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introduce dm-snap-mv

From:  Cong Meng <mcpacino@gmail.com>
To:  linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, dm-devel@redhat.com, Andrew Morton <akpm@linux-foundation.org>, Alexander Viro <viro@zeniv.linux.org.uk>, Christoph Hellwig <hch@lst.de>, Nick Piggin <npiggin@kernel.dk>
Subject:  introduce dm-snap-mv
Date:  Wed, 6 Oct 2010 16:31:50 +0800
Message-ID:  <20101006083150.GA15758@zhongling>
Archive-link:  Article

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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