Use ftruncate on Unix to create hard disk images quickly #4792
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Instead of writing out disk blocks slowly across the entire volume, just use the ftruncate function to create a file instantly at the desired size. Depending on file system, this can either result in identical results to the old code just faster (eg: ZFS and btrfs with compression enabled), sparse files (most native Unix file systems without compression, eg ext4 and UFS), or a full non-sparse file like before (creating an image on FAT).
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Summary
Instead of writing out disk blocks slowly across the entire volume, just use the ftruncate function to create a file instantly at the desired size.
Depending on file system, this can either result in identical results to the old code just faster (eg: ZFS and btrfs with compression enabled), sparse files (most native Unix file systems without compression, eg ext4 and UFS), or a full non-sparse file like before (creating an image on FAT and exFAT).
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