On Thu, Jun 23, 2016 at 8:07 PM, Zygo Blaxell <ce3g8jdj@xxxxxxxxxxxxxxxxxxxxx> wrote: >> With simple files changing one character with vi and gedit, >> I get completely different logical and physical numbers with each >> change, so it's clearly cowing the entire stripe (192KiB in my 3 dev >> raid5). > > You are COWing the entire file because vi and gedit do truncate followed > by full-file write. I'm seeing the file inode changes with either a vi or gedit modification, even when file size is exactly the same, just character substitute. So as far as VFS and Btrfs are concerned, it's an entirely different file, so it's like faux-CoW that would have happened on any file system, not an overwrite. > Try again with 'dd conv=notrunc bs=4k count=1 seek=N of=...' or > edit the file with a sector-level hex editor. The inode is now the same, one of the 4096 byte blocks is dereferenced, a new 4096 byte block is referenced, and written, the other 3 blocks remain untouched, the other files in the stripe remain untouched. So it's pretty clearly cow'd in this case. [root@f24s ~]# filefrag -v /mnt/5/* Filesystem type is: 9123683e File size of /mnt/5/a.txt is 16383 (4 blocks of 4096 bytes) ext: logical_offset: physical_offset: length: expected: flags: 0: 0.. 3: 2931712.. 2931715: 4: last,eof /mnt/5/a.txt: 1 extent found File size of /mnt/5/b.txt is 16383 (4 blocks of 4096 bytes) ext: logical_offset: physical_offset: length: expected: flags: 0: 0.. 3: 2931716.. 2931719: 4: last,eof /mnt/5/b.txt: 1 extent found File size of /mnt/5/c.txt is 16383 (4 blocks of 4096 bytes) ext: logical_offset: physical_offset: length: expected: flags: 0: 0.. 3: 2931720.. 2931723: 4: last,eof /mnt/5/c.txt: 1 extent found File size of /mnt/5/d.txt is 16383 (4 blocks of 4096 bytes) ext: logical_offset: physical_offset: length: expected: flags: 0: 0.. 3: 2931724.. 2931727: 4: last,eof /mnt/5/d.txt: 1 extent found File size of /mnt/5/e.txt is 16383 (4 blocks of 4096 bytes) ext: logical_offset: physical_offset: length: expected: flags: 0: 0.. 3: 2931728.. 2931731: 4: last,eof /mnt/5/e.txt: 1 extent found [root@f24s ~]# ls -li /mnt/5/* 285 -rw-r--r--. 1 root root 16383 Jun 23 22:57 /mnt/5/a.txt 286 -rw-r--r--. 1 root root 16383 Jun 23 22:57 /mnt/5/b.txt 287 -rw-r--r--. 1 root root 16383 Jun 23 22:57 /mnt/5/c.txt 288 -rw-r--r--. 1 root root 16383 Jun 23 22:57 /mnt/5/d.txt 289 -rw-r--r--. 1 root root 16383 Jun 23 22:57 /mnt/5/e.txt [root@f24s ~]# btrfs-map-logical -l $[4096*2931712] /dev/VG/a mirror 1 logical 12008292352 physical 34603008 device /dev/mapper/VG-a mirror 2 logical 12008292352 physical 1108344832 device /dev/mapper/VG-c [root@f24s ~]# btrfs-map-logical -l $[4096*2931716] /dev/VG/a mirror 1 logical 12008308736 physical 34619392 device /dev/mapper/VG-a mirror 2 logical 12008308736 physical 1108361216 device /dev/mapper/VG-c [root@f24s ~]# btrfs-map-logical -l $[4096*2931720] /dev/VG/a mirror 1 logical 12008325120 physical 34635776 device /dev/mapper/VG-a mirror 2 logical 12008325120 physical 1108377600 device /dev/mapper/VG-c [root@f24s ~]# btrfs-map-logical -l $[4096*2931724] /dev/VG/a mirror 1 logical 12008341504 physical 34652160 device /dev/mapper/VG-a mirror 2 logical 12008341504 physical 1108393984 device /dev/mapper/VG-c [root@f24s ~]# btrfs-map-logical -l $[4096*2931728] /dev/VG/a mirror 1 logical 12008357888 physical 1048576 device /dev/mapper/VG-b mirror 2 logical 12008357888 physical 1108344832 device /dev/mapper/VG-c [root@f24s ~]# echo -n "g" | dd of=/mnt/5/a.txt conv=notrunc 0+1 records in 0+1 records out 1 byte copied, 0.000314582 s, 3.2 kB/s [root@f24s ~]# ls -li /mnt/5/* 285 -rw-r--r--. 