[Bug 1806] New: disks stats not kept for DM (device mapper) devices

[Martin J. Bligh]

http://bugme.osdl.org/show_bug.cgi?id=1806

           Summary: disks stats not kept for DM (device mapper) devices
    Kernel Version: 2.6.0
            Status: NEW
          Severity: normal
             Owner: axboe@xxxxxxx
         Submitter: slpratt@xxxxxxxxxx


Distribution:all
Hardware Environment:all
Software Environment:all
Problem Description:
Disk stats as reported through sysfs are empty for all DM (device mapper)
devices.  This appears to be due to the fact that the stats are traced via
request structs which are not generated until below the device mapper layer.  It
seems it would be possible to add code to device mapper to track the stats since
the actual location of the stats is in the gendisk entry which does exsist for
DM deivices.  Only problem I see is in tracking ticks for IO since in the non DM
case this is done by storing a start time in the request struct on driving the
request.  Since DM has no request struct (only the BIO) it has no place to
record the start time.

Steps to reproduce:
create a DM device using dmsetup, lvm2 or EVMS.  Do IO to device, look at
/sys/block/dm-xxx/stat.


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