[dm-devel] [PATCH v4 1/2] md raid0/linear: Mark array as 'broken' and fail BIOs if a member is gone

[Song Liu]

> On Sep 3, 2019, at 12:49 PM, Guilherme G. Piccoli <gpiccoli at canonical.com> wrote:
> 
> 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 at fb.com>
> Reviewed-by: NeilBrown <neilb at suse.de>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli at canonical.com>

Applied to md-next. 

Thanks!