Function call chain __btrfs_map_block()->find_live_mirror() uses thread pid to determine the %mirror_num when the mirror_num=0.
This patch introduces a framework so that we can add policies to determine the %mirror_num. And also adds the devid as the readmirror policy. The new property is stored as an item in the device tree as show below. (BTRFS_READMIRROR_OBJECTID, BTRFS_PERSISTENT_ITEM_KEY, devid) To be able to set and get this new property also introduces new ioctls BTRFS_IOC_GET_READMIRROR and BTRFS_IOC_SET_READMIRROR. The ioctl argument is defined as struct btrfs_ioctl_readmirror_args { __u64 type; /* RW */ __u64 device_bitmap; /* RW */ } An usage example as follows: btrfs property set /btrfs readmirror devid:1,3 btrfs property get /btrfs readmirror readmirror devid:1 3 btrfs property set /btrfs readmirror "" btrfs property get /btrfs readmirror readmirror default This patchset has been tested completely, however marked as RFC for the following reasons and comments on them (or any other) are appreciated as usual. . The new objectid is defined as #define BTRFS_READMIRROR_OBJECTID -1ULL Need consent we are fine to use this value, and with this value it shall be placed just before the DEV_STATS_OBJECTID item which is more frequently used only during the device errors. . I am using a u64 bitmap to represent the devices id, so the max device id that we could represent is 63, its a kind of limitation which should be addressed before integration, I wonder if there is any suggestion? Kindly note that, multiple ioctls with each time representing a set of device(s) is not a choice because we need to make sure the readmirror changes happens in a commit transaction. v1->RFC v2: . Property is stored as a dev-tree item instead of root inode extended attribute. . Rename BTRFS_DEV_STATE_READ_OPRIMIZED to BTRFS_DEV_STATE_READ_PREFERRED. . Changed format specifier from devid1,2,3.. to devid:1,2,3.. RFC->v1: Drops pid as one of the readmirror policy choices and as usual remains as default. And when the devid is reset the readmirror policy falls back to pid. Drops the mount -o readmirror idea, it can be added at a later point of time. Property now accepts more than 1 devid as readmirror device. As shown in the example above. Anand Jain (1): btrfs: add readmirror framework and policy devid fs/btrfs/ctree.h | 3 + fs/btrfs/disk-io.c | 9 ++ fs/btrfs/ioctl.c | 108 ++++++++++++++++++++++++ fs/btrfs/transaction.c | 3 + fs/btrfs/volumes.c | 145 +++++++++++++++++++++++++++++++- fs/btrfs/volumes.h | 9 +- include/uapi/linux/btrfs.h | 15 +++- include/uapi/linux/btrfs_tree.h | 11 +++ 8 files changed, 300 insertions(+), 3 deletions(-) Anand Jain (1): btrfs_progs: add readmirror property and ioctl to set get ioctl.h | 14 +++++++++ props.c | 98 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 112 insertions(+)