Thanks, Liu, that clarifies. Alex.
On Tue, May 22, 2012 at 4:42 AM, Liu Bo <liubo2...@cn.fujitsu.com> wrote: > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
