Re: Stray 4k extents with slow buffered writes

["Holger Hoffstätte" <holger.hoffstaette@googlemail.com>]

On 03/03/16 21:47, Austin S. Hemmelgarn wrote:
>> $mount | grep sdf
>> /dev/sdf1 on /mnt/usb type btrfs (rw,relatime,space_cache=v2,subvolid=5,subvol=/)
> Do you still see the same behavior with the old space_cache format?
> This appears to be an issue of space management and allocation, so
> this may be playing a part.

I just did the clear_cache,space_cache=v1 dance. Now a download with
bandwidth-limit=1M, dirty_expire=20s, commit=30 and *no* autodefrag
first ended up looking like this:

$filefrag -ek linux-4.5-rc6.tar.xz 
Filesystem type is: 9123683e
File size of linux-4.5-rc6.tar.xz is 88362576 (86292 blocks of 1024 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..    7427:  227197920.. 227205347:   7428:            
   1:     7428..   33027:  227205348.. 227230947:  25600:            
   2:    33028..   53011:  227271164.. 227291147:  19984:  227230948:
   3:    53012..   72995:  227291148.. 227311131:  19984:            
   4:    72996..   86291:  227311132.. 227324427:  13296:             last,eof
linux-4.5-rc6.tar.xz: 2 extents found

Yay! But wait, there's more!

$sync
$filefrag -ek linux-4.5-rc6.tar.xz
Filesystem type is: 9123683e
File size of linux-4.5-rc6.tar.xz is 88362576 (86292 blocks of 1024 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..    7423:  227197920.. 227205343:   7424:            
   1:     7424..    7427:  227169600.. 227169603:      4:  227205344:
   2:     7428..   33023:  227205348.. 227230943:  25596:  227169604:
   3:    33024..   33027:  227169604.. 227169607:      4:  227230944:
   4:    33028..   53007:  227271164.. 227291143:  19980:  227169608:
   5:    53008..   53011:  227230948.. 227230951:      4:  227291144:
   6:    53012..   72991:  227291148.. 227311127:  19980:  227230952:
   7:    72992..   72995:  227230952.. 227230955:      4:  227311128:
   8:    72996..   86291:  227311132.. 227324427:  13296:  227230956: last,eof
linux-4.5-rc6.tar.xz: 9 extents found

Now I'm like ¯\(ツ)/¯

With autodefrag the same happens, though it then eventually does the
merging from 4k -> 256k. I went searching for that hardcoded 256k value
and found it as default in ioctl.c:btrfs_defrag_file() when no threshold
has been passed, as is the case for autodefrag. I'll try to increase that
and see how much I can destroy.

Also, rsync with --bwlimit=1m does _not_ seem to create files like this:

$rsync (..)
$filefrag -ek linux-4.4.4.tar.bz2 
Filesystem type is: 9123683e
File size of linux-4.4.4.tar.bz2 is 105008928 (102548 blocks of 1024 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..    4095:  227197920.. 227202015:   4096:            
   1:     4096..   25599:  227202016.. 227223519:  21504:            
   2:    25600..   51199:  227271164.. 227296763:  25600:  227223520:
   3:    51200..   76799:  227296764.. 227322363:  25600:            
   4:    76800..  102547:  227322364.. 227348111:  25748:             last,eof
linux-4.4.4.tar.bz2: 2 extents found

Which looks exactly as one would expect, probably - as Chris' mail
just explained - it doesn't use O_APPEND, whereas wget apparently does.

> I'd be somewhat curious to see if something similar happens on other
> filesystems with such low writeback timeouts.  My thought in this
> case is that the issue is that BTRFS's allocator isn't smart enough
> to try and merge new extents into existing ones when possible.

ext4 creates 1-2 extents, regardless of method.

Holger