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Re: [Qemu-block] [PATCH] block/gluster: add support for SEEK_DATA/SEEK_H


From: Niels de Vos
Subject: Re: [Qemu-block] [PATCH] block/gluster: add support for SEEK_DATA/SEEK_HOLE
Date: Tue, 8 Mar 2016 14:14:28 +0100
User-agent: Mutt/1.5.24 (2015-08-30)

On Tue, Mar 08, 2016 at 07:33:26AM -0500, Jeff Cody wrote:
> On Tue, Mar 08, 2016 at 05:21:48AM +0100, Niels de Vos wrote:
> > On Mon, Mar 07, 2016 at 01:27:38PM -0500, Jeff Cody wrote:
> > > On Mon, Mar 07, 2016 at 07:04:15PM +0100, Niels de Vos wrote:
> > > > GlusterFS 3.8 contains support for SEEK_DATA and SEEK_HOLE. This makes
> > > > it possible to detect sparse areas in files.
> > > > 
> > > > Signed-off-by: Niels de Vos <address@hidden>
> > > > 
> > > > --
> > > > Tested by compiling and running "qemu-img map gluster://..." with a
> > > > build of the current master branch of glusterfs. Using a Fedora
> > > > cloud image (in raw format) shows many SEEK procudure calls going back
> > > > and forth over the network. The output of "qemu map" matches the output
> > > > when run against the image on the local filesystem.
> > > > ---
> > > >  block/gluster.c | 159 
> > > > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > > >  configure       |  25 +++++++++
> > > >  2 files changed, 184 insertions(+)
> > > > 
> > > > diff --git a/block/gluster.c b/block/gluster.c
> > > > index 65077a0..1430010 100644
> > > > --- a/block/gluster.c
> > > > +++ b/block/gluster.c
> > > > @@ -677,6 +677,153 @@ static int 
> > > > qemu_gluster_has_zero_init(BlockDriverState *bs)
> > > >      return 0;
> > > >  }
> > > >  
> > > > +#ifdef CONFIG_GLUSTERFS_SEEK_DATA
> > > 
> > > Why do we need to make this a compile-time option?  Version checking
> > > is problematic; for instance, different distributions may have
> > > backported bug fixes / features, that are not reflected by the
> > > reported version number, etc..  Ideally, we can determine
> > > functionality during runtime, and behave accordingly.
> > 
> > This will not get backported to older Gluster versions, it required a
> > protocol change.
> > 
> > > If SEEK_DATA and SEEK_HOLE are not supported,
> > > qemu_gluster_co_get_block_status can return that sectors are all
> > > allocated (which is what happens in block/io.c anyway if the driver
> > > doesn't support the function).
> > 
> > Ok, good to know.
> > 
> > > As long as glfs_lseek() will return error (e.g. EINVAL) for an invalid
> > > whence value,  we can handle it runtime.  Does glfs_lseek() behave
> > > sanely?
> > 
> > Unfortunately older versions of libgfapi do not return EINVAL when
> > SEEK_DATA/HOLE is used. It is something we'll need to fix in the stable
> > releases. We can not assume that all users have installed a version of
> > the library that handles SEEK_DATA/HOLE correctly (return EINVAL) when
> > there is no support in the network protocol or on the server.
> > 
> > To be sure that we don't get some undefined behaviour, the compile time
> > check is needed.
> >
> 
> That's unfortunate.
> 
> However, we may still be able to work around that with some probing in
> the qemu_gluster_open() function.  I peeked at the libgfapi code, and
> it looks like for an unknown whence, the current offset is just
> returned by glfs_lseek() - is that correct?

Yes, that is correct.

> With the same offset input, an lseek should always return something
> different for a SEEK_DATA and SEEK_HOLE whence (SEEK_HOLE will return
> EOF offset if there are no further holes).
> 
> In qemu_gluster_open(), we can then probe for support.  if we call
> glfs_lseek() twice with the same offset, first with SEEK_DATA and
> then with SEEK_HOLE, then we can see if the libgfapi version
> supports SEEK_DATA/SEEK_HOLE.  If the resulting offsets from the calls
> are equal, it doesn't support it.  If the offsets differ, then
> presumably it does.  We can then set and remember that in the
> BDRVGlusterState struct (e.g. bool supports_seek_data).

