Re: [RFC PATCH v2 0/9] Block/XFS: Support alternative mirror device retry

["jianchao.wang" <jianchao.w.wang@oracle.com>]

On 2/18/19 4:08 PM, jianchao.wang wrote:
> Hi Bob
> 
> On 2/13/19 5:50 PM, Bob Liu wrote:
>> Motivation:
>> When fs data/metadata checksum mismatch, lower block devices may have other
>> correct copies. e.g. If XFS successfully reads a metadata buffer off a raid1 but
>> decides that the metadata is garbage, today it will shut down the entire
>> filesystem without trying any of the other mirrors.  This is a severe
>> loss of service, and we propose these patches to have XFS try harder to
>> avoid failure.
>>
>> This patch prototype this mirror retry idea by:
>> * Adding @nr_mirrors to struct request_queue which is similar as
>>   blk_queue_nonrot(), filesystem can grab device request queue and check max
>>   mirrors this block device has.
>>   Helper functions were also added to get/set the nr_mirrors.
>>
>> * Introducing bi_rd_hint just like bi_write_hint, but bi_rd_hint is a long bitmap
>> in order to support stacked layer case.
> 
> Why does we need a bitmap to know which underlying device has been tried ?
> For example, the following scenario,
> 
>                     md8
>                    / | \
>                sda sdb sdc
> 
> If the the raid read the data from sda and fs check and find the data is corrupted.
> Then we may just need to let raid1 know that the data is from sda. Then based on this
> hint, raid1 could handle it with handle_read_error to try other replica and fix the
> error.

This doesn't work.
The md raid1 can only see IO success or failure, so fix_read_error won't fix this.
Sorry for the noise.

Thanks
Jianchao

> 
> If this is feasible, we just need to modify the bio as following and needn't add any
> bytes in it.
> 
> struct bio {
>     ...
>     union {
>         unsigned short bi_write_hint;
>         unsigned short bi_read_hint;
>     }
>     ...
> }
> 
> Thanks
> Jianchao
>>
>> * Modify md/raid1 to support this retry feature.
>>
>> * Adapter xfs to use this feature.
>>   If the read verify fails, we loop over the available mirrors and retry the read.
>>
>> * Rewrite retried read
>>   When the read verification fails, but the retry succeedes
>>   write the buffer back to correct the bad mirror
>>
>> * Add tracepoints and logging to alternate device retry.
>>   This patch adds new log entries and trace points to the alternate device retry
>>   error path.
>>
>> Changes v2:
>> - No more reuse bi_write_hint
>> - Stacked layer support(see patch 4/9)
>> - Other feedback fix
>>
>> Allison Henderson (5):
>>   Add b_alt_retry to xfs_buf
>>   xfs: Add b_rd_hint to xfs_buf
>>   xfs: Add device retry
>>   xfs: Rewrite retried read
>>   xfs: Add tracepoints and logging to alternate device retry
>>
>> Bob Liu (4):
>>   block: add nr_mirrors to request_queue
>>   block: add rd_hint to bio and request
>>   md:raid1: set mirrors correctly
>>   md:raid1: rd_hint support and consider stacked layer case
>>
>>  Documentation/block/biodoc.txt |   3 +
>>  block/bio.c                    |   1 +
>>  block/blk-core.c               |   4 ++
>>  block/blk-merge.c              |   6 ++
>>  block/blk-settings.c           |  24 +++++++
>>  block/bounce.c                 |   1 +
>>  drivers/md/raid1.c             | 123 ++++++++++++++++++++++++++++++++-
>>  fs/xfs/xfs_buf.c               |  58 +++++++++++++++-
>>  fs/xfs/xfs_buf.h               |  14 ++++
>>  fs/xfs/xfs_trace.h             |   6 +-
>>  include/linux/blk_types.h      |   1 +
>>  include/linux/blkdev.h         |   4 ++
>>  include/linux/types.h          |   3 +
>>  13 files changed, 244 insertions(+), 4 deletions(-)
>>
>