Re: using GPT support in BIOS boot

[Steven Bellovin <smb%cs.columbia.edu@localhost>]

On Jan 9, 2011, at 10:20 11AM, Joerg Sonnenberger wrote:

> On Sun, Jan 09, 2011 at 10:09:18AM -0500, Greg Troxel wrote:
>> I understand why 4KB alignment makes sense (larger physical disk blocks
>> on disks that present 512 byte sectors), and 128KB/256KB/1MB seem
>> broadly sensible, but I would appreciate understanding what bad thing
>> would happen with starting the first GPT partition at:
>> 
>>   34 sectors/17KB    (messes up 4KB alignment, splitting reads/writes)
>>   36 sectors/18KB    (ditto)
>>   40 sectors/20KB    ?
>>   64 sectors/32KB    ?
>>  128 sectors/64KB    ?
> 
> The central idea is that the underlaying device may have a write unit
> that is larger than one sector or 4KB. Both for RAID 5 and SSD this is
> often the case. It will work, but it can easily trash performance.
> 
Is there any way to query the device to learn what size the write unit is?
> 


                --Steve Bellovin, http://www.cs.columbia.edu/~smb