[Top][All Lists]
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH] qcow2: Add full image preallocation option
From: |
Daniel P. Berrange |
Subject: |
Re: [Qemu-devel] [PATCH] qcow2: Add full image preallocation option |
Date: |
Thu, 27 Jan 2011 15:58:24 +0000 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Thu, Jan 27, 2011 at 04:52:14PM +0100, Kevin Wolf wrote:
> This adds a preallocation=full mode to qcow2 image creation, which does not
> only allocate metadata for the whole image, but also writes zeros to it,
> creating a non-sparse image file.
>
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
> block/qcow2.c | 45 ++++++++++++++++++++++++++++++++++++++++-----
> 1 files changed, 40 insertions(+), 5 deletions(-)
>
> diff --git a/block/qcow2.c b/block/qcow2.c
> index a1773e4..90cf2ca 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -838,7 +838,15 @@ static int qcow2_change_backing_file(BlockDriverState
> *bs,
> return qcow2_update_ext_header(bs, backing_file, backing_fmt);
> }
>
> -static int preallocate(BlockDriverState *bs)
> +enum prealloc_mode {
> + PREALLOC_OFF = 0,
> + PREALLOC_METADATA,
> + PREALLOC_FULL,
> +};
> +
> +#define IO_BUF_SIZE (2 * 1024 * 1024)
> +
> +static int preallocate(BlockDriverState *bs, enum prealloc_mode mode)
> {
> uint64_t nb_sectors;
> uint64_t offset;
> @@ -846,11 +854,14 @@ static int preallocate(BlockDriverState *bs)
> int ret;
> QCowL2Meta meta;
>
> + assert(mode != PREALLOC_OFF);
> +
> nb_sectors = bdrv_getlength(bs) >> 9;
> offset = 0;
> QLIST_INIT(&meta.dependent_requests);
> meta.cluster_offset = 0;
>
> + /* First allocate metadata in _really_ big chunks */
> while (nb_sectors) {
> num = MIN(nb_sectors, INT_MAX >> 9);
> ret = qcow2_alloc_cluster_offset(bs, offset, 0, num, &num, &meta);
> @@ -874,6 +885,28 @@ static int preallocate(BlockDriverState *bs)
> offset += num << 9;
> }
>
> + /* Then write zeros to the cluster data, if requested */
> + if (mode == PREALLOC_FULL) {
> + void *buf = qemu_mallocz(IO_BUF_SIZE);
> +
> + nb_sectors = bdrv_getlength(bs) >> BDRV_SECTOR_BITS;
> + offset = 0;
> +
> + while (nb_sectors) {
> + num = MIN(nb_sectors, IO_BUF_SIZE / BDRV_SECTOR_SIZE);
> + ret = bdrv_write(bs, offset >> BDRV_SECTOR_BITS, buf, num);
Is there a way you can calculate the total size of the qcow2
file upfront, and just use a single posix_fallocate() call to
do the zero-filled allocation of all the data blocks. It is
many orders of magnitude faster than truely writing blocks of
zero'd data on modern filesystems. I guess if you're using
compression or encryption, we'd really have to go the slow
path, but for regular usage it'd be better to take a fast
path.
Daniel