On Thu, Jun 23, 2016 at 05:37:09PM -0600, Chris Murphy wrote: > > So in your example of degraded writes, no matter what the on disk > > format makes it discoverable there is a problem: > > > > A. The "updating" is still always COW so there is no overwriting. > > There is RMW code in btrfs/raid56.c but I don't know when that gets > triggered.
RMW seems to be for cases where part of a stripe is modified but the entire stripe has not yet been read into memory. It reads the remaining blocks (reconstructing missing blocks if necessary) then calculates new parity blocks. > 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. Try again with 'dd conv=notrunc bs=4k count=1 seek=N of=...' or edit the file with a sector-level hex editor. > [root@f24s ~]# filefrag -v /mnt/5/64k-a-then64k-b.txt > Filesystem type is: 9123683e > File size of /mnt/5/64k-a-then64k-b.txt is 131072 (32 blocks of 4096 bytes) > ext: logical_offset: physical_offset: length: expected: flags: > 0: 0.. 31: 2931744.. 2931775: 32: last,eof > /mnt/5/64k-a-then64k-b.txt: 1 extent found > [root@f24s ~]# btrfs-map-logical -l $[4096*2931744] /dev/VG/a > mirror 1 logical 12008423424 physical 1114112 device /dev/mapper/VG-b > mirror 2 logical 12008423424 physical 34668544 device /dev/mapper/VG-a > [root@f24s ~]# vi /mnt/5/64k-a-then64k-b.txt > [root@f24s ~]# filefrag -v /mnt/5/64k-a-then64k-b.txt > Filesystem type is: 9123683e > File size of /mnt/5/64k-a-then64k-b.txt is 131072 (32 blocks of 4096 bytes) > ext: logical_offset: physical_offset: length: expected: flags: > 0: 0.. 31: 2931776.. 2931807: 32: last,eof > /mnt/5/64k-a-then64k-b.txt: 1 extent found > [root@f24s ~]# btrfs-map-logical -l $[4096*29317776] /dev/VG/a > No extent found at range [120085610496,120085626880) > [root@f24s ~]# btrfs-map-logical -l $[4096*2931776] /dev/VG/a > mirror 1 logical 12008554496 physical 1108475904 device /dev/mapper/VG-c > mirror 2 logical 12008554496 physical 1179648 device /dev/mapper/VG-b > [root@f24s ~]# > > There is a neat bug/rfe I found for btrfs-map-logical, it doesn't > report back the physical locations for all num_stripes on the volume. > It only spits back two, and sometimes it's the two data strips, > sometimes it's one data and one parity strip. > > > [1] > https://bugzilla.kernel.org/show_bug.cgi?id=120941 > > > -- > Chris Murphy >
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