[parted-devel] [rfc] SSD partition alignment

[Jim Meyering]

Daniel J Blueman wrote:
> On Fri, Jan 23, 2009 at 1:56 PM, Colin Watson <cjwatson at ubuntu.com> wrote:
>> On Fri, Jan 23, 2009 at 12:04:33AM +0000, Daniel J Blueman wrote:
>>> In order to maximise user experience thus performance, we need to
>>> ensure the disk partitions created on SSDs/USB flash drives/RAID
>>> arrays are 4KB (perhaps up to 128KB) aligned. There are considerable
>>> benefits with the flash/RAID controller opening half as many
>>> pages/stripes on small reads. This pays better with slower SSDs in eg
>>> netbooks.
>>>
>>> Patches have entered the upstream kernel to detect when drives are
>>> solid-state [1], though the dust hasn't settled on the interface.
>>>
>>> How acceptable would getting the userspace partitioner changes into
>>> Jaunty if proven to be stable and robust?
>>
>> This seems like something that could be added as a libparted constraint
>> without *too* much pain. If and only if the corresponding kernel changes
>> go into the Ubuntu kernel (work with kernel-team at lists.ubuntu.com, I
>> think), I agree that we should change the partitioner too, and pass the
>> changes to parted upstream, although I would add that the changes need
>> to be reasonably simple and elegant as well as stable and robust; I only
>> like partitioner code that I can understand. :-)

...and accompanied by thorough tests.

> 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.