Re: [Qemu-devel] [PATCH v10 6/6] qcow2: Add full image preallocation option

[Max Reitz]

On 20.06.2014 10:25, Hu Tao wrote:
> On Sat, Jun 14, 2014 at 10:37:33PM +0200, Max Reitz wrote:
> > On 12.06.2014 05:54, Hu Tao wrote:
> > > This adds a preallocation=full mode to qcow2 image creation, which
> > > creates a non-sparse image file.
> > >
> > > Signed-off-by: Hu Tao <address@hidden>
> > > ---
> > >   block/qcow2.c              | 90 ++++++++++++++++++++++++++++++++++++++++------
> > >   tests/qemu-iotests/082.out | 54 ++++++++++++++--------------
> > >   2 files changed, 107 insertions(+), 37 deletions(-)
> > >
> > > diff --git a/block/qcow2.c b/block/qcow2.c
> > > index 2e0b83c..8c87c1a 100644
> > > --- a/block/qcow2.c
> > > +++ b/block/qcow2.c
> > > @@ -30,6 +30,7 @@
> > >   #include "qemu/error-report.h"
> > >   #include "qapi/qmp/qerror.h"
> > >   #include "qapi/qmp/qbool.h"
> > > +#include "qapi/util.h"
> > >   #include "trace.h"
> > >   /*
> > > @@ -1597,6 +1598,7 @@ static int qcow2_create2(const char *filename, int64_t 
> > > total_size,
> > >                            QEMUOptionParameter *options, int version,
> > >                            Error **errp)
> > >   {
> > > +    QEMUOptionParameter *alloc_options = NULL;
> > >       /* Calculate cluster_bits */
> > >       int cluster_bits;
> > >       cluster_bits = ffs(cluster_size) - 1;
> > > @@ -1626,10 +1628,78 @@ static int qcow2_create2(const char *filename, int64_t 
> > > total_size,
> > >       Error *local_err = NULL;
> > >       int ret;
> > > +    if (prealloc == PREALLOC_MODE_FULL || prealloc == PREALLOC_MODE_METADATA) {
> > > +        int64_t meta_size = 0;
> > > +        unsigned nreftablee, nrefblocke, nl1e, nl2e;
> > > +        BlockDriver *drv;
> > > +
> > > +        total_size = align_offset(total_size, cluster_size);
> > > +
> > > +        drv = bdrv_find_protocol(filename, true);
> > > +        if (drv == NULL) {
> > > +            error_setg(errp, "Could not find protocol for file '%s'", 
> > > filename);
> > > +            return -ENOENT;
> > > +        }
> > > +
> > > +        alloc_options = append_option_parameters(alloc_options,
> > > +                                                 drv->create_options);
> > > +        alloc_options = append_option_parameters(alloc_options, options);
> > > +
> > > +        /* header: 1 cluster */
> > > +        meta_size += cluster_size;
> > > +
> > > +        /* total size of L2 tables */
> > > +        nl2e = total_size / cluster_size;
> > > +        nl2e = align_offset(nl2e, cluster_size / sizeof(uint64_t));
> > > +        meta_size += nl2e * sizeof(uint64_t);
> > > +
> > > +        /* total size of L1 tables */
> > > +        nl1e = nl2e * sizeof(uint64_t) / cluster_size;
> > > +        nl1e = align_offset(nl1e, cluster_size / sizeof(uint64_t));
> > > +        meta_size += nl1e * sizeof(uint64_t);
> > > +
> > > +        /* total size of refcount blocks
> > > +         *
> > > +         * note: every host cluster is reference-counted, including metadata
> > > +         * (even refcount blocks are recursively included).
> > > +         * Let:
> > > +         *   a = total_size (this is the guest disk size)
> > > +         *   m = meta size not including refcount blocks and refcount tables
> > > +         *   c = cluster size
> > > +         *   y1 = number of refcount blocks entries
> > > +         *   y2 = meta size including everything
> > > +         * then,
> > > +         *   y1 = (y2 + a)/c
> > > +         *   y2 = y1 * sizeof(u16) + y1 * sizeof(u16) * sizeof(u64) / c + m
