You want to set readahead to tune the performance of you disk reads and you find that in your server there are several levels of devices, block devices, RAID devices, then LVM with device-mapper, etc.
You can set the readahead in any of them, which one is the right one?
I came up with this Stackoverflow question: https://serverfault.com/questions/418352/readahead-settings-for-lvm-device-mapper-software-raid-and-block-devices-wha
And i decided to do some tests to prove what wojciechz was saying, and he is right, let me show you:
My setup is a server with RAID10 and LVM with a /db partition mounted on the logical volume
# cat /proc/mdstat Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] md127 : active raid10 sdj sdi sdh sdg sdf sde sdd sdc 3906525184 blocks super 1.2 512K chunks 2 near-copies [8/8] [UUUUUUUU] # pvdisplay --- Physical volume --- PV Name /dev/md127 VG Name vg1 PV Size 3.64 TiB / not usable 0 Allocatable yes PE Size 4.00 MiB Total PE 953741 Free PE 489746 Allocated PE 463995 PV UUID KH4RjS-lgAN-2OdI-hiYQ-HuR1-naDM-nSmc5S # mount | grep db /dev/mapper/vg1-db on /db type ext4 (rw,noatime,nodiratime,discard,stripe=512,data=ordered)