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August 2008
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2008.08.25 06:28 "Re: creating sparse files......", by Andrey Kiselev
Roger,
My primary concern about this issue is a portability. I am not sure this
approach is totally portable even across the POSIX-compatible systems
(and there are multiple levels of compatibility, that is what is the
certification for). Also I think the compression will better results
overall. Fast methods, such as LZW and Packbits should be great for this
particular purpose.
Despite of that it is possible to add such a functionality to libtiff.
It can be user-controlled using the TIFFOpen() parameter, just like we
are controlling memory-mapped files usage. If not enabled this code
path should not be used at all. On some systems it should be disabled
during the compile time, just like it is for memory-mapping. Our
documentation states:
M Enable the use of memory-mapped files for images opened read-
only. If the underlying system does not support memory-mapped
files or if the specific image being opened cannot be memory-
mapped then the library will fallback to using the normal system
interface for reading information. By default the library will
attempt to use memory-mapped files.
We can use the same approach with the sparse files, though it should be
disabled by default.
Roger, if you like to see this in libtiff, please, prepare the patch and
submit it via our Bugzilla. But I would like to see it tested under
various conditions, the FAT and network file systems are mandatory, I see
there is a small likelihood of accidental data loss.
The flag 's' ('sparse') seems to be free for usage in TIFFOpen().
Best regards,
Andrey
--
Andrey V. Kiselev
ICQ# 26871517