1 root root 16383 Jun 23 23:06 /mnt/5/a.txt 286 -rw-r--r--. 1 root root 16383 Jun 23 22:57 /mnt/5/b.txt 287 -rw-r--r--. 1 root root 16383 Jun 23 22:57 /mnt/5/c.txt 288 -rw-r--r--. 1 root root 16383 Jun 23 22:57 /mnt/5/d.txt 289 -rw-r--r--. 1 root root 16383 Jun 23 22:57 /mnt/5/e.txt [root@f24s ~]# filefrag -v /mnt/5/* Filesystem type is: 9123683e File size of /mnt/5/a.txt is 16383 (4 blocks of 4096 bytes) ext: logical_offset: physical_offset: length: expected: flags: 0: 0.. 0: 2931732.. 2931732: 1: 1: 1.. 3: 2931713.. 2931715: 3: 2931733: last,eof /mnt/5/a.txt: 2 extents found File size of /mnt/5/b.txt is 16383 (4 blocks of 4096 bytes) ext: logical_offset: physical_offset: length: expected: flags: 0: 0.. 3: 2931716.. 2931719: 4: last,eof /mnt/5/b.txt: 1 extent found File size of /mnt/5/c.txt is 16383 (4 blocks of 4096 bytes) ext: logical_offset: physical_offset: length: expected: flags: 0: 0.. 3: 2931720.. 2931723: 4: last,eof /mnt/5/c.txt: 1 extent found File size of /mnt/5/d.txt is 16383 (4 blocks of 4096 bytes) ext: logical_offset: physical_offset: length: expected: flags: 0: 0.. 3: 2931724.. 2931727: 4: last,eof /mnt/5/d.txt: 1 extent found File size of /mnt/5/e.txt is 16383 (4 blocks of 4096 bytes) ext: logical_offset: physical_offset: length: expected: flags: 0: 0.. 3: 2931728.. 2931731: 4: last,eof /mnt/5/e.txt: 1 extent found [root@f24s ~]# btrfs-map-logical -l $[4096*2931732] /dev/VG/a mirror 1 logical 12008374272 physical 1064960 device /dev/mapper/VG-b mirror 2 logical 12008374272 physical 1108361216 device /dev/mapper/VG-c It has been cow'd. [root@f24s ~]# dd if=/dev/VG/b bs=1 skip=1064960 count=4096 2>/dev/null | hexdump -C 00000000 67 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 |gaaaaaaaaaaaaaaa| 00000010 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 |aaaaaaaaaaaaaaaa| * 00001000 [root@f24s ~]# dd if=/dev/VG/c bs=1 skip=1108361216 count=4096 2>/dev/null | hexdump -C 00000000 05 03 03 03 03 03 03 03 03 03 03 03 03 03 03 03 |................| 00000010 03 03 03 03 03 03 03 03 03 03 03 03 03 03 03 03 |................| * 00001000 [root@f24s ~]# btrfs-map-logical -l $[4096*2931712] /dev/VG/a mirror 1 logical 12008292352 physical 34603008 device /dev/mapper/VG-a mirror 2 logical 12008292352 physical 1108344832 device /dev/mapper/VG-c [root@f24s ~]# dd if=/dev/VG/a bs=1 skip=34603008 count=4096 2>/dev/null | hexdump -C 00000000 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 |aaaaaaaaaaaaaaaa| * 00001000 So at the old address, it shows the "aaaaa..." is still there. And at the added single block for this file at new logical and physical addresses, is the modification substituting the first "a" for "g". In this case, no rmw, no partial stripe modification, and no data already on-disk is at risk. Even the metadata leaf/node is cow'd, it has a new logical and physical address as well, which contains information for all five files. -- Chris Murphy -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