> > > +         * we can get y1:
> > > +         *   y1 = (a + m) / (c - sizeof(u16) - sizeof(u16) * sizeof(u64) / c)
> > > +         */
> > > +        nrefblocke = (total_size + meta_size + cluster_size) /
> > > +            (cluster_size - sizeof(uint16_t) -
> > > +             1.0 * sizeof(uint16_t) * sizeof(uint64_t) / cluster_size);
> > > +        nrefblocke = align_offset(nrefblocke, cluster_size / sizeof(uint16_t));
> > > +        meta_size += nrefblocke * sizeof(uint16_t);
> > > +
> > > +        /* total size of refcount tables */
> > > +        nreftablee = nrefblocke * sizeof(uint16_t) / cluster_size;
> > > +        nreftablee = align_offset(nreftablee, cluster_size / sizeof(uint64_t));
> > > +        meta_size += nreftablee * sizeof(uint64_t);
> > Hm, you could use the function minimal_blob_size() from my
> > "qemu-img: Implement commit like QMP" series. It calculates the
> > minimal size required for an L1 table, the refcount table and all
> > refcount blocks for an image with a given number of guest sectors
> > and a given number of "overhead", which are clusters that should be
> > covered by the refcount structures in addition to
> > L1+Reftable+Refblocks. To use it here, you would have to use it
> > probably like this:
> >
> > meta_size = (1 + minimal_blob_size(total_size / BDRV_SECTOR_SIZE,
> > cluster_bits, cluster_bits - BDRV_SECTOR_BITS, 1 + l2_clusters +
> > (total_size >> cluster_bits)) + l2_clusters) << cluster_bits;
> >
> > The first parameter is the number of guest sectors; the second is
> > cluster_bits; the third is parameter x, so that (1 << x) == number
> > of sectors per cluster; and the fourth is the number of clusters
> > which should be covered by the refcount structures in addition to
> > L1+Reftable+Refblocks. These are the image header (1 cluster), the
> > L2 tables (l2_clusters, which you'd have to calculate yourself (as
> > you are already doing in this patch)) and the clusters for guest
> > data (total_size >> cluster_bits).
> >
> > The function then returns the number of clusters required from
> > L1+Reftable+Refblocks to which the image header cluster and the L2
> > clusters have to be added to get the full metadata size.
> >
> > I'm suggesting you use this function (even though it's not merged
> > yet, but fully reviewed), because I guess it's always likely to have
> > some kind of small mistake in the size calculation for the refcount
> > structures; and therefore it's better to have a single place where
> > these mistakes can occur.
> Thanks! I used minimal_blob_size() from your patch. But tests show that
> the calculated meta size (and the image file size) is larger than the
> number calculated from my patch and without patch(file size).
>
> test command:
> qemu-img create -f qcow2 -o preallocation=metadata delme.img 8588490000
>
> result:
>
> without patch:
>
> Formatting 'delme.img', fmt=qcow2 size=8588490000 encryption=off 
> cluster_size=65536 preallocation='metadata' lazy_refcounts=off
> -rw-r--r-- 1 root root 8590065664 Jun 20 15:02 delme.img
>
>
> my patch:
>
> Formatting 'delme.img', fmt=qcow2 size=8588490000 encryption=off 
> cluster_size=65536 preallocation='metadata' lazy_refcounts=off
> meta_size: 1507328, 1507328
> total_size: 8588492800, 8588492800
> -rw-r--r-- 1 root root 8590065664 Jun 20 15:01 delme.img
>
>
> using minimal_blob_size:
>
> Formatting 'delme.img', fmt=qcow2 size=8588490000 encryption=off 
> cluster_size=65536 preallocation='metadata' lazy_refcounts=off
> -rw-r--r-- 1 root root 8590131200 Jun 20 15:03 delme.img

