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Diffstat (limited to 'convert/main.c')
| -rw-r--r-- | convert/main.c | 64 |
1 files changed, 64 insertions, 0 deletions
diff --git a/convert/main.c b/convert/main.c index dc6047e6..38cfdead 100644 --- a/convert/main.c +++ b/convert/main.c @@ -16,6 +16,70 @@ * 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 pointing 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 metadata/data for + * converted fs. + * + * We must reserve and relocate 3 ranges for btrfs: + * * [0, 1M) - area never used for any data except the first + * superblock + * * [btrfs_sb_offset(1), +64K) - 1st superblock backup copy + * * [btrfs_sb_offset(2), +64K) - 2nd, dtto + * + * Most work is 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 used space of old fs + * + * 1.2) Calculate data chunk layout - this is the hard part + * New data chunks must meet 3 conditions using result fomr 1.1 + * a. Large enough to be a chunk + * b. Doesn't intersect reserved ranges + * c. Covers all the remaining old fs used space + * + * NOTE: This can be simplified if we don't need to handle backup supers + * + * 1.3) Calculate usable space for new btrfs chunks + * Btrfs chunk usable space must meet 3 conditions using result from 1.2 + * a. Large enough to be a chunk + * b. Doesn't intersect reserved ranges + * c. Doesn't cover any data chunks in 1.1 + * + * 2) Create basic btrfs filesystem structure + * Initial metadata and sys chunks are inserted in the first availabe + * space found in step 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 file extents from old fs to newly created files on + * btrfs. + */ + #include "kerncompat.h" #include <stdio.h> |