88 lines
3.1 KiB
C
88 lines
3.1 KiB
C
/* Test for file system hole support.
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Copyright (C) 2018-2019 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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#include <stdbool.h>
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#include <support.h>
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#include <support/check.h>
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#include <sys/stat.h>
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#include <xunistd.h>
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int
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support_descriptor_supports_holes (int fd)
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{
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enum
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{
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/* Write offset for the enlarged file. This value is arbitrary
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and hopefully large enough to trigger the creation of holes.
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We cannot use the file system block size as a reference here
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because it is incorrect for network file systems. */
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write_offset = 16 * 1024 * 1024,
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/* Our write may add this number of additional blocks (see
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block_limit below). */
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block_headroom = 8,
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};
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struct stat64 st;
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xfstat (fd, &st);
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if (!S_ISREG (st.st_mode))
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FAIL_EXIT1 ("descriptor %d does not refer to a regular file", fd);
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if (st.st_size != 0)
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FAIL_EXIT1 ("descriptor %d does not refer to an empty file", fd);
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if (st.st_blocks > block_headroom)
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FAIL_EXIT1 ("descriptor %d refers to a pre-allocated file (%lld blocks)",
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fd, (long long int) st.st_blocks);
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/* Write a single byte at the start of the file to compute the block
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usage for a single byte. */
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xlseek (fd, 0, SEEK_SET);
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char b = '@';
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xwrite (fd, &b, 1);
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/* Attempt to bypass delayed allocation. */
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TEST_COMPARE (fsync (fd), 0);
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xfstat (fd, &st);
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/* This limit is arbitrary. The file system needs to store
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somewhere that data exists at the write offset, and this may
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moderately increase the number of blocks used by the file, in
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proportion to the initial block count, but not in proportion to
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the write offset. */
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unsigned long long int block_limit = 2 * st.st_blocks + block_headroom;
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/* Write a single byte at 16 megabytes. */
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xlseek (fd, write_offset, SEEK_SET);
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xwrite (fd, &b, 1);
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/* Attempt to bypass delayed allocation. */
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TEST_COMPARE (fsync (fd), 0);
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xfstat (fd, &st);
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bool supports_holes = st.st_blocks <= block_limit;
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/* Also check that extending the file does not fill up holes. */
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xftruncate (fd, 2 * write_offset);
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/* Attempt to bypass delayed allocation. */
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TEST_COMPARE (fsync (fd), 0);
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xfstat (fd, &st);
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supports_holes = supports_holes && st.st_blocks <= block_limit;
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/* Return to a zero-length file. */
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xftruncate (fd, 0);
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xlseek (fd, 0, SEEK_SET);
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return supports_holes;
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}
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