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Re: [Qemu-devel] [PATCH v6] block:add-cow file format
From: |
Marcelo Tosatti |
Subject: |
Re: [Qemu-devel] [PATCH v6] block:add-cow file format |
Date: |
Thu, 5 Jan 2012 13:46:08 -0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Thu, Dec 29, 2011 at 05:36:59PM +0800, Dong Xu Wang wrote:
> From: Dong Xu Wang <address@hidden>
>
> Introduce a new file format: add-cow. The usage can be found in add-cow.txt of
> this patch.
>
> CC: Kevin Wolf <address@hidden>
> CC: Stefan Hajnoczi <address@hidden>
> Signed-off-by: Dong Xu Wang <address@hidden>
> ---
> After applying this patch, qemu might can not compile, need apply this patch
> first:
> http://lists.gnu.org/archive/html/qemu-devel/2011-12/msg02527.html
>
> Makefile.objs | 1 +
> block.c | 2 +-
> block.h | 1 +
> block/add-cow.c | 429
> ++++++++++++++++++++++++++++++++++++++++++++++++
> block_int.h | 1 +
> docs/specs/add-cow.txt | 72 ++++++++
> 6 files changed, 505 insertions(+), 1 deletions(-)
> create mode 100644 block/add-cow.c
> create mode 100644 docs/specs/add-cow.txt
>
> + s->bitmap_size = ((bs->total_sectors + 7) >> 3);
> + s->bitmap = qemu_blockalign(bs, s->bitmap_size);
> +
> + ret = bdrv_pread(bs->file, sizeof(header), s->bitmap,
> + s->bitmap_size);
> + if (ret != s->bitmap_size) {
> + goto fail;
> + }
As noted previously, it is not acceptable to read the entire bitmap in
memory since it might be very large. A cache, which limits the in-memory
size of the bitmap, must be created. In the qcow2-cache.c file you can
find an example (thats for qcow2 metadata cache). You can divide the
bitmap in chunks of say, 4k, and have:
int is_bit_set(int64_t bitnum, BlockDriverState *bs)
{
int64_t bitmap_entry = bitnum >> bits_per_entry;
if (!is_in_bitmap_cache(bs, bitmap_entry))
read_from_disk(bs, bitmap_entry);
return lookup_bitmap_cache(bs, bitnum);
}
And then limit the cache to a few megabytes.
Also when setting a bit you must update cache and write
to disk.
> +
> + if (s->image_file[0] == '\0') {
> + ret = -ENOENT;
> + goto fail;
> + }
> +
> + ret = bdrv_file_open(&backing_bs, bs->backing_file, 0);
> + if (ret < 0) {
> + return ret;
> + }
> + bdrv_delete(backing_bs);
> +
> + s->image_hd = bdrv_new("");
> +
> + if (path_has_protocol(s->image_file)) {
> + pstrcpy(image_filename, sizeof(image_filename),
> + s->image_file);
> + } else {
> + path_combine(image_filename, sizeof(image_filename),
> + bs->filename, s->image_file);
> + }
> +
> + image_drv = bdrv_find_format("raw");
> + ret = bdrv_open(s->image_hd, image_filename, flags, image_drv);
> + if (ret < 0) {
> + bdrv_delete(s->image_hd);
> + s->image_hd = NULL;
> + goto fail;
> + }
Please make sure it is possible to create a snapshot with the
snapshot_blkdev command, of a raw image. It is necessary for live block
copy, as described here:
http://patchwork.ozlabs.org/patch/134257/
Also please update that document, later, with raw examples.
Thanks