git: kernel - disklabel64 - Increase partition start alignment to 1 megabyte.

Matthew Dillon dillon at
Sat Feb 20 10:18:36 PST 2010

commit 4921cba1f62ca52955e529f9b60fa2fd23821d80
Author: Matthew Dillon <dillon at>
Date:   Sat Feb 20 10:11:28 2010 -0800

    kernel - disklabel64 - Increase partition start alignment to 1 megabyte.
    * Someone suggested that instead of using a 32K alignment we use a larger
      alignment.  I forgot who suggested it but after thinking about it a bit
      and messing around with swapcache on a SSD I decided it was a good idea.
      SSDs using MLC flash have a physical block size of 128K.  SLC flash has
      a physical block size of 64K.  Most typical cluster operations in
      DragonFly are 64K to 128K but clustered writes are often linear on disk
      leading to larger linear writes from the point of view of the physical
      drive's write cache.
      swapcache and swap operation tends to have even larger write linearities
      and write amplification effects on SSDs can be reduced to better than 1:2
      (verses the 1:10 the vendor typically assumes).
    * Virgin disklabel64's layed down by the kernel will now align the
      start of the partition space to 1MB (1024 * 1024).  In for a penny,
      in for a pound.
    * Adjust the manual page and note the benefits of using a larger alignment,
      particularly when swapcache is used with SSDs.

Summary of changes:
 sbin/disklabel64/disklabel64.8 |   24 ++++++++++++++++++++++++
 sys/kern/subr_disklabel64.c    |   15 ++++++++++++++-
 2 files changed, 38 insertions(+), 1 deletions(-)

DragonFly BSD source repository

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