There was a deliberate race condition in dm_stat_for_entry() to avoid the
overhead of disabling and enabling interrupts. The race could result in
some events not being counted on 64-bit architectures.
However, on 32-bit architectures, operations on long long variables are
not atomic, so the race condition could cause the counter to jump by 2^32.
Such jumps could be disruptive, so we need to do proper locking on 32-bit
architectures.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: Alasdair G. Kergon <agk@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Support the collection of I/O statistics on user-defined regions of
a DM device. If no regions are defined no statistics are collected so
there isn't any performance impact. Only bio-based DM devices are
currently supported.
Each user-defined region specifies a starting sector, length and step.
Individual statistics will be collected for each step-sized area within
the range specified.
The I/O statistics counters for each step-sized area of a region are
in the same format as /sys/block/*/stat or /proc/diskstats but extra
counters (12 and 13) are provided: total time spent reading and
writing in milliseconds. All these counters may be accessed by sending
the @stats_print message to the appropriate DM device via dmsetup.
The creation of DM statistics will allocate memory via kmalloc or
fallback to using vmalloc space. At most, 1/4 of the overall system
memory may be allocated by DM statistics. The admin can see how much
memory is used by reading
/sys/module/dm_mod/parameters/stats_current_allocated_bytes
See Documentation/device-mapper/statistics.txt for more details.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
