What is Sparse File?
A sparse file has an
attribute that causes the I/O subsystem to allocate only meaningful (nonzero)
data. Nonzero data is allocated on disk, and
non-meaningful data (large strings of data composed of zeros) is not. When a
sparse file is read, allocated data is returned as it
was stored; non-allocated data is returned, by default, as zeros.
NTFS deallocates sparse data streams and only maintains other data as allocated. When a program
accesses a sparse file, the file system yields allocated data as actual data
and deallocated data as zeros.
NTFS includes full sparse
file support for both compressed and uncompressed files. NTFS handles read
operations on sparse files by returning allocated data and sparse data. It is
possible to read a sparse file as allocated data and a range of data without
retrieving the entire data set, although NTFS returns the entire data set by
default.
With the sparse file
attribute set, the file system can deallocate data
from anywhere in the file and, when an application calls, yield the zero data
by range instead of storing and returning the actual data. File system
application programming interfaces (APIs) allow for the file to be copied or
backed as actual bits and sparse stream ranges. The net result is efficient
file system storage and access. Next figure shows how data is stored with and
without the sparse file attribute set.
Figure 5-4 Windows 2000 Data Storage
