
> >> Because bios will merge at block-layer,so bios-error may caused by other
> >> bio which be merged into to the same request.
> >> Using this flag,it will find exactly error-sector and not do redundant
> >> operation like re-write and re-read.
> >> 
> >> Signed-off-by: majianpeng <majianpeng@gmail.com>
> >
> >Hi,
> > I think this patch needs a more detailed explanation.
> >
> >What exactly is the situation that causes a problem, and what exactly is the
> >problem that it causes?
> >
> >Pretend that I don't know anything about what happens below the md level..
> >
> >Thanks,
> >NeilBrown
> >
> >
> >> ---
> >>  block/blk-core.c   |    8 ++++++++
> >>  drivers/md/raid5.c |   16 +++++++++++++---
> >>  drivers/md/raid5.h |    1 +
> >>  3 files changed, 22 insertions(+), 3 deletions(-)
> >> 
> >> diff --git a/block/blk-core.c b/block/blk-core.c
> >> index 3c923a7..ee04bfc 100644
> >> --- a/block/blk-core.c
> >> +++ b/block/blk-core.c
> >> @@ -1401,6 +1401,9 @@ void init_request_from_bio(struct request *req, struct bio *bio)
> >>  	if (bio->bi_rw & REQ_RAHEAD)
> >>  		req->cmd_flags |= REQ_FAILFAST_MASK;
> >>  
> >> +	if (unlikely(bio->bi_rw & REQ_NOMERGE))
> >> +		req->cmd_flags |= REQ_NOMERGE;
> >> +
> >>  	req->errors = 0;
> >>  	req->__sector = bio->bi_sector;
> >>  	req->ioprio = bio_prio(bio);
> >> @@ -1428,6 +1431,11 @@ void blk_queue_bio(struct request_queue *q, struct bio *bio)
> >>  		goto get_rq;
> >>  	}
> >>  
> >> +	if (unlikely(bio->bi_rw & REQ_NOMERGE)) {
> >> +		spin_lock_irq(q->queue_lock);
> >> +		where = ELEVATOR_INSERT_BACK;
> >> +		goto get_rq;
> >> +	}
> >>  	/*
> >>  	 * Check if we can merge with the plugged list before grabbing
> >>  	 * any locks.
> >> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> >> index d267672..04f78d2 100644
> >> --- a/drivers/md/raid5.c
> >> +++ b/drivers/md/raid5.c
> >> @@ -632,6 +632,9 @@ static void ops_run_io(struct stripe_head *sh, struct stripe_head_state *s)
> >>  			else
> >>  				bi->bi_sector = (sh->sector
> >>  						 + rdev->data_offset);
> >> +			if (test_bit(R5_ReadNoMerge, &sh->dev[i].flags))
> >> +				bi->bi_rw |= REQ_NOMERGE;
> >> +
> >>  			bi->bi_flags = 1 << BIO_UPTODATE;
> >>  			bi->bi_idx = 0;
> >>  			bi->bi_io_vec[0].bv_len = STRIPE_SIZE;
> >> @@ -1731,7 +1734,9 @@ static void raid5_end_read_request(struct bio * bi, int error)
> >>  			atomic_add(STRIPE_SECTORS, &rdev->corrected_errors);
> >>  			clear_bit(R5_ReadError, &sh->dev[i].flags);
> >>  			clear_bit(R5_ReWrite, &sh->dev[i].flags);
> >> -		}
> >> +		} else if (test_bit(R5_ReadNoMerge, &sh->dev[i].flags))
> >> +			clear_bit(R5_ReadNoMerge, &sh->dev[i].flags);
> >> +
> >>  		if (atomic_read(&rdev->read_errors))
> >>  			atomic_set(&rdev->read_errors, 0);
> >>  	} else {
> >> @@ -1773,7 +1778,11 @@ static void raid5_end_read_request(struct bio * bi, int error)
> >>  		else
> >>  			retry = 1;
> >>  		if (retry)
> >> -			set_bit(R5_ReadError, &sh->dev[i].flags);
> >> +			if (test_bit(R5_ReadNoMerge, &sh->dev[i].flags)) {
> >> +				set_bit(R5_ReadError, &sh->dev[i].flags);
> >> +				clear_bit(R5_ReadNoMerge, &sh->dev[i].flags);
> >> +			} else
> >> +				set_bit(R5_ReadNoMerge, &sh->dev[i].flags);
> >>  		else {
> >>  			clear_bit(R5_ReadError, &sh->dev[i].flags);
> >>  			clear_bit(R5_ReWrite, &sh->dev[i].flags);
> >> @@ -4481,7 +4490,8 @@ static int  retry_aligned_read(struct r5conf *conf, struct bio *raid_bio)
> >>  			conf->retry_read_aligned = raid_bio;
> >>  			return handled;
> >>  		}
> >> -
> >> +		if (likely(raid_bio->bi_size >> 9) > STRIPE_SECTORS)
> >> +			set_bit(R5_ReadNoMerge, &sh->dev[dd_idx].flags);
> >>  		handle_stripe(sh);
> >>  		release_stripe(sh);
> >>  		handled++;
> >> diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
> >> index 2164021..6767d07 100644
> >> --- a/drivers/md/raid5.h
> >> +++ b/drivers/md/raid5.h
> >> @@ -273,6 +273,7 @@ enum r5dev_flags {
> >>  	R5_Wantwrite,
> >>  	R5_Overlap,	/* There is a pending overlapping request
> >>  			 * on this block */
> >> +	R5_ReadNoMerge, /* prevent bio from merging in block-layer */
> >>  	R5_ReadError,	/* seen a read error here recently */
> >>  	R5_ReWrite,	/* have tried to over-write the readerror */
> >>  
> >
> >
> How about the below explanation:
> 
> Because bio will be merged at block-layer,so bios-error may caused by other
> bio which be merged into to the same request.
> For example: if chunk_aligned_read failed, it will add bio to some stipe.
> But because the bio-merge function,those bios at most be merged.It will like
> the chunk_aligned_read and returen error.Then it will re-write and re-read.
> Suppose RAID5 created by n disk and chunk-size is 512K.
> If read 512k chunk_aligned and met error(sector 0 is media error), 
> then add 512/4=128 stipe.If those bios merged and must be error.The rewrite operation
> will read (n-1) * 128 and computer 128 stripe.But using this flag,we only exec one rewrite.
> This occur in resync/repair situation and chunk-aligned-read.
> 
> May be using this flag,we can find exact bad-sector.Not recorded the whole request bios 
> as bad-sector

