RAID1 physical stripe placement

Two different physical stripe allocation polices have been tried out on a RAID1 device. One policy allocates successive physical stripes for a logical RAID1 stripe, if possible. Another policy allocates physical stripes of a logical stripe separated be some distance, if possible. The performance of TSS under the two policies is summarized in Table 2.

The results show that allocating the physical stripes apart performs better. This can be due to the fact that, with physical stripes away from one another, the successive logical stripes can be successive physical stripes also. Due to this the clustering can happen across the stripes at the lower level drivers (SCSI) both during reads and writes.

All of the following trace runs have been taken with two RAID1 stripes allocated half-way across on the physical devices.

Table 2: Effect of physical stripe placement on RAID1 device TSS (trace data from 6 [/var])

Type of	Type of	Avg. time
Device	I/O	per block (ms)
		cont.	fixed dist.
		placement	placement
RAID1	READ	5.42	4.94
	WRITE	8.79	8.45
	Avg. I/O	7.43	7.04