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Date: Fri, 4 Dec 2015 15:11:03 -0800
From: "Darrick J. Wong" <darrick.wong@...cle.com>
To: Andreas Dilger <adilger@...ger.ca>
Cc: "Theodore Ts'o" <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH v2] filefrag: accommodate holes when calculating expected
values
On Fri, Dec 04, 2015 at 03:06:04PM -0700, Andreas Dilger wrote:
> On Dec 3, 2015, at 1:37 PM, Darrick J. Wong <darrick.wong@...cle.com> wrote:
> >
> > Currently, filefrag's "expected physical block" column expects extent
> > records to be physically adjacent regardless of the amount of logical
> > block space between the two records. This means that if we punch a
> > hole in a file, we get reports like this:
> >
> > ext: logical_offset: physical_offset: length: expected: flags:
> > 4: 4096.. 8343: 57376.. 61623: 4248:
> > 5: 8345.. 10313: 61625.. 63593: 1969: 61624:
> >
> > Notice how it expects 8345 to map to 61624, and scores this against
> > the fragmentation of the file. Flagging this as "unexpected" is
> > incorrect because the gap in the logical mapping is exactly the same
> > size as the gap in the physical extents.
> >
> > Furthermore, this particular mapping leaves the door open to the
> > optimal mapping -- if a write to block 8344 causes it to be mapped to
> > 61624, the entire range 4096-10313 can be mapped with a single extent.
> > Until that happens, there's no way to combine extents 4 and 5 because
> > of the gap in the logical mapping at block 8344.
> >
> > Therefore, tweak the extent report to account for holes.
> >
> > v2: Make it work for extents crossing FIEMAP calls, and clean up the
> > FIBMAP version to report correct expected values.
> >
> > Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> > ---
> > misc/filefrag.c | 74 +++++++++++++++++++++++++++++++++----------------------
> > 1 file changed, 45 insertions(+), 29 deletions(-)
> >
> > diff --git a/misc/filefrag.c b/misc/filefrag.c
> > index 5bcde91..5ad6ee0 100644
> > --- a/misc/filefrag.c
> > +++ b/misc/filefrag.c
> > @@ -208,6 +208,7 @@ static int filefrag_fiemap(int fd, int blk_shift, int *num_extents,
> > __u64 buf[2048]; /* __u64 for proper field alignment */
> > struct fiemap *fiemap = (struct fiemap *)buf;
> > struct fiemap_extent *fm_ext = &fiemap->fm_extents[0];
> > + struct fiemap_extent fm_last;
> > int count = (sizeof(buf) - sizeof(*fiemap)) /
> > sizeof(struct fiemap_extent);
> > unsigned long long expected = 0;
> > @@ -219,6 +220,7 @@ static int filefrag_fiemap(int fd, int blk_shift, int *num_extents,
> > int rc;
> >
> > memset(fiemap, 0, sizeof(struct fiemap));
> > + memset(&fm_last, 0, sizeof(struct fiemap_extent));
>
> This could just be an initializer at declaration time?
Yeah. I was tempted to just fm_last = {0} but couldn't remember if that's
valid C or a gcc-ism. :)
>
> > if (sync_file)
> > flags |= FIEMAP_FLAG_SYNC;
> > @@ -254,6 +256,8 @@ static int filefrag_fiemap(int fd, int blk_shift, int *num_extents,
> > }
> >
> > for (i = 0; i < fiemap->fm_mapped_extents; i++) {
> > + expected = fm_last.fe_physical +
> > + fm_ext[i].fe_logical - fm_last.fe_logical;
>
> Does it make sense to allow two "expected" values? Either the sparse one that
> leaves a gap for the block, or the dense one that packs physical blocks adjacent
> to each other seem acceptable, depending on the application. It doesn't make
> sense to preserve holes in files that are never going to be modified in-place
> (e.g. core dump or something). Something like:
>
> expected_dense = fm_last.fe_physical + fm_last.fe_length;
> expected_sparse = fm_last.fe_physical +
> fm_ext[i].fe_logical - fm_last.fe_logical;
>
> if (fm_ext[i].fe_logical != 0 &&
> fm_ext[i].fe_physical != expected_dense &&
> fm_ext[i].fe_physical != expected_sparse) {
> tot_extents++;
Hmm. I like that idea, what does everyone else think?
