Re: impact of 4k sector size on the IO & FS stack

[Jeff Garzik <jeff@garzik.org>]

Jan Engelhardt wrote:
> On Mar 11 2007 22:45, Ric Wheeler wrote:
>> Jan Engelhardt wrote:
>>> On Mar 11 2007 18:51, Ric Wheeler wrote:
>>>
>>>> During the recent IO/FS workshop, we spoke briefly about the
>>>> coming change to a 4k sector size for disks on linux. If I
>>>> recall correctly, the general feeling was that the impact was
>>>> not significant since we already do most file system IO in 4k
>>>> page sizes and should be fine as long as we partition drives
>>>> correctly and avoid non-4k aligned partitions.
>>>>
>>> Sorry about jumping right in, but what about an 'old-style'
>>> partition table that relies on 512 as a unit?
>>>
>>>
>> I think that the normal case would involve new drives which
>> would need to be partitioned in 4k aligned partitions.
>> Shouldn't that work regardless of the unit used in the
>> partition table?
> 
> Assume this partition table on my current HD:
> 
> 	Disk /dev/hdc: 251.0 GB, 251000193024 bytes
> 	255 heads, 63 sectors/track, 30515 cylinders
> 	Units = cylinders of 16065 * 512 = 8225280 bytes
> 	
> 	   Device Start  End      Blocks   Id  System
> 	/dev/hdc1   1     33      265041   82  Linux swap / Solaris
> 	/dev/hdc2  34  30515   244846665    5  Extended
> 
> That is, 255 * 63 * 30515 * 512 == roughly 251 GB.
> 
> Now, if this disk was copied byte per byte (/bin/dd) to a
> 4096-based disk, and Linux would start using a sector size of
> 4096, then I would suddenly have
> 
> 255 * 63 * 30515 * 4096 == 2 TB
> 
> Although I would not mind the 2 TB, the partition table would
> read quite differently (note the Blocks column which is
> multiplied by 4 (512x4=4096))

At this level, for RMW drives, nothing changes.  The partition software, 
ATA driver, and all other bits continue to think that sector size == 512 
bytes.

The partition software /hopefully/ becomes smart enough to understand 
the alignment necessary, but that is not a requirement.

This is the key to understanding the difference between a physical 
(==platters) sector size change without a logical (==ATA interface) 
sector size change.


>            Device Start  End      Blocks   Id  System
>         /dev/hdc1   1     33     1060164   82  Linux swap / Solaris
>         /dev/hdc2  34  30515   979386660    5  Extended
> 
> Which would mean that the swap partition reaches into the real
> data partition and would corrupt it.

For RMW drives, RMW cycles would occur but not corruption.

For non-RMW drives, this just wouldn't occur.

	Jeff