Re: 4Kn drives, allocation block size
Re: 4Kn drives, allocation block size
- Subject: Re: 4Kn drives, allocation block size
- From: Chris Murphy <email@hidden>
- Date: Fri, 15 Mar 2013 12:35:51 -0600
On Mar 15, 2013, at 11:44 AM, Mark Day <email@hidden> wrote:
> Ok, this time I went at looked at the source (available at www.opensource.com). ;-) It's in the diskdev_cmds project, in the file newfs_hfs.tproj/newfs_hfs.c; look for the array named alloc_blocksize.
>
> Under 2TiB, the default is 4096 bytes. From 2TiB up to (but not including) 8TiB, the default is 8192 bytes. From 8TiB up to 16TiB, the default is 16384 bytes; it scales linearly above here (for example, 512TiB up to 1024TiB defaults to 1024KiB = 1MiB allocation block size).
OK thanks.
It seems universally SSDs are fibbing about their physical sector size (page size), which are either 4KB or 8KB apparently but is reported as 512 bytes like a conventional non-AF HDD. Is the 8KB page size and alignment for an SSD is relevant? If it is, the Disk Utility default first partition start LBA of 40 seems not ideal. Further I'm not sure if alignment to the erase block size is more ideal or irrelevant for SSDs, but as far as I know this isn't specified anywhere by any manufacturer anyway.
Other partitioning tools seem to get around the question by defaulting to LBA 2048 for the start of the 1st partition.
For 4Kn HDDs reporting 4096 logical and physical sector sizes, the Disk Utility first partition start LBA is 6 instead of 40. That's both 4KB and 8KB aligned, but I don't think we're anticipating 4096 byte logical sector SSDs, are we?
Chris Murphy
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