--D
>
> Cheers, Andreas
>
> > @@ -265,10 +269,9 @@ static int filefrag_fiemap(int fd, int blk_shift, int *num_extents,
> > if (verbose)
> > print_extent_info(&fm_ext[i], n, expected,
> > blk_shift, st);
> > -
> > - expected = fm_ext[i].fe_physical + fm_ext[i].fe_length;
> > if (fm_ext[i].fe_flags & FIEMAP_EXTENT_LAST)
> > last = 1;
> > + fm_last = fm_ext[i];
> > n++;
> > }
> >
> > @@ -287,14 +290,15 @@ static int filefrag_fibmap(int fd, int blk_shift, int *num_extents,
> > ext2fs_struct_stat *st,
> > unsigned long numblocks, int is_ext2)
> > {
> > - struct fiemap_extent fm_ext;
> > + struct fiemap_extent fm_ext, fm_last;
> > unsigned long i, last_block;
> > - unsigned long long logical;
> > + unsigned long long logical, expected = 0;
> > /* Blocks per indirect block */
> > const long bpib = st->st_blksize / 4;
> > int count;
> >
> > memset(&fm_ext, 0, sizeof(fm_ext));
> > + memset(&fm_last, 0, sizeof(fm_last));
>
> These could be declaration initializers.
>
> > if (force_extent) {
> > fm_ext.fe_flags = FIEMAP_EXTENT_MERGED;
> > }
> > @@ -322,40 +326,52 @@ static int filefrag_fibmap(int fd, int blk_shift, int *num_extents,
> > return rc;
> > if (block == 0)
> > continue;
> > - if (*num_extents == 0) {
> > - (*num_extents)++;
> > - if (force_extent) {
> > +
> > + if (*num_extents == 0 || block != last_block + 1 ||
> > + fm_ext.fe_logical + fm_ext.fe_length != logical) {
> > + /*
> > + * This is the start of a new extent; figure out where
> > + * we expected it to be and report the extent.
> > + */
> > + if (*num_extents != 0 && fm_last.fe_length) {
> > + expected = fm_last.fe_physical +
> > + (fm_ext.fe_logical - fm_last.fe_logical);
> > + if (expected == fm_ext.fe_physical)
> > + expected = 0;
> > + }
> > + if (force_extent && *num_extents == 0)
> > print_extent_header();
> > - fm_ext.fe_physical = block * st->st_blksize;
> > + if (force_extent && *num_extents != 0) {
> > + print_extent_info(&fm_ext, *num_extents - 1,
> > + expected, blk_shift, st);
> > }
> > - }
> > - count++;
> > - if (force_extent && last_block != 0 &&
> > - (block != last_block + 1 ||
> > - fm_ext.fe_logical + fm_ext.fe_length != logical)) {
> > - print_extent_info(&fm_ext, *num_extents - 1,
> > - (last_block + 1) * st->st_blksize,
> > - blk_shift, st);
> > - fm_ext.fe_length = 0;
> > + if (verbose && expected != 0) {
> > + printf("Discontinuity: Block %llu is at %llu "
> > + "(was %llu)\n",
> > + fm_ext.fe_logical / st->st_blksize,
> > + fm_ext.fe_physical / st->st_blksize,
> > + expected / st->st_blksize);
> > + }
> > + /* create the new extent */
> > + fm_last = fm_ext;
> > (*num_extents)++;
> > - fm_ext.fe_logical = logical;
> > fm_ext.fe_physical = block * st->st_blksize;
> > - } else if (last_block && (block != last_block + 1)) {
> > - if (verbose)
> > - printf("Discontinuity: Block %ld is at %lu (was "
> > - "%lu)\n", i, block, last_block + 1);
> > - fm_ext.fe_length = 0;
> > - (*num_extents)++;
> > fm_ext.fe_logical = logical;
> > - fm_ext.fe_physical = block * st->st_blksize;
> > + fm_ext.fe_length = 0;
> > }
> > fm_ext.fe_length += st->st_blksize;
> > last_block = block;
> > }
> > -
> > - if (force_extent)
> > - print_extent_info(&fm_ext, *num_extents - 1,
> > - last_block * st->st_blksize, blk_shift, st);
> > + if (force_extent && *num_extents != 0) {
> > + if (fm_last.fe_length) {
> > + expected = fm_last.fe_physical +
> > + (fm_ext.fe_logical - fm_last.fe_logical);
> > + if (expected == fm_ext.fe_physical)
> > + expected = 0;
> > + }
> > + print_extent_info(&fm_ext, *num_extents - 1, expected,
> > + blk_shift, st);
> > + }
> >
> > return count;
> > }
> > --
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>
>
> Cheers, Andreas
>
>
>
>
>
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