@@ -16,6 +16,63 @@
* Boston, MA 021110-1307, USA.
*/
+/*
+ * btrfs convert design:
+ * The overall design of btrfs convert is like the following:
+ * |<------------------Old fs----------------------------->|
+ * |<- used ->| |<- used ->| |<- used ->|
+ * ||
+ * \/
+ * |<---------------Btrfs fs------------------------------>|
+ * |<- Old data chunk ->|< new chunk (D/M/S)>|<- ODC ->|
+ * |<-Old-FE->| |<-Old-FE->|<- Btrfs extents ->|<-Old-FE->|
+ *
+ * ODC = Old data chunk, btrfs chunks containing old fs data
+ * Mapped 1:1 (logical address == device offset)
+ * Old-FE = file extents points to old fs.
+ *
+ * So old fs used space is (mostly) kept as is, while btrfs will insert
+ * its chunk(Data/Meta/Sys) into large enough free space.
+ * In this way, we can create different profiles for meta/data for converted fs.
+ *
+ * The DIRTY work is, we must reserve and relocate 3 ranges for btrfs:
+ * [0, 1M), [btrfs_sb_offset(1), +64K), [btrfs_sb_offset(2), +64K)
+ *
+ * Most work are spent handling corner cases around these reserved ranges.
+ *
+ * Detailed workflow is:
+ * 1) Scan old fs used space and calculate data chunk layout
+ * 1.1) Scan old fs
+ * We can a map of used space of old fs
+ *
+ * 1.2) Calculate data chunk layout <<< Complex work
+ * New data chunks must meet 3 conditions using result fomr 1.1)
+ * a. Large enough to be a chunk
+ * b. Doesn't cover reserved ranges
+ * c. Covers all the remaining old fs used space
+ *
+ * XXX: This can be simplified if we don't need to handle backup supers
+ *
+ * 1.3) Calculate usable space for btrfs new chunks
+ * Btrfs chunk usable space must meet 3 conditions using result from 1.2)
+ * a. Large enough to be a chunk
+ * b. Doesn't cover reserved ranges
+ * c. Doesn't cover any data chunks in 1.1)
+ *
+ * 2) Create basic btrfs
+ * Initial metadata and sys chunks are inserted in the first availabe
+ * space of result of 1.3)
+ * Then insert all data chunks into the basic btrfs
+ *
+ * 3) Create convert image
+ * We need to relocate reserved ranges here.
+ * After this step, the convert image is done, and we can use the image
+ * as reflink source to create old files
+ *
+ * 4) Iterate old fs to create files
+ * We just reflink any offset in old fs to new file extents
+ */
+
#include "kerncompat.h"
#include <sys/ioctl.h>
@@ -276,6 +333,9 @@ static void init_blk_iterate_data(struct blk_iterate_data *data,
* The special point is, old disk_block can point to a reserved range.
* So here, we don't use disk_block directly but search convert_root
* to get the real disk_bytenr.
+ *
+ * XXX: Better extract this function as btrfs_reflink(), as in fact we are
+ * just reflinking from convert_image.
*/
static int record_file_blocks(struct blk_iterate_data *data,
u64 file_block, u64 disk_block, u64 num_blocks)
Convert is now a little complex due to that fact we need to separate meta and data chunks for different profiles. Add a comment with ascii art explaining the whole workflow and points out the really complex part, so any newcomers interested in convert can get a quick overview of it before digging into the hard to read code. Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com> --- convert/main.c | 60 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 60 insertions(+)