Re: Newbie questions on some of btrfs code...

[Liu Bo <liubo2009@cn.fujitsu.com>]

On 05/21/2012 06:05 PM, Alex Lyakas wrote:

> Hi Liu,
> thanks for the clarifications.
> 
> I did not understand the dd example of yours, though.
> 
>> So for the following situation:
>>>       item 23 key (266 EXTENT_DATA 4096) itemoff 2269 itemsize 53
>>>               extent data disk byte 0 nr 0
>>>               extent data offset 0 nr 4096 ram 8192
>>>               extent compression 0
>> As your case, after the first 'size 5' inline extent is written,
>> "nr 4096 < ram 8192" could come from:
>> 1) dd if=/dev/zero of=/mnt/btrfs/foobar bs=1k seek=12 count=4 conv=notrunc;sync
>> 2) dd if=/dev/zero of=/mnt/btrfs/foobar bs=1k seek=8 count=4 conv=notrunc;sync
>>
>> 1) makes
>>>       item 23 key (266 EXTENT_DATA 4096) itemoff 2269 itemsize 53
>>>               extent data disk byte 0 nr 0
>>>               extent data offset 0 nr 8192 ram 8192
>>>               extent compression 0
>> 2) makes
>>>       item 23 key (266 EXTENT_DATA 4096) itemoff 2269 itemsize 53
>>>               extent data disk byte 0 nr 0
>>>               extent data offset 0 nr 4096 ram 8192
>>>               extent compression 0
> 
> You talk about the "ram_bytes" field. But do I need to look at it, if
> I don't use compression or another encoding? Shouldn't I always look
> at btrfs_file_extent_item::offset/num_bytes for the real data, and at
> btrfs_file_extent_item::disk_bytenr/disk_num_bytes for finding
> CHUNK_ITEM? Any reason I should be aware of "ram_bytes" field?
> 

> The first dd created a 4k extent at offset 12k. How did we end up with
> "nr 8192 ram 8192" and offset 4k?
> The second dd added a 4k extent at 8k offset. But still EXTENT_DATA
> has 4k offset.
> So now we should have have twp 4k extents or one 8k extent. What am I missing?
> 
> Alex.
> 


As I mentioned, disk_bytenr == 0 means dummy extents, which we have not yet allocate
a range of space for it.

After your first 'size=5' inline extent, we'll start allocating extents from _4096_, cause
it is _4k aligned_.

>> 1) dd if=/dev/zero of=/mnt/btrfs/foobar bs=1k seek=12 count=4 conv=notrunc;sync
: we need a dummy extent for [4k, 12k], which starts from 4096, and nr is 8192

>> 2) dd if=/dev/zero of=/mnt/btrfs/foobar bs=1k seek=8 count=4 conv=notrunc;sync
: we break [4k, 12k] into a dummy one [4k, 8k] and a real one [8k, 12k].

More details, plz refer to btrfs_drop_extents();

thanks,
liubo