Hmm, yes, that should work. And it'll obviously catch the case where
glfs_lseek() returns EINVAL too.

Thanks for the idea, I'll try to get this done over the next few days.

Niels


> 
> > > > +/*
> > > > + * Find allocation range in @bs around offset @start.
> > > > + * May change underlying file descriptor's file offset.
> > > > + * If @start is not in a hole, store @start in @data, and the
> > > > + * beginning of the next hole in @hole, and return 0.
> > > > + * If @start is in a non-trailing hole, store @start in @hole and the
> > > > + * beginning of the next non-hole in @data, and return 0.
> > > > + * If @start is in a trailing hole or beyond EOF, return -ENXIO.
> > > > + * If we can't find out, return a negative errno other than -ENXIO.
> > > > + *
> > > > + * (Shamefully copied from raw-posix.c, only miniscule adaptions.)
> > > > + */
> > > > +static int find_allocation(BlockDriverState *bs, off_t start,
> > > > +                           off_t *data, off_t *hole)
> > > > +{
> > > > +    BDRVGlusterState *s = bs->opaque;
> > > > +    off_t offs;
> > > > +
> > > > +    /*
> > > > +     * SEEK_DATA cases:
> > > > +     * D1. offs == start: start is in data
> > > > +     * D2. offs > start: start is in a hole, next data at offs
> > > > +     * D3. offs < 0, errno = ENXIO: either start is in a trailing hole
> > > > +     *                              or start is beyond EOF
> > > > +     *     If the latter happens, the file has been truncated behind
> > > > +     *     our back since we opened it.  All bets are off then.
> > > > +     *     Treating like a trailing hole is simplest.
> > > > +     * D4. offs < 0, errno != ENXIO: we learned nothing
> > > > +     */
> > > > +    offs = glfs_lseek(s->fd, start, SEEK_DATA);
> > > > +    if (offs < 0) {
> > > > +        return -errno;          /* D3 or D4 */
> > > > +    }
> > > > +    assert(offs >= start);
> > > > +
> > > > +    if (offs > start) {
> > > > +        /* D2: in hole, next data at offs */
> > > > +        *hole = start;
> > > > +        *data = offs;
> > > > +        return 0;
> > > > +    }
> > > > +
> > > > +    /* D1: in data, end not yet known */
> > > > +
> > > > +    /*
> > > > +     * SEEK_HOLE cases:
> > > > +     * H1. offs == start: start is in a hole
> > > > +     *     If this happens here, a hole has been dug behind our back
> > > > +     *     since the previous lseek().
> > > > +     * H2. offs > start: either start is in data, next hole at offs,
> > > > +     *                   or start is in trailing hole, EOF at offs
> > > > +     *     Linux treats trailing holes like any other hole: offs ==
> > > > +     *     start.  Solaris seeks to EOF instead: offs > start (blech).
> > > > +     *     If that happens here, a hole has been dug behind our back
> > > > +     *     since the previous lseek().
> > > > +     * H3. offs < 0, errno = ENXIO: start is beyond EOF
> > > > +     *     If this happens, the file has been truncated behind our
> > > > +     *     back since we opened it.  Treat it like a trailing hole.
> > > > +     * H4. offs < 0, errno != ENXIO: we learned nothing
> > > > +     *     Pretend we know nothing at all, i.e. "forget" about D1.
> > > > +     */
> > > > +    offs = glfs_lseek(s->fd, start, SEEK_HOLE);
> > > > +    if (offs < 0) {
> > > > +        return -errno;          /* D1 and (H3 or H4) */
> > > > +    }
> > > > +    assert(offs >= start);
> > > > +
> > > > +    if (offs > start) {
> > > > +        /*
