Quoting Qu Wenruo <quwenruo.bt...@gmx.com>:
On 2019/9/10 上午12:38, webmas...@zedlx.com wrote:
Quoting Qu Wenruo <quwenruo.bt...@gmx.com>:
2) Sensible defrag
The defrag is currently a joke.
Maybe there are such cases, but I would say that a vast majority of
users (99,99%) in a vast majority of cases (99,99%) don't want the
defrag operation to reduce free disk space.
What's wrong with current file based defrag?
If you want to defrag a subvolume, just iterate through all files.
I repeat: The defrag should not decrease free space. That's the 'normal'
expectation.
Since you're talking about btrfs, it's going to do CoW for metadata not
matter whatever, as long as you're going to change anything, btrfs will
cause extra space usage.
(Although the final result may not cause extra used disk space as freed
space is as large as newly allocated space, but to maintain CoW, newly
allocated space can't overlap with old data)
It is OK for defrag to temporarily decrease free space while defrag
operation is in progress. That's normal.
Further more, talking about snapshots with space wasted by extent
booking, it's definitely possible user want to break the shared extents:
Subvol 257, inode 257 has the following file extents:
(257 EXTENT_DATA 0)
disk bytenr X len 16M
offset 0 num_bytes 4k << Only 4k is referred in the whole 16M extent.
Subvol 258, inode 257 has the following file extents:
(257 EXTENT_DATA 0)
disk bytenr X len 16M
offset 0 num_bytes 4K << Shared with that one in subv 257
(257 EXTENT_DATA 4K)
disk bytenr Y len 16M
offset 0 num_bytes 4K << Similar case, only 4K of 16M is used.
In that case, user definitely want to defrag file in subvol 258, as if
that extent at bytenr Y can be freed, we can free up 16M, and allocate a
new 8K extent for subvol 258, ino 257.
(And will also want to defrag the extent in subvol 257 ino 257 too)
You are confusing the actual defrag with a separate concern, let's call
it 'reserved space optimization'. It is about partially used extents.
The actual name 'reserved space optimization' doesn't matter, I just
made it up.
Then when it's not snapshotted, it's plain defrag.
How things go from "reserved space optimization" to "plain defrag" just
because snapshots?
I'm not sure that I'm still following you here.
I'm just saying that when you have some unused space within an extent
and you want the defrag to free it up, that is OK, but such thing is
not the main focus of the defrag operation. So you are giving me some
edge case here which is half-relevant and it can be easily solved. The
extent just needs to be split up into pieces, it's nothing special.
'reserved space optimization' is usually performed as a part of the
defrag operation, but it doesn't have to be, as the actual defrag is
something separate.
Yes, 'reserved space optimization' can break up extents.
'reserved space optimization' can either decrease or increase the free
space. If the algorithm determines that more space should be reserved,
than free space will decrease. If the algorithm determines that less
space should be reserved, than free space will increase.
The 'reserved space optimization' can be accomplished such that the free
space does not decrease, if such behavior is needed.
Also, the defrag operation can join some extents. In my original example,
the extents e33 and e34 can be fused into one.
Btrfs defrag works by creating new extents containing the old data.
So if btrfs decides to defrag, no old extents will be used.
It will all be new extents.
That's why your proposal is freaking strange here.
Ok, but: can the NEW extents still be shared? If you had an extent E88
shared by 4 files in different subvolumes, can it be copied to another
place and still be shared by the original 4 files? I guess that the
answer is YES. And, that's the only requirement for a good defrag
algorithm that doesn't shrink free space.
Perhaps the metadata extents need to be unshared. That is OK. But I
guess that after a typical defrag, the sharing ratio in metadata
woudn't change much.
That's why knowledge in btrfs tech details can make a difference.
Sometimes you may find some ideas are brilliant and why btrfs is not
implementing it, but if you understand btrfs to some extent, you will
know the answer by yourself.
Yes, it is true, but what you are posting so far are all 'red
herring'-type arguments. It's just some irrelevant concerns, and you
just got me explaining thinks like I would to a little baby. I don't
know whether I stumbled on some rookie member of btrfs project, or you
are just lazy and you don't want to think or consider my proposals.
Go check my name in git log.
I didn't check yet. Ok, let's just try to communicate here, I'm dead serious.
I can't understand a defrag that substantially decreases free space. I
mean, each such defrag is a lottery, because you might end up with
practically unusable file system if the partition fills up.
CURRENT DEFRAG IS A LOTTERY!
How bad is that?
