Re: HFS and FAT FS-resizing returning soon (but to a separate library)

[Chris Murphy]

On Jan 24, 2012, at 2:26 PM, Chris Murphy wrote:
> 
> So something has changed, and the jhdr_size, can apparently be 4096 bytes on 
> a 512 byte sector disk. Again, I don't know at what point Disk Utility 
> chooses to go from a 512 byte to 4096 byte journal header, but it seems it's 
> related to volume size. Not sector size.

An Apple engineer is reporting to an inquiry I posted on this discrepancy. He's 
saying that for any disks formatted in the past ~5-7 years, they will have a 
4096 byte jhdr_size if the volume is 1+TB. And a 512 byte jhdr_size if less 
than 1TB.

For volumes formatted with the current (~7 month old) Disk Utility in Lion, 
a.k.a. 10.7.x, the jhdr_size should be equal to physical sector size even on 
volumes greater than 1TB, due to code changes they've made in diskutil.

It's reasonably safe to say Apple laptops will have jhdr_size of 512 since all 
of their disks are less than 1TB in size. But for desktop computers, I estimate 
it is much more likely to find 1+TB disks that will have an jhdr_size of 4096 
bytes yet physical sectors of 512 bytes, and therefore not be resizable with 
the current sizing code.

Full text of the post is available here:
http://lists.apple.com/archives/filesystem-dev/2012/Feb/msg00001.html


Chris