Re: [PATCH v3 1/8] MD: add a new disk role to present cache device

[Neil Brown <neilb@suse.de>]

On Wed, 3 Jun 2015 15:48:36 -0700
Shaohua Li <shli@fb.com> wrote:

> From: Song Liu <songliubraving@fb.com>
> 
> Next patches will use a disk as raid5/6 caching. We need a new disk role
> to present the cache device
> 
> Not sure if we should bump up the MD superblock version for the disk
> role.

No need to increase the superblock version, but you would need to add a
feature flag (for feature_map) which was set whenever the array had a
caching device.

NeilBrown

> 
> Signed-off-by: Song Liu <songliubraving@fb.com>
> Signed-off-by: Shaohua Li <shli@fb.com>
> ---
>  drivers/md/md.c                | 14 +++++++++++++-
>  drivers/md/md.h                |  4 ++++
>  include/uapi/linux/raid/md_p.h |  1 +
>  3 files changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 2750630..6297087 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -1656,6 +1656,9 @@ static int super_1_validate(struct mddev *mddev, struct md_rdev *rdev)
>  		case 0xfffe: /* faulty */
>  			set_bit(Faulty, &rdev->flags);
>  			break;
> +		case 0xfffd: /* cache device */
> +			set_bit(WriteCache, &rdev->flags);
> +			break;
>  		default:
>  			rdev->saved_raid_disk = role;
>  			if ((le32_to_cpu(sb->feature_map) &
> @@ -1811,6 +1814,8 @@ static void super_1_sync(struct mddev *mddev, struct md_rdev *rdev)
>  			sb->dev_roles[i] = cpu_to_le16(0xfffe);
>  		else if (test_bit(In_sync, &rdev2->flags))
>  			sb->dev_roles[i] = cpu_to_le16(rdev2->raid_disk);
> +		else if (test_bit(WriteCache, &rdev2->flags))
> +			sb->dev_roles[i] = cpu_to_le16(0xfffd);
>  		else if (rdev2->raid_disk >= 0)
>  			sb->dev_roles[i] = cpu_to_le16(rdev2->raid_disk);
>  		else
> @@ -5780,7 +5785,8 @@ static int get_disk_info(struct mddev *mddev, void __user * arg)
>  		else if (test_bit(In_sync, &rdev->flags)) {
>  			info.state |= (1<<MD_DISK_ACTIVE);
>  			info.state |= (1<<MD_DISK_SYNC);
> -		}
> +		} else if (test_bit(WriteCache, &rdev->flags))
> +			info.state |= (1<<MD_DISK_WRITECACHE);
>  		if (test_bit(WriteMostly, &rdev->flags))
>  			info.state |= (1<<MD_DISK_WRITEMOSTLY);
>  	} else {
> @@ -5895,6 +5901,8 @@ static int add_new_disk(struct mddev *mddev, mdu_disk_info_t *info)
>  		else
>  			clear_bit(WriteMostly, &rdev->flags);
>  
> +		if (info->state & (1<<MD_DISK_WRITECACHE))
> +			set_bit(WriteCache, &rdev->flags);
>  		/*
>  		 * check whether the device shows up in other nodes
>  		 */
> @@ -7263,6 +7271,10 @@ static int md_seq_show(struct seq_file *seq, void *v)
>  				seq_printf(seq, "(F)");
>  				continue;
>  			}
> +			if (test_bit(WriteCache, &rdev->flags)) {
> +				seq_printf(seq, "(C)");
> +				continue;
> +			}
>  			if (rdev->raid_disk < 0)
>  				seq_printf(seq, "(S)"); /* spare */
>  			if (test_bit(Replacement, &rdev->flags))
> diff --git a/drivers/md/md.h b/drivers/md/md.h
> index 4046a6c..6857592 100644
> --- a/drivers/md/md.h
> +++ b/drivers/md/md.h
> @@ -175,6 +175,10 @@ enum flag_bits {
>  				 * This device is seen locally but not
>  				 * by the whole cluster
>  				 */
> +	WriteCache,		/* This device is used as write cache.
> +				 * Usually, this device should be faster
> +				 * than other devices in the array
> +				 */
>  };
>  
>  #define BB_LEN_MASK	(0x00000000000001FFULL)
> diff --git a/include/uapi/linux/raid/md_p.h b/include/uapi/linux/raid/md_p.h
> index 2ae6131..9d36b91 100644
> --- a/include/uapi/linux/raid/md_p.h
> +++ b/include/uapi/linux/raid/md_p.h
> @@ -89,6 +89,7 @@
>  				   * read requests will only be sent here in
>  				   * dire need
>  				   */
> +#define MD_DISK_WRITECACHE      18 /* disk is used as the write cache in RAID-5/6 */
>  
>  typedef struct mdp_device_descriptor_s {
>  	__u32 number;		/* 0 Device number in the entire set	      */