> > > > +         * D1 and H2: either in data, next hole at offs, or it was in
> > > > +         * data but is now in a trailing hole.  In the latter case,
> > > > +         * all bets are off.  Treating it as if it there was data all
> > > > +         * the way to EOF is safe, so simply do that.
> > > > +         */
> > > > +        *data = start;
> > > > +        *hole = offs;
> > > > +        return 0;
> > > > +    }
> > > > +
> > > > +    /* D1 and H1 */
> > > > +    return -EBUSY;
> > > > +}
> > > > +
> > > > +/*
> > > > + * Returns the allocation status of the specified sectors.
> > > > + *
> > > > + * If 'sector_num' is beyond the end of the disk image the return 
> > > > value is 0
> > > > + * and 'pnum' is set to 0.
> > > > + *
> > > > + * 'pnum' is set to the number of sectors (including and immediately 
> > > > following
> > > > + * the specified sector) that are known to be in the same
> > > > + * allocated/unallocated state.
> > > > + *
> > > > + * 'nb_sectors' is the max value 'pnum' should be set to.  If 
> > > > nb_sectors goes
> > > > + * beyond the end of the disk image it will be clamped.
> > > > + *
> > > > + * (Based on raw_co_get_block_status() from raw-posix.c.)
> > > > + */
> > > > +static int64_t coroutine_fn qemu_gluster_co_get_block_status(
> > > > +        BlockDriverState *bs, int64_t sector_num, int nb_sectors, int 
> > > > *pnum)
> > > > +{
> > > > +    BDRVGlusterState *s = bs->opaque;
> > > > +    off_t start, data = 0, hole = 0;
> > > > +    int64_t total_size;
> > > > +    int ret = -EINVAL;
> > > > +
> > > > +    if (!s->fd) {
> > > > +        return ret;
> > > > +    }
> > > > +
> > > > +    start = sector_num * BDRV_SECTOR_SIZE;
> > > > +    total_size = bdrv_getlength(bs);
> > > > +    if (total_size < 0) {
> > > > +        return total_size;
> > > > +    } else if (start >= total_size) {
> > > > +        *pnum = 0;
> > > > +        return 0;
> > > > +    } else if (start + nb_sectors * BDRV_SECTOR_SIZE > total_size) {
> > > > +        nb_sectors = DIV_ROUND_UP(total_size - start, 
> > > > BDRV_SECTOR_SIZE);
> > > > +    }
> > > > +
> > > > +    ret = find_allocation(bs, start, &data, &hole);
> > > > +    if (ret == -ENXIO) {
> > > > +        /* Trailing hole */
> > > > +        *pnum = nb_sectors;
> > > > +        ret = BDRV_BLOCK_ZERO;
> > > > +    } else if (ret < 0) {
> > > > +        /* No info available, so pretend there are no holes */
> > > > +        *pnum = nb_sectors;
> > > > +        ret = BDRV_BLOCK_DATA;
> > > > +    } else if (data == start) {
> > > > +        /* On a data extent, compute sectors to the end of the extent,
> > > > +         * possibly including a partial sector at EOF. */
> > > > +        *pnum = MIN(nb_sectors, DIV_ROUND_UP(hole - start, 
> > > > BDRV_SECTOR_SIZE));
> > > > +        ret = BDRV_BLOCK_DATA;
> > > > +    } else {
> > > > +        /* On a hole, compute sectors to the beginning of the next 
> > > > extent.  */
> > > > +        assert(hole == start);
> > > > +        *pnum = MIN(nb_sectors, (data - start) / BDRV_SECTOR_SIZE);
> > > > +        ret = BDRV_BLOCK_ZERO;
> > > > +    }
> > > > +    return ret | BDRV_BLOCK_OFFSET_VALID | start;
> > > > +}
> > > > +#endif /* CONFIG_GLUSTERFS_SEEK_DATA */
> > > > +
> > > > +
> > > >  static QemuOptsList qemu_gluster_create_opts = {
> > > >      .name = "qemu-gluster-create-opts",
> > > >      .head = QTAILQ_HEAD_INITIALIZER(qemu_gluster_create_opts.head),
> > > > @@ -719,6 +866,9 @@ static BlockDriver bdrv_gluster = {