Well, then I'll need to fix minimal_blob_size() anyway. ;-)

But, well, it's just one cluster too much, so I guess it's still fine (I 
mostly made sure to allocate enough space in any case without using 
(potentially imprecise) floating point operations; so it may indeed 
sometimes require too much space, but as long as it's rather 
insignificant, I guess we can live with it).

> I need to review minimal_blob_size() further.

In http://lists.nongnu.org/archive/html/qemu-devel/2014-04/msg04820.html 
I attached a document which explains how I ended up with my formula, 
maybe it'll prove helpful to you.

Max

> > As for your own calculation code: You should round all structure
> > sizes separately to full clusters (the L1 table, the L2 tables, the
> > refcount table and the refcount blocks). Especially this makes the
> > refcount structure size so "hard" to calculate (or at least
> > error-prone), as this has to be considered for the recursive
> > calculation you demonstrated in your comment.
> >
> > > +
> > > +        set_option_parameter_int(alloc_options, BLOCK_OPT_SIZE,
> > > +                                 total_size + meta_size);
> > Wouldn't it suffice for preallocation=metadata to only allocate meta_size?
> >
> > If we allocate total_size + meta_size for metadata preallocation as
> > well, both metadata and full preallocation are the same (as far as I
> > can see). But if we only allocate meta_size, we have to rework
> > preallocate(): This function really "allocates" the data clusters
> > from qcow2's point of view, which means they are holes in the raw
> > file whereas all metadata should ideally be contained in the blob
> > sized meta_size at the start of the file and the rest should be
> > completely unused.
> >
> > With the current preallocate(), we either have to allocate
> > everything (including data), or can't guarantee (and it will in fact
> > never happen) that all metadata clusters fit into the meta_size
> > sized blob, because it creates a mixture of interleaved data and
> > metadata. The only way I can think of to really fix this is to
> > rework preallocate() so that it doesn't use the regular
> > qcow2_alloc_cluster_offset() function anymore but fills all metadata
> > tables by itself (which should not be too hard for a simple linear
> > preallocation of everything) and places all data clusters behind the
> > metadata blob.
> >
> > > +        if (prealloc == PREALLOC_MODE_FULL) {
> > > +            set_option_parameter(alloc_options, BLOCK_OPT_PREALLOC, "full");
> > > +        } else if (prealloc == PREALLOC_MODE_METADATA) {
> > > +            set_option_parameter(alloc_options, BLOCK_OPT_PREALLOC, 
> > > "metadata");
> > > +        }
> > How about set_option_parameter(alloc_options, BLOCK_OPT_PREALLOC,
> > PreallocMode_lookup[prealloc]); ?
> >
> > Max
> >
> > > +
> > > +        options = alloc_options;
> > > +    }
> > > +
> > >       ret = bdrv_create_file(filename, options, &local_err);
> > >       if (ret < 0) {
> > >           error_propagate(errp, local_err);
> > > -        return ret;
> > > +        goto out_options;
> > >       }
> > >       bs = NULL;
> > > @@ -1637,7 +1707,7 @@ static int qcow2_create2(const char *filename, int64_t 
> > > total_size,
> > >                       NULL, &local_err);
> > >       if (ret < 0) {
> > >           error_propagate(errp, local_err);
> > > -        return ret;
> > > +        goto out_options;
> > >       }
> > >       /* Write the header */
> > > @@ -1759,6 +1829,8 @@ out:
> > >       if (bs) {
> > >           bdrv_unref(bs);
> > >       }
> > > +out_options:
> > > +    free_option_parameters(alloc_options);
> > >       return ret;
> > >   }
> > > @@ -1790,13 +1862,11 @@ static int qcow2_create(const char *filename, 
> > > QEMUOptionParameter *options,
> > >                   cluster_size = options->value.n;
> > >               }
> > >           } else if (!strcmp(options->name, BLOCK_OPT_PREALLOC)) {
> > > -            if (!options->value.s || !strcmp(options->value.s, "off")) {
> > > -                prealloc = PREALLOC_MODE_OFF;
> > > -            } else if (!strcmp(options->value.s, "metadata")) {
> > > -                prealloc = PREALLOC_MODE_METADATA;
> > > -            } else {
> > > -                error_setg(errp, "Invalid preallocation mode: '%s'",
> > > -                           options->value.s);
> > > +            prealloc = qapi_enum_parse(PreallocMode_lookup, options->value.s,
> > > +                                       PREALLOC_MODE_MAX, PREALLOC_MODE_OFF,
> > > +                                       &local_err);
> > > +            if (local_err) {
> > > +                error_propagate(errp, local_err);
> > >                   return -EINVAL;
> > >               }
