A good post by Bob Plankers (thank you) – reminds me of a good post by Stephen Foskett
This is post #1 in my December-long series on Linux VM performance tuning,Tuningmas.
I wrote about it back in 2006, and lots of others have written about it since (Duncan Epping has a nice vendor-agnostic post with diagrams): misaligned storage trashing your I/O performance.
What’s the big deal? In short, it is killing your I/O performance. Logical Block Addressing on your disk drive makes the Master Boot Record 63 bytes long. This means it occupies sectors 0-62 on disk, and the first partition will start at sector 63. The number 63 is a persona non grata in the computer world. It isn’t a power of 2, and it certainly doesn’t line up with your storage’s idea of the world (no matter if it’s a local SSD, a RAID controller, or a big enterprise array). The misaligned partition has blocks that straddle the stripes on the array, and instead of reading a single stripe the array has to read from, or write to, two stripes.
This isn’t a big problem on one or two VMs, but when hundreds of VMs have misaligned I/O the effect is crippling. For every I/O operation you do at the OS level, you’re really doing two on the back end. That hurts performance, deduplication, and on SSD disks it reduces lifespan because SSDs have a limited number of writes they can do. Do twice as many writes as you meant to and your SSD lives half as long. Seriously. It also fills your disk cache with twice as much stuff, which means it’ll be half as useful (or less). Read on here
Storage CH Blog 