[parted-devel] [rfc] SSD partition alignment

[Colin Watson]

On Sun, Feb 22, 2009 at 01:40:16AM +0100, Jim Meyering wrote:
> Daniel J Blueman wrote:
> > I've checked into this, and since libparted sees the SATA block device
> > as SCSI, it doesn't perform the expected ATA 'identify' command to
> > fill out the 512 bytes of device info, of which (short) word 217 is
> > device RPM, defined to be 1 on newer compliant SSDs. The kernel uses
> > this word to detect if a device is an SSD or not, so I suggest we use
> > the same.
> >
> > Anyone think of objections to calling the ATA identify ioctl to fill
> > out the structure, then storing this flat for later use in constraint
> > checking? If the SCSI device supports it also, fine, else nothing
> > lost.
> >
> > For now, a 1MB starting offset for an SSD seems safest, and is what MS
> > Windows 7 and Server 2008 use, thus a number of vendors will also be
> > testing/optimising with this case too.
> 
> Does this really need to be SSD-specific?
> 
> I hear that this (alignment) is high priority also for many
> of the big new disks, since they have 4k-byte sectors.
> Without better alignment, their performance will suffer, too.

Well, one step at a time. We can detect SSD; can we detect those big new
disks (or, in general, the desired sector size)?

Or are you saying that we should increase the alignment to 4KB in
general? It seems that SSDs actually really want 128KB, which starts to
feel like a bit much to apply to all disks:

  http://thunk.org/tytso/blog/2009/02/20/aligning-filesystems-to-an-ssds-erase-block-size/

-- 
Colin Watson                                       [cjwatson at ubuntu.com]