Thanks.  That make some sense.

I wonder if we can just use REQ_FUA to stop requests being merged, rather
than create a new flag.  In general we should probably be using REQ_FUA in
all the cases where we are checking for, and trying to fix, read errors.

Thanks,
NeilBrown


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Re: Re: [RFC PATCH] raid5: Add R5_ReadNoMerge flag which prevent bio from merging at block layer

[majianpeng]

On 2012-07-04 10:30 NeilBrown <neilb@suse.de> Wrote:
>On Tue, 3 Jul 2012 16:52:49 +0800 majianpeng <majianpeng@gmail.com> wrote:
>
>> On 2012-07-03 14:38 NeilBrown <neilb@suse.de> Wrote:
>> >On Tue, 3 Jul 2012 14:23:41 +0800 majianpeng <majianpeng@gmail.com> wrote:
>> >
>> >> Because bios will merge at block-layer,so bios-error may caused by other
>> >> bio which be merged into to the same request.
>> >> Using this flag,it will find exactly error-sector and not do redundant
>> >> operation like re-write and re-read.
>> >> 
>> >> Signed-off-by: majianpeng <majianpeng@gmail.com>
>> >
>> >Hi,
>> > I think this patch needs a more detailed explanation.
>> >
>> >What exactly is the situation that causes a problem, and what exactly is the
>> >problem that it causes?
>> >
>> >Pretend that I don't know anything about what happens below the md level..
>> >
>> >Thanks,
>> >NeilBrown
>> >
>> >
....
>> >>  
>> >
>> >
>> How about the below explanation:
>> 
>> Because bio will be merged at block-layer,so bios-error may caused by other
>> bio which be merged into to the same request.
>> For example: if chunk_aligned_read failed, it will add bio to some stipe.
>> But because the bio-merge function,those bios at most be merged.It will like
>> the chunk_aligned_read and returen error.Then it will re-write and re-read.
>> Suppose RAID5 created by n disk and chunk-size is 512K.
>> If read 512k chunk_aligned and met error(sector 0 is media error), 
>> then add 512/4=128 stipe.If those bios merged and must be error.The rewrite operation
>> will read (n-1) * 128 and computer 128 stripe.But using this flag,we only exec one rewrite.
>> This occur in resync/repair situation and chunk-aligned-read.
>> 
>> May be using this flag,we can find exact bad-sector.Not recorded the whole request bios 
>> as bad-sector
>
>Thanks.  That make some sense.
>
>I wonder if we can just use REQ_FUA to stop requests being merged, rather
>than create a new flag.  In general we should probably be using REQ_FUA in
>all the cases where we are checking for, and trying to fix, read errors.
>
>Thanks,
>NeilBrown
>
>
Using REQ_FUA/REQ_FLUSH can have same effect.
But i think REQ_FUA/REQ_FLUSH operation do more work for example, pre_flush/post_pflush and the harddisk will do more
work.But our purpose only exec bio slonely.