On 2016-04-25 23:48, Qu Wenruo wrote:
The branch can be fetched from my github:
https://github.com/adam900710/btrfs-progs.git low_mem_fsck_rebasing
Original btrfsck checks extent tree in a very efficient method, by
recording every checked extent in extent record tree to ensure every
extent will be iterated for at most 2 times.
However extent records are all stored in heap memory, and consider how
large a btrfs file system can be, it can easily eat up all memory and
cause OOM for TB-sized metadata.
Instead of such heap memory usage, we introduce low memory usage fsck
mode.
In this mode, we will use btrfs_search_slot() only and avoid any heap
memory allocation.
The work flow is:
1) Iterate extent tree (backref check)
And check whether the referencer of every backref exists.
2) Iterate other trees (forward ref check)
And check whether the backref of every tree block/data exists in
extent tree.
So in theory, every extent is iterated twice just as original one.
But since we don't have extent record, but use btrfs_search_slot() every
time we check, it will cause extra IO.
I assume the extra IO is reasonable and should make btrfsck able to
handle super large fs.
TODO features:
1) Repair
Repair should be the same as old btrfsck, but still need to determine
the repair principle.
Current repair sometimes uses backref to repair data extent,
sometimes uses data extent to fix backref.
We need a consistent principle, or we will screw things up.
2) Replace current fsck code
We assume the low memory mode has less lines of code, and may be
easier for review and expand.
If low memory mode is stable enough, we will consider to replace
current extent and chunk tree check codes to free a lot of lines.
3) Further code refining
Reduce duplicated codes
4) Unify output
Make the output of low-memory mode same as the normal one.
Lu Fengqi (16):
btrfs-progs: fsck: Introduce function to check tree block backref in
extent tree
btrfs-progs: fsck: Introduce function to check data backref in extent
tree
btrfs-progs: fsck: Introduce function to query tree block level
btrfs-progs: fsck: Introduce function to check referencer of a backref
btrfs-progs: fsck: Introduce function to check shared block ref
btrfs-progs: fsck: Introduce function to check referencer for data
backref
btrfs-progs: fsck: Introduce function to check shared data backref
btrfs-progs: fsck: Introduce function to check an extent
btrfs-progs: fsck: Introduce function to check dev extent item
btrfs-progs: fsck: Introduce function to check dev used space
btrfs-progs: fsck: Introduce function to check block group item
btrfs-progs: fsck: Introduce function to check chunk item
btrfs-progs: fsck: Introduce hub function for later fsck
btrfs-progs: fsck: Introduce function to speed up fs tree check
btrfs-progs: fsck: Introduce traversal function for fsck
btrfs-progs: fsck: Introduce low memory mode
Documentation/btrfs-check.asciidoc | 2 +
cmds-check.c | 1667 +++++++++++++++++++++++++++++++++---
ctree.h | 2 +
extent-tree.c | 2 +-
4 files changed, 1536 insertions(+), 137 deletions(-)
I don't really have a stock of broken FS images to test this with, but
I've checked it against known good ones and it correctly identifies them
as good (I've tested all the profiles except raid5 and raid6 in both
normal and mixed-bg variants, with all combinations of profiles between
data and metadata, and with 2-8 devices for the multi-device levels,
most of the involved filesystems were on LVM thinp storage with mostly
sparse files), and it properly repairs the couple of broken filesystems
I can make by hand (mostly stuff with orphaned inodes or bad ref-counts)
in the same way the existing code repairs them, all while using
measurably less memory as advertised, so you can add:
Tested-by: Austin S. Hemmelgarn <ahferroin7@xxxxxxxxx>
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