Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
tjh.dev
kernel
os
linux
Documentation/iostats.txt: bit-size reference etc.
- correction that disk stats values are native-word-sized
32-bit or 64-bit values, not always 32-bi values
- drop "Last modified" entry; use git for that
- fix a few typos
- change "cpu" to "CPU"
Reported-by: Linda Walsh <lkml@tlinx.org>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
authored by Randy Dunlap and committed by Jens Axboe 9d2e157d c4ade94f
+8
-9
Documentation/iostats.txt
+8
-9
Documentation/iostats.txt
···
1
I/O statistics fields
2
---------------
3
4
-
Last modified Sep 30, 2003
5
-
6
Since 2.4.20 (and some versions before, with patches), and 2.5.45,
7
more extensive disk statistics have been introduced to help measure disk
8
activity. Tools such as sar and iostat typically interpret these and do
···
44
By contrast, in 2.6 if you look at /sys/block/hda/stat, you'll
45
find just the eleven fields, beginning with 446216. If you look at
46
/proc/diskstats, the eleven fields will be preceded by the major and
47
-
minor device numbers, and device name. Each of these formats provide
48
eleven fields of statistics, each meaning exactly the same things.
49
All fields except field 9 are cumulative since boot. Field 9 should
50
-
go to zero as I/Os complete; all others only increase. Yes, these are
51
-
32 bit unsigned numbers, and on a very busy or long-lived system they
52
may wrap. Applications should be prepared to deal with that; unless
53
your observations are measured in large numbers of minutes or hours,
54
they should not wrap twice before you notice them.
···
95
read I/Os issued per partition should equal those made to the disks ...
96
but due to the lack of locking it may only be very close.
97
98
-
In 2.6, there are counters for each cpu, which made the lack of locking
99
-
almost a non-issue. When the statistics are read, the per-cpu counters
100
-
are summed (possibly overflowing the unsigned 32-bit variable they are
101
summed to) and the result given to the user. There is no convenient
102
-
user interface for accessing the per-cpu counters themselves.
103
104
Disks vs Partitions
105
-------------------
···
1
I/O statistics fields
2
---------------
3
4
Since 2.4.20 (and some versions before, with patches), and 2.5.45,
5
more extensive disk statistics have been introduced to help measure disk
6
activity. Tools such as sar and iostat typically interpret these and do
···
46
By contrast, in 2.6 if you look at /sys/block/hda/stat, you'll
47
find just the eleven fields, beginning with 446216. If you look at
48
/proc/diskstats, the eleven fields will be preceded by the major and
49
+
minor device numbers, and device name. Each of these formats provides
50
eleven fields of statistics, each meaning exactly the same things.
51
All fields except field 9 are cumulative since boot. Field 9 should
52
+
go to zero as I/Os complete; all others only increase (unless they
53
+
overflow and wrap). Yes, these are (32-bit or 64-bit) unsigned long
54
+
(native word size) numbers, and on a very busy or long-lived system they
55
may wrap. Applications should be prepared to deal with that; unless
56
your observations are measured in large numbers of minutes or hours,
57
they should not wrap twice before you notice them.
···
96
read I/Os issued per partition should equal those made to the disks ...
97
but due to the lack of locking it may only be very close.
98
99
+
In 2.6, there are counters for each CPU, which make the lack of locking
100
+
almost a non-issue. When the statistics are read, the per-CPU counters
101
+
are summed (possibly overflowing the unsigned long variable they are
102
summed to) and the result given to the user. There is no convenient
103
+
user interface for accessing the per-CPU counters themselves.
104
105
Disks vs Partitions
106
-------------------