Misaligned partitions (and filesystems) cause increased i/o.

See eg 3Par doc:
http://h20195.www2.hp.com/V2/GetPDF.aspx%2F4AA4-4519ENW.pdf
"All RAID devices, including HP 3PAR arrays, incur the overhead of calculating new parity after a write. A single write that spans two RAID sets will require two parity calculations. Aligning Linux disk partitions to the HP 3PAR RAID set reduces the frequency of RAID parity calculation, resulting in improved performance.
"Best-practice example: Linux fdisk partitioning
Figure 3 (below) shows how a 1 MiB Oracle write spans three HP 3PAR StoreServ RAID sets because the Linux partition is offset by 31.5 KiB. The result of this misalignment is that one Oracle write requires three RAID sets to recalculate parity.
"In figure 4, the Linux partition has been created at an offset equal to the size of one RAID set (512 KiB). This aligns the Oracle writes to the RAID set boundary—resulting in only two parity recalculations—which can improve random write performance by as muchans two RAID sets will require two parity calculations. Aligning Linux disk partitions to the HP 3PAR RAID set reduces the frequency of RAID parity calculation, resulting in improved performance.
"Best-practice example: Linux fdisk partitioning as 20 percent."

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