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btrfs-progs: convert: Add comment for the overall convert design

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Convert is now a little complex due to that fact we need to separate
metadata and data chunks for different profiles.

Add a comment with ascii art explaining the whole design and point
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>
[ wording and formatting adjustments ]
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Qu Wenruo 2017-02-22 15:50:32 +08:00 committed by David Sterba
parent 21e8582d32
commit 0117fdf1fc
1 changed files with 64 additions and 0 deletions
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64
convert/main.c
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@ -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>