Re: [PATCH 3/4] btrfs: introduce new read_policy device

[Josef Bacik <josef@toxicpanda.com>]

On 10/28/20 9:26 AM, Anand Jain wrote:
> Read-policy type 'device' and device flag 'read-preferred':
> 
> The read-policy type device picks the device(s) flagged as
> read-preferred for reading chunks of type raid1, raid10,
> raid1c3 and raid1c4.
> 
> A system might contain SSD, nvme, iscsi or san lun, and which are all
> a non-rotational device, so it is not a good idea to set the read-preferred
> automatically. Instead device read-policy along with the read-preferred
> flag provides an ability to do it manually. This advance tuning is
> useful in more than one situation, for example,
>   - In heterogeneous-disk volume, it provides an ability to manually choose
>      the low latency disks for reading.
>   - Useful for more accurate testing.
>   - Avoid known problematic device from reading the chunk until it is
>     replaced (by marking the other good devices as read-preferred).
> 
> Note:
> 
> If the read-policy type is set to 'device', but there isn't any device
> which is flagged as read-preferred, then stripe 0 is used for reading.
> 
> The device replace won't migrate the read-preferred flag to the new
> replace the target device.
> 
> As of now, this is an in-memory only feature.
> 
> It's pointless to set the read-preferred flag on the missing device,
> as IOs aren't submitted to the missing device.
> 
> If there is more than one read-preferred device in a chunk, the read IO
> shall go to the stripe 0 (as of now, when depth patches are integrated
> we will use the least busy device among the read-preferred devices).
> 
> Usage example:
> 
> Consider a typical two disks raid1.
> 
> Configure devid1 for reading.
> 
> $ echo 1 > devinfo/1/read_preferred
> $ cat devinfo/1/read_preferred; cat devinfo/2/read_preferred
> 1
> 0
> 
> $ pwd
> /sys/fs/btrfs/12345678-1234-1234-1234-123456789abc
> 
> $ cat read_policy; echo device > ./read_policy; cat read_policy
> [pid] device
> pid [device]
> 
> Now read IOs are sent to devid 1 (sdb).
> 
> $ echo 3 > /proc/sys/vm/drop_caches; md5sum /btrfs/YkZI
> 
> $ iostat -zy 1 | egrep 'sdb|sdc' (from another terminal)
> sdb              50.00     40048.00         0.00      40048          0
> 
> Change the read-preferred device from devid 1 to devid 2 (sdc).
> 
> $ echo 0 > ./devinfo/1/read_preferred; echo 1 > ./devinfo/2/read_preferred;
> 
> [ 3343.918658] BTRFS info (device sdb): reset read preferred on devid 1 (1334)
> [ 3343.919876] BTRFS info (device sdb): set read preferred on devid 2 (1334)
> 
> Further read ios are sent to devid 2 (sdc).
> 
> $ echo 3 > /proc/sys/vm/drop_caches; md5sum /btrfs/YkZI
> 
> $ iostat -zy 1 | egrep 'sdb|sdc' (from another terminal)
> sdc              49.00     40048.00         0.00      40048          0
> 
> Signed-off-by: Anand Jain <anand.jain@oracle.com>
> ---
>   fs/btrfs/sysfs.c   |  3 ++-
>   fs/btrfs/volumes.c | 22 ++++++++++++++++++++++
>   fs/btrfs/volumes.h |  1 +
>   3 files changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
> index 52b4c9bef673..d2a974e1a1c4 100644
> --- a/fs/btrfs/sysfs.c
> +++ b/fs/btrfs/sysfs.c
> @@ -907,7 +907,8 @@ static bool btrfs_strmatch(const char *given, const char *golden)
>   }
>   
>   /* Must follow the order as in enum btrfs_read_policy */
> -static const char * const btrfs_read_policy_name[] = { "pid", "latency" };
> +static const char * const btrfs_read_policy_name[] = { "pid", "latency",
> +						       "device" };
>   
>   static ssize_t btrfs_read_policy_show(struct kobject *kobj,
>   				      struct kobj_attribute *a, char *buf)
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 48587009b656..7ac675504051 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -5502,6 +5502,25 @@ static int btrfs_find_best_stripe(struct btrfs_fs_info *fs_info,
>   	return best_stripe;
>   }
>   
> +static int btrfs_find_read_preferred(struct map_lookup *map, int first, int num_stripe)
> +{
> +	int stripe_index;
> +	int last = first + num_stripe;
> +
> +	/*
> +	 * If there are more than one read preferred devices, then just pick the
> +	 * first found read preferred device as of now.
> +	 */
> +	for (stripe_index = first; stripe_index < last; stripe_index++) {
> +		if (test_bit(BTRFS_DEV_STATE_READ_PREFERRED,
> +			     &map->stripes[stripe_index].dev->dev_state))
> +			return stripe_index;
> +        }

b4 isn't working for me because these patches didn't make it to linux-btrfs 
proper for some reason, so I could be wrong here, but it looks like this } is 
off?  There's spaces here instead of a tab maybe?  If not then just ignore me. 
Thanks,

Josef