Re: 4Kn drives, allocation block size
Re: 4Kn drives, allocation block size
- Subject: Re: 4Kn drives, allocation block size
- From: Mark Day <email@hidden>
- Date: Fri, 15 Mar 2013 10:44:18 -0700
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).
-Mark
On Mar 15, 2013, at 10:23 AM, Chris Murphy <email@hidden> wrote:
>
> On Mar 15, 2013, at 11:15 AM, Mark Day <email@hidden> wrote:
>>
>> For volumes in the gigabyte range, the default allocation block size is 4KiB. As you get to terabytes and larger, we start to increase the default allocation block size. I think it was in Mountain Lion that we adjusted the algorithm used by newfs_hfs to determine the default allocation block size. We wanted to pick an allocation block size such that the volume could still grow its total size without running out of allocation blocks (so that if the user had partitioned a large drive, and then decided to merge the partitions, we could simply grow the file system and continue to use the same allocation block size). Off hand, I'm not positive about that growth factor; I think it was 4X.
>
> OK so for a 3TB 4Kn drive, it might be a 16KB allocation block size?
>
> Apparently there are now 4096 logical/4096 physical drives appearing in the wild.
>
>
> Chris Murphy
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