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    Btrfs: introduce GET_READ_MIRRORS functionality for btrfs_map_block() · 29a8d9a0
    Stefan Behrens 提交于
    Before this commit, btrfs_map_block() was called with REQ_WRITE
    in order to retrieve the list of mirrors for a disk block.
    This needs to be changed for the device replace procedure since
    it makes a difference whether you are asking for read mirrors
    or for locations to write to.
    GET_READ_MIRRORS is introduced as a new interface to call
    btrfs_map_block().
    In the current commit, the functionality is not yet changed,
    only the interface for GET_READ_MIRRORS is introduced and all
    the places that should use this new interface are adapted.
    
    The reason that REQ_WRITE cannot be abused anymore to retrieve
    a list of read mirrors is that during a running dev replace
    operation all write requests to the live filesystem are
    duplicated to also write to the target drive.
    Keep in mind that the target disk is only partially a valid
    copy of the source disk while the operation is ongoing. All
    writes go to the target disk, but not all reads would return
    valid data on the target disk. Therefore it is not possible
    anymore to abuse a REQ_WRITE interface to find valid mirrors
    for a REQ_READ.
    Signed-off-by: NStefan Behrens <sbehrens@giantdisaster.de>
    Signed-off-by: NChris Mason <chris.mason@fusionio.com>
    29a8d9a0
ctree.h 119.3 KB