3fc3dc1b_dmsetup___Unix__Linux_Command__Description
[Tag] --readahead [+]|auto|none [Description] 
Specify read ahead size in units of sectors. The default value is "auto" which allows the kernel to choose a suitable value automatically. The + prefix lets you specify a minimum value which will not be used if it is smaller than the value chosen by the kernel. "None" is equivalent to specifying zero.
dmsetup - Unix, Linux Command