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Date: Thu, 24 Aug 2006 15:53:04 -0500
From: Eric Sandeen <sandeen@sandeen.net>
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Subject: Re: how to understand allocsp
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To: mingz@ele.uri.edu
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Ming Zhang wrote:
> Hi All
> 
> Play with xfs_io and feel confused about this allocsp.
> 
> Starting with a empty file.
> 

I honestly never keep this all straight without going back to the code, 
but I'd start by looking at the comments for, and code in, 
xfs_change_file_space and xfs_alloc_file_space.

         /*
          * XFS_IOC_RESVSP and XFS_IOC_UNRESVSP will reserve or unreserve
          * file space.
          * These calls do NOT zero the data space allocated to the file,
          * nor do they change the file size.
          *
          * XFS_IOC_ALLOCSP and XFS_IOC_FREESP will allocate and free file
          * space.
          * These calls cause the new file data to be zeroed and the file
          * size to be changed.
          */

The semantics of all this could be clearer, IMHO.

-Eric