[PATCH] Btrfs: disable FUA if mounted with nobarrier

[PATCH] Btrfs: disable FUA if mounted with nobarrier

[Omar Sandoval]

From: Omar Sandoval <osandov@fb.com>

I was seeing disk flushes still happening when I mounted a Btrfs
filesystem with nobarrier for testing. This is because we use FUA to
write out the first super block, and on devices without FUA support, the
block layer translates FUA to a flush. Even on devices supporting true
FUA, using FUA when we asked for no barriers is surprising.

Signed-off-by: Omar Sandoval <osandov@fb.com>
---
 fs/btrfs/disk-io.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 10a2a579cc7f..a8ecccfc36de 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3231,6 +3231,7 @@ static int write_dev_supers(struct btrfs_device *device,
 	int errors = 0;
 	u32 crc;
 	u64 bytenr;
+	int op_flags;
 
 	if (max_mirrors == 0)
 		max_mirrors = BTRFS_SUPER_MIRROR_MAX;
@@ -3273,13 +3274,10 @@ static int write_dev_supers(struct btrfs_device *device,
 		 * we fua the first super.  The others we allow
 		 * to go down lazy.
 		 */
-		if (i == 0) {
-			ret = btrfsic_submit_bh(REQ_OP_WRITE,
-				REQ_SYNC | REQ_FUA | REQ_META | REQ_PRIO, bh);
-		} else {
-			ret = btrfsic_submit_bh(REQ_OP_WRITE,
-				REQ_SYNC | REQ_META | REQ_PRIO, bh);
-		}
+		op_flags = REQ_SYNC | REQ_META | REQ_PRIO;
+		if (i == 0 && !btrfs_test_opt(device->fs_info, NOBARRIER))
+			op_flags |= REQ_FUA;
+		ret = btrfsic_submit_bh(REQ_OP_WRITE, op_flags, bh);
 		if (ret)
 			errors++;
 	}
-- 
2.15.1

Re: [PATCH] Btrfs: disable FUA if mounted with nobarrier

[Qu Wenruo]

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On 2017年12月06日 14:54, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> I was seeing disk flushes still happening when I mounted a Btrfs
> filesystem with nobarrier for testing. This is because we use FUA to
> write out the first super block, and on devices without FUA support, the
> block layer translates FUA to a flush. Even on devices supporting true
> FUA, using FUA when we asked for no barriers is surprising.
> 
> Signed-off-by: Omar Sandoval <osandov@fb.com>

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu

> ---
>  fs/btrfs/disk-io.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 10a2a579cc7f..a8ecccfc36de 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -3231,6 +3231,7 @@ static int write_dev_supers(struct btrfs_device *device,
>  	int errors = 0;
>  	u32 crc;
>  	u64 bytenr;
> +	int op_flags;
>  
>  	if (max_mirrors == 0)
>  		max_mirrors = BTRFS_SUPER_MIRROR_MAX;
> @@ -3273,13 +3274,10 @@ static int write_dev_supers(struct btrfs_device *device,
>  		 * we fua the first super.  The others we allow
>  		 * to go down lazy.
>  		 */
> -		if (i == 0) {
> -			ret = btrfsic_submit_bh(REQ_OP_WRITE,
> -				REQ_SYNC | REQ_FUA | REQ_META | REQ_PRIO, bh);
> -		} else {
> -			ret = btrfsic_submit_bh(REQ_OP_WRITE,
> -				REQ_SYNC | REQ_META | REQ_PRIO, bh);
> -		}
> +		op_flags = REQ_SYNC | REQ_META | REQ_PRIO;
> +		if (i == 0 && !btrfs_test_opt(device->fs_info, NOBARRIER))
> +			op_flags |= REQ_FUA;
> +		ret = btrfsic_submit_bh(REQ_OP_WRITE, op_flags, bh);
>  		if (ret)
>  			errors++;
>  	}
> 


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Re: [PATCH] Btrfs: disable FUA if mounted with nobarrier

[David Sterba]

On Tue, Dec 05, 2017 at 10:54:02PM -0800, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> I was seeing disk flushes still happening when I mounted a Btrfs
> filesystem with nobarrier for testing. This is because we use FUA to
> write out the first super block, and on devices without FUA support, the
> block layer translates FUA to a flush. Even on devices supporting true
> FUA, using FUA when we asked for no barriers is surprising.
> 
> Signed-off-by: Omar Sandoval <osandov@fb.com>

Reviewed-by: David Sterba <dsterba@suse.com>

The documented nobarrier behaviour matches the updated code.