Re: [dm-devel] introduce dm-snap-mv

[Mikulas Patocka <mpatocka@redhat.com>]

Hi

Thanks for the development!

I'm already developing something similar 
paper: http://people.redhat.com/mpatocka/papers/shared-snapshots.pdf
kernel code: http://people.redhat.com/mpatocka/patches/kernel/new-snapshots/r22/
user code: http://people.redhat.com/mpatocka/patches/userspace/new-snapshots/lvm-2.02.73/

Please change your code to conform to the existing interface. My kernel 
code is extendible, you can plug arbitrary on-disk formats to it, there's 
another example of Daniel Phillips' zumastore snapshots. Use this 
interface so that it could be used with the existing userspace 
infrastructure.

Mikulas

On Wed, 6 Oct 2010, Cong Meng wrote:

> Hello everyone,
> 
> I am very glad to introduce my work dm-snap-mv here.
> 
> 
> what is dm-snap-mv
> -------------------
> The dm-snap-mv is a target module for device-mapper, which can take
> multiple snapshots against an existing block device(origin device).
> 
> All snapshots are saved in an independent block device(COW device).
> 
> The copy-on-write is used, so only diff-data will be saved in the COW device.
> 
> 
> features
> --------
> 1. snapshot of origin
> 2. snapshot of snapshot
> 3. instant snapshot creation and deletion
> 4. origin and snapshots are concurrent readable/writable
> 5. rollback snapshot to origin
> 6. multiple origin devices share a COW device
> 7. multiple COW devices are supported
> 
> 
> diagram
> -------
> 
>            +---------------------+    +---------------+     +--------------+
>    read <--|                     |-----               |     |              |
>            |    origin dm_dev    |    |   origin_dev  | COW |              |
>   write ---|                     |---->               |----->  snapshot-1  |
>            +---------------------+    +--|------------+     |     ...      | 
>                                          |                  |     ...      |        
>            +---------------------+    +--|------------------+     ...      |
>    read <--|                     |-------?-                    snapshot-N  |
>            |  snapshot-X dm_dev  |    |         cow_dev                    |
>   write ---|                     |---->                                    |
>            +---------------------+    +------------------------------------+
> 
>   dm_dev: device-mapper device, created by "dmsetup create ..."
>      COW: copy-on-write
> 
> 
> download the source
> -----------------------
> http://github.com/mcpacino/dm-snap-mv
> 
> git clone git://github.com/mcpacino/dm-snap-mv.git
> 
> 
> a kernel patch
> --------------
> Now, dm-snap-mv highly depends on a kernel patch below, which make __getblk()
> can get a 4K buffer head while block size of the disk is NOT 4K.
> 
> Signed-off-by: Cong Meng <mcpacino@gmail.com>
> ---
>  fs/buffer.c |    7 ++-----
>  1 files changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 3e7dca2..f7f9d33 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -1051,10 +1051,7 @@ grow_buffers(struct block_device *bdev, sector_t block, int size)
>  	pgoff_t index;
>  	int sizebits;
>  
> -	sizebits = -1;
> -	do {
> -		sizebits++;
> -	} while ((size << sizebits) < PAGE_SIZE);
> +	sizebits = PAGE_CACHE_SHIFT - bdev->bd_inode->i_blkbits;
>  
>  	index = block >> sizebits;
>  
> @@ -2924,7 +2921,7 @@ int submit_bh(int rw, struct buffer_head * bh)
>  	 */
>  	bio = bio_alloc(GFP_NOIO, 1);
>  
> -	bio->bi_sector = bh->b_blocknr * (bh->b_size >> 9);
> +	bio->bi_sector = bh->b_blocknr << (bh->b_bdev->bd_inode->i_blkbits - 9);
>  	bio->bi_bdev = bh->b_bdev;
>  	bio->bi_io_vec[0].bv_page = bh->b_page;
>  	bio->bi_io_vec[0].bv_len = bh->b_size;
> 
> --
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