Strage buffer behaviour

[Tino Keitel <tino.keitel@gmx.de>]

Hi folks,

I noticed that the kernel (2.6.23.1) seems to buffer only certain
partitions on my system:

$ for i in 1 2 3 4 ; do for j in 1 2 ; do dd if=/dev/sda$i of=/dev/null
bs=1024k count=100 ; done ; done
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 3.01471 seconds, 34.8 MB/s
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 2.86945 seconds, 36.5 MB/s
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 2.92038 seconds, 35.9 MB/s
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 3.00272 seconds, 34.9 MB/s
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 4.07722 seconds, 25.7 MB/s
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 0.0944248 seconds, 1.1 GB/s
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 5.61527 seconds, 18.7 MB/s
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 0.331822 seconds, 316 MB/s

The dd command reads 100 MB from each partition two times in a row. It
looks like sda1 and sda2 are not bufferd (the first 4 dd runs), but
sda3 and sda4 are (the last 4 dd runs).

The computer is a Mac mini with a 2,5" SATA hard disk. The first 2
partitions contain EFI and MacOS X, and are unused in Linux. The last 2
partitions are an ext3 partition for / and an LVM for the rest of the
sytem.

Any hints how the dd/buffering behaviour could be explained? The system
was mostly idle, and the numbers are reproducible across reboots.

Regards,
Tino