Specifying the size ranges for I/O histograms

The I/O histogram size ranges are used to categorize the I/O according to the size, in bytes, of the I/O operation.

The size ranges are specified using a string of positive integers separated by semicolons (;). No white space is allowed within the size range operand. Each number represents the upper bound, in bytes, of the I/O request size for that range. The numbers must be monotonically increasing. Each number may be optionally followed by the letters K or k to denote multiplication by 1024, or by the letters M or m to denote multiplication by 1048576 (1024*1024).

For example, the size range operand:

512;1m;4m

represents these four size ranges

0        to     512 bytes
513      to 1048576 bytes
1048577  to 4194304 bytes
4194305 and greater bytes

In this example, a read of size 3 MB would fall in the third size range, a write of size 20 MB would fall in the fourth size range.

A size range operand of = (equal sign) indicates that the current size range is not to be changed. A size range operand of * (asterisk) indicates that the current size range is to be changed to the default size range. A maximum of 15 numbers may be specified, which produces 16 total size ranges.

The default request size ranges are:

0        to      255 bytes
256      to      511 bytes
512      to     1023 bytes
1024     to     2047 bytes
2048     to     4095 bytes
4096     to     8191 bytes
8192     to    16383 bytes
16384    to    32767 bytes
32768    to    65535 bytes
65536    to   131071 bytes
131072   to   262143 bytes
262144   to   524287 bytes
524288   to  1048575 bytes
1048576  to  2097151 bytes
2097152  to  4194303 bytes
4194304  and greater bytes

The last size range collects all request sizes greater than or equal to 4 MB. The request size ranges can be changed by using the rhist nr request.

For more information, see Processing of rhist nr.