> > >           } else if (!strcmp(options->name, BLOCK_OPT_COMPAT_LEVEL)) {
> > > @@ -2359,7 +2429,7 @@ static QEMUOptionParameter qcow2_create_options[] = {
> > >       {
> > >           .name = BLOCK_OPT_PREALLOC,
> > >           .type = OPT_STRING,
> > > -        .help = "Preallocation mode (allowed values: off, metadata)"
> > > +        .help = "Preallocation mode (allowed values: off, metadata, full)"
> > >       },
> > >       {
> > >           .name = BLOCK_OPT_LAZY_REFCOUNTS,
> > > diff --git a/tests/qemu-iotests/082.out b/tests/qemu-iotests/082.out
> > > index 28309a0..5689802 100644
> > > --- a/tests/qemu-iotests/082.out
> > > +++ b/tests/qemu-iotests/082.out
> > > @@ -64,7 +64,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: create -f qcow2 -o ? TEST_DIR/t.qcow2 128M
> > > @@ -75,7 +75,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: create -f qcow2 -o cluster_size=4k,help TEST_DIR/t.qcow2 128M
> > > @@ -86,7 +86,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: create -f qcow2 -o cluster_size=4k,? TEST_DIR/t.qcow2 128M
> > > @@ -97,7 +97,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: create -f qcow2 -o help,cluster_size=4k TEST_DIR/t.qcow2 128M
> > > @@ -108,7 +108,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: create -f qcow2 -o ?,cluster_size=4k TEST_DIR/t.qcow2 128M
> > > @@ -119,7 +119,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: create -f qcow2 -o cluster_size=4k -o help TEST_DIR/t.qcow2 128M
> > > @@ -130,7 +130,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: create -f qcow2 -o cluster_size=4k -o ? TEST_DIR/t.qcow2 128M
> > > @@ -141,7 +141,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: create -f qcow2 -o backing_file=TEST_DIR/t.qcow2,,help 
> > > TEST_DIR/t.qcow2 128M
> > > @@ -167,7 +167,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: create -o help
> > > @@ -245,7 +245,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: convert -O qcow2 -o ? TEST_DIR/t.qcow2 TEST_DIR/t.qcow2.base
> > > @@ -256,7 +256,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: convert -O qcow2 -o cluster_size=4k,help TEST_DIR/t.qcow2 
> > > TEST_DIR/t.qcow2.base
> > > @@ -267,7 +267,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: convert -O qcow2 -o cluster_size=4k,? TEST_DIR/t.qcow2 
> > > TEST_DIR/t.qcow2.base
> > > @@ -278,7 +278,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: convert -O qcow2 -o help,cluster_size=4k TEST_DIR/t.qcow2 
> > > TEST_DIR/t.qcow2.base
> > > @@ -289,7 +289,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: convert -O qcow2 -o ?,cluster_size=4k TEST_DIR/t.qcow2 
> > > TEST_DIR/t.qcow2.base
> > > @@ -300,7 +300,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: convert -O qcow2 -o cluster_size=4k -o help TEST_DIR/t.qcow2 
> > > TEST_DIR/t.qcow2.base
> > > @@ -311,7 +311,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: convert -O qcow2 -o cluster_size=4k -o ? TEST_DIR/t.qcow2 
> > > TEST_DIR/t.qcow2.base
> > > @@ -322,7 +322,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: convert -O qcow2 -o backing_file=TEST_DIR/t.qcow2,,help 
> > > TEST_DIR/t.qcow2 TEST_DIR/t.qcow2.base
> > > @@ -348,7 +348,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: convert -o help
> > > @@ -415,7 +415,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: amend -f qcow2 -o ? TEST_DIR/t.qcow2
> > > @@ -426,7 +426,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: amend -f qcow2 -o cluster_size=4k,help TEST_DIR/t.qcow2
> > > @@ -437,7 +437,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: amend -f qcow2 -o cluster_size=4k,? TEST_DIR/t.qcow2
> > > @@ -448,7 +448,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: amend -f qcow2 -o help,cluster_size=4k TEST_DIR/t.qcow2
> > > @@ -459,7 +459,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: amend -f qcow2 -o ?,cluster_size=4k TEST_DIR/t.qcow2
> > > @@ -470,7 +470,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: amend -f qcow2 -o cluster_size=4k -o help TEST_DIR/t.qcow2
> > > @@ -481,7 +481,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: amend -f qcow2 -o cluster_size=4k -o ? TEST_DIR/t.qcow2
> > > @@ -492,7 +492,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: amend -f qcow2 -o backing_file=TEST_DIR/t.qcow2,,help 
> > > TEST_DIR/t.qcow2
> > > @@ -520,7 +520,7 @@ backing_file     File name of a base image
> > >   backing_fmt      Image format of the base image
> > >   encryption       Encrypt the image
> > >   cluster_size     qcow2 cluster size
> > > -preallocation    Preallocation mode (allowed values: off, metadata)
> > > +preallocation    Preallocation mode (allowed values: off, metadata, full)
> > >   lazy_refcounts   Postpone refcount updates
> > >   Testing: convert -o help