On Tue, Jun 16, 2015 at 01:20:37PM +0800, Tom Yan wrote: > The thing is, why any io/transfer size/length should be considered > when it comes to partition alignment? From what I understand, > partition alignment is only to make sure partition starts at physical > boundaries of the disk because of the mismatch between logicial sector > (512 bytes) and physical sectors (4096 bytes) or pages/erase blocks of > SSDs. It's more complicated, the I/O limits are the most important for RAIDs where optimal I/O size is usually stripe size and you want to use it for partitions alignment for better performance (if you align to sector size then read/write on RAID maybe performed on more disks on unaligned partitions). And it's not only fdisk who cares, it's also important for mkfs.<type> (for example XFS align according to I/O limits). And because all this is mess and sometimes HW does not provide relevant information and because people use dd(1) to copy partition tables we have decided to use 1MiB granularity if possible. If 1MiB is useless then we use optimal_io_size, if undefined then minimal_io_size and if undefined then sector_size. http://people.redhat.com/msnitzer/docs/io-limits.txt Unfortunately the current code does not check if optimal_io_size makes sense, so thing like 33553920 for 4k device is blindly accepted ;-( Karel -- Karel Zak <kzak@xxxxxxxxxx> http://karelzak.blogspot.com -- To unsubscribe from this list: send the line "unsubscribe util-linux" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
