Commit a71835a0 authored by Kevin Wolf's avatar Kevin Wolf
Browse files

qcow2: Set zero flag for discarded clusters 



Instead of making the backing file contents visible again after a discard
request, set the zero flag if possible (i.e. on version >= 3).

Signed-off-by: default avatarKevin Wolf <kwolf@redhat.com>
Reviewed-by: default avatarEric Blake <eblake@redhat.com>
parent 3e890c77

block/qcow2-cluster.c

+20 −2
Original line number Diff line number Diff line
@@ -1367,13 +1367,31 @@ static int discard_single_l2(BlockDriverState *bs, uint64_t offset, 
        uint64_t old_offset;



        old_offset = be64_to_cpu(l2_table[l2_index + i]);

        if ((old_offset & L2E_OFFSET_MASK) == 0) {



        /*

         * Make sure that a discarded area reads back as zeroes for v3 images

         * (we cannot do it for v2 without actually writing a zero-filled

         * buffer). We can skip the operation if the cluster is already marked

         * as zero, or if it's unallocated and we don't have a backing file.

         *

         * TODO We might want to use bdrv_get_block_status(bs) here, but we're

         * holding s->lock, so that doesn't work today.

         */

        if (old_offset & QCOW_OFLAG_ZERO) {

            continue;

        }



        if ((old_offset & L2E_OFFSET_MASK) == 0 && !bs->backing_hd) {

            continue;

        }



        /* First remove L2 entries */

        qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);

        if (s->qcow_version >= 3) {

            l2_table[l2_index + i] = cpu_to_be64(QCOW_OFLAG_ZERO);

        } else {

            l2_table[l2_index + i] = cpu_to_be64(0);

        }



        /* Then decrease the refcount */

        qcow2_free_any_clusters(bs, old_offset, 1, type);

tests/qemu-iotests/046

+14 −4
Original line number Diff line number Diff line
@@ -193,6 +193,16 @@ echo "== Verify image content ==" 


function verify_io()

{

    if ($QEMU_IMG info -f "$IMGFMT" "$TEST_IMG" | grep "compat: 0.10" > /dev/null); then

        # For v2 images, discarded clusters are read from the backing file

        # Keep the variable empty so that the backing file value can be used as

        # the default below

        discarded=

    else

        # Discarded clusters are zeroed for v3 or later

        discarded=0

    fi



    echo read -P 0 0 0x10000



    echo read -P 1  0x10000 0x2000
@@ -221,16 +231,16 @@ function verify_io() 
    echo read -P 70 0x78000 0x6000

    echo read -P 7  0x7e000 0x2000



    echo read -P 8  0x80000 0x6000

    echo read -P ${discarded:-8} 0x80000 0x6000

    echo read -P 80 0x86000 0x2000

    echo read -P 8  0x88000 0x2000

    echo read -P ${discarded:-8} 0x88000 0x2000

    echo read -P 81 0x8a000 0xe000

    echo read -P 90 0x98000 0x6000

    echo read -P 9  0x9e000 0x2000



    echo read -P 10  0xa0000 0x6000

    echo read -P ${discarded:-10} 0xa0000 0x6000

    echo read -P 100 0xa6000 0x2000

    echo read -P 10  0xa8000 0x2000

    echo read -P ${discarded:-10} 0xa8000 0x2000

    echo read -P 101 0xaa000 0xe000

    echo read -P 110 0xb8000 0x8000