> > > >  #ifdef CONFIG_GLUSTERFS_ZEROFILL
> > > >      .bdrv_co_write_zeroes         = qemu_gluster_co_write_zeroes,
> > > >  #endif
> > > > +#ifdef CONFIG_GLUSTERFS_SEEK_DATA
> > > > +    .bdrv_co_get_block_status     = qemu_gluster_co_get_block_status,
> > > > +#endif
> > > >      .create_opts                  = &qemu_gluster_create_opts,
> > > >  };
> > > >  
> > > > @@ -746,6 +896,9 @@ static BlockDriver bdrv_gluster_tcp = {
> > > >  #ifdef CONFIG_GLUSTERFS_ZEROFILL
> > > >      .bdrv_co_write_zeroes         = qemu_gluster_co_write_zeroes,
> > > >  #endif
> > > > +#ifdef CONFIG_GLUSTERFS_SEEK_DATA
> > > > +    .bdrv_co_get_block_status     = qemu_gluster_co_get_block_status,
> > > > +#endif
> > > >      .create_opts                  = &qemu_gluster_create_opts,
> > > >  };
> > > >  
> > > > @@ -773,6 +926,9 @@ static BlockDriver bdrv_gluster_unix = {
> > > >  #ifdef CONFIG_GLUSTERFS_ZEROFILL
> > > >      .bdrv_co_write_zeroes         = qemu_gluster_co_write_zeroes,
> > > >  #endif
> > > > +#ifdef CONFIG_GLUSTERFS_SEEK_DATA
> > > > +    .bdrv_co_get_block_status     = qemu_gluster_co_get_block_status,
> > > > +#endif
> > > >      .create_opts                  = &qemu_gluster_create_opts,
> > > >  };
> > > >  
> > > > @@ -800,6 +956,9 @@ static BlockDriver bdrv_gluster_rdma = {
> > > >  #ifdef CONFIG_GLUSTERFS_ZEROFILL
> > > >      .bdrv_co_write_zeroes         = qemu_gluster_co_write_zeroes,
> > > >  #endif
> > > > +#ifdef CONFIG_GLUSTERFS_SEEK_DATA
> > > > +    .bdrv_co_get_block_status     = qemu_gluster_co_get_block_status,
> > > > +#endif
> > > >      .create_opts                  = &qemu_gluster_create_opts,
> > > >  };
> > > >  
> > > > diff --git a/configure b/configure
> > > > index 0c0472a..ca3821c 100755
> > > > --- a/configure
> > > > +++ b/configure
> > > > @@ -3351,6 +3351,9 @@ if test "$glusterfs" != "no" ; then
> > > >      if $pkg_config --atleast-version=6 glusterfs-api; then
> > > >        glusterfs_zerofill="yes"
> > > >      fi
> > > > +    if $pkg_config --atleast-version=7.3.8 glusterfs-api; then
> > > > +      glusterfs_seek_data="yes"
> > > > +    fi
> > > >    else
> > > >      if test "$glusterfs" = "yes" ; then
> > > >        feature_not_found "GlusterFS backend support" \
> > > > @@ -3660,6 +3663,24 @@ if compile_prog "" "" ; then
> > > >    fiemap=yes
> > > >  fi
> > > >  
> > > > +# check for SEEK_DATA and SEEK_HOLE
> > > > +seek_data=no
> > > > +cat > $TMPC << EOF
> > > > +#define _GNU_SOURCE
> > > > +#include <sys/types.h>
> > > > +#include <unistd.h>
> > > > +
> > > > +int main(void)
> > > > +{
> > > > +    lseek(0, 0, SEEK_DATA);
> > > > +    lseek(0, 0, SEEK_HOLE);
> > > > +    return 0;
> > > > +}
> > > > +EOF
> > > > +if compile_prog "" "" ; then
> > > > +  seek_data=yes
> > > > +fi
> > > > +
> > > >  # check for dup3
> > > >  dup3=no
> > > >  cat > $TMPC << EOF
> > > > @@ -5278,6 +5299,10 @@ if test "$glusterfs_zerofill" = "yes" ; then
> > > >    echo "CONFIG_GLUSTERFS_ZEROFILL=y" >> $config_host_mak
> > > >  fi
> > > >  
> > > > +if test "$glusterfs_seek_data" = "yes" && test "$seek_data" = "yes" ; 
> > > > then
> > > > +  echo "CONFIG_GLUSTERFS_SEEK_DATA=y" >> $config_host_mak
> > > > +fi
> > > > +
> > > >  if test "$archipelago" = "yes" ; then
> > > >    echo "CONFIG_ARCHIPELAGO=m" >> $config_host_mak
> > > >    echo "ARCHIPELAGO_LIBS=$archipelago_libs" >> $config_host_mak
> > > > -- 
> > > > 2.5.0
> > > > 
> > > > 



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