• Guilherme G. Piccoli's avatar
    md raid0/linear: Mark array as 'broken' and fail BIOs if a member is gone · 62f7b198
    Guilherme G. Piccoli authored
    Currently md raid0/linear are not provided with any mechanism to validate
    if an array member got removed or failed. The driver keeps sending BIOs
    regardless of the state of array members, and kernel shows state 'clean'
    in the 'array_state' sysfs attribute. This leads to the following
    situation: if a raid0/linear array member is removed and the array is
    mounted, some user writing to this array won't realize that errors are
    happening unless they check dmesg or perform one fsync per written file.
    Despite udev signaling the member device is gone, 'mdadm' cannot issue the
    STOP_ARRAY ioctl successfully, given the array is mounted.
    
    In other words, no -EIO is returned and writes (except direct ones) appear
    normal. Meaning the user might think the wrote data is correctly stored in
    the array, but instead garbage was written given that raid0 does stripping
    (and so, it requires all its members to be working in order to not corrupt
    data). For md/linear, writes to the available members will work fine, but
    if the writes go to the missing member(s), it'll cause a file corruption
    situation, whereas the portion of the writes to the missing devices aren't
    written effectively.
    
    This patch changes this behavior: we check if the block device's gendisk
    is UP when submitting the BIO to the array member, and if it isn't, we flag
    the md device as MD_BROKEN and fail subsequent I/Os to that device; a read
    request to the array requiring data from a valid member is still completed.
    While flagging the device as MD_BROKEN, we also show a rate-limited warning
    in the kernel log.
    
    A new array state 'broken' was added too: it mimics the state 'clean' in
    every aspect, being useful only to distinguish if the array has some member
    missing. We rely on the MD_BROKEN flag to put the array in the 'broken'
    state. This state cannot be written in 'array_state' as it just shows
    one or more members of the array are missing but acts like 'clean', it
    wouldn't make sense to write it.
    
    With this patch, the filesystem reacts much faster to the event of missing
    array member: after some I/O errors, ext4 for instance aborts the journal
    and prevents corruption. Without this change, we're able to keep writing
    in the disk and after a machine reboot, e2fsck shows some severe fs errors
    that demand fixing. This patch was tested in ext4 and xfs filesystems, and
    requires a 'mdadm' counterpart to handle the 'broken' state.
    
    Cc: Song Liu <songliubraving@fb.com>
    Reviewed-by: default avatarNeilBrown <neilb@suse.de>
    Signed-off-by: default avatarGuilherme G. Piccoli <gpiccoli@canonical.com>
    Signed-off-by: default avatarSong Liu <songliubraving@fb.com>
    62f7b198
md-linear.c 8.67 KB