Re: SEEK_HOLE and SEEK_DATA support?

[Phillip Susi <psusi@cfl.rr.com>]

Aren't there already apis to query for the holes in the file, and 
doesn't tar already use them to efficiently back up sparse files?  I 
seem to remember seeing that somewhere.

Jim Dennis wrote:
>  Perhaps I should have been a bit more clear.  /var/log/lastlog has
>  been a sparse file in most implementation for ... well ... forever.
>
>  The example issue is that the support for large UIDs and the convention
>  of setting nfsnobody to -1 (4294967294) combine to create a file whose
>  size is very large.  The du of the file is (in my case) only about
>  100KiB.  So there's a small cluster of used blocks for the valid
>  corporate UIDs that have ever accessed this machine ... then a huge
>  allocate hole, and then one block storing the lastlog timestamp for
>  nfsnobody.
>
>  However, this message was not intended to dwell on the cause of that
>  huge sparse file ... but rather to inquire as to the core issue; 
>  how do we efficiently handle skipping over (potentially huge)
>  allocation holes in a portable fashion that might be adopted by
>  archiving and other tools?  I provided this example simply to point
>  out that it does happen, in the real world and has a significant
>  cost (40 minutes to scan through NULs with which the filesystem fills
>  the hole for read()s).
>
>  OpenSolaris has implemented a mechanism for doing this and it sounds
>  reasonable from my admittedly superficial perspective.

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