Re: xfs: Temporary extra disk space consumption?

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

On 2022/03/24 4:16, Dave Chinner wrote:
> On Wed, Mar 23, 2022 at 08:21:52PM +0900, Tetsuo Handa wrote:
>> Hello.
>>
>> I found that running a sample program shown below on xfs filesystem
>> results in consuming extra disk space until close() is called.
>> Is this expected result?
> 
> Yes. It's an anti-fragmentation mechanism that is intended to
> prevent ecessive fragmentation when many files are being written at
> once.

OK, this is an xfs specific behavior.

> Looks like specualtive preallocation for sequential writes is
> behaving exactly as designed....

Here is the result of "filefrag -v my_testfile" before close().

Filesystem type is: 58465342
File size of my_testfile is 1073741824 (262144 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..   65519:   33363497..  33429016:  65520:
   1:    65520..  229915:   62724762..  62889157: 164396:   33429017:
   2:   229916..  262143:   63132138..  63164365:  32228:   62889158: eof
   3:   262144..  294895:   63164366..  63197117:  32752:             unwritten,eof
my_testfile: 3 extents found

Filesystem type is: 58465342
File size of my_testfile is 1073741824 (262144 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..  131055:   62724762..  62855817: 131056:
   1:   131056..  240361:   63813369..  63922674: 109306:   62855818:
   2:   240362..  262143:   32448944..  32470725:  21782:   63922675: eof
   3:   262144..  349194:   32470726..  32557776:  87051:             unwritten,eof
   4:   349195..  524271:          0..    175076: 175077:   32557777: unknown,delalloc,eof
my_testfile: 4 extents found



An interesting behavior I noticed is that, since "filefrag -v" opens this file
for reading and then closes this file descriptor opened for reading, injecting
close(open(filename, O_RDONLY)) like below causes consumption by speculative
preallocation gone; close() of a file descriptor opened for writing is not
required.

----------
diff -u my_write_unlink.c my_write_unlink2.c
--- my_write_unlink.c
+++ my_write_unlink2.c
@@ -23,6 +23,8 @@
                return 1;
        printf("Before close().\n");
        system("/bin/df -m .");
+       close(open(filename, O_RDONLY));
+       system("/bin/df -m .");
        if (close(fd))
                return 1;
        printf("Before unlink().\n");
----------

----------
Before write().
Filesystem     1M-blocks   Used Available Use% Mounted on
/dev/sda1         255875 130396    125479  51% /
Before close().
Filesystem     1M-blocks   Used Available Use% Mounted on
/dev/sda1         255875 132447    123428  52% /
Filesystem     1M-blocks   Used Available Use% Mounted on
/dev/sda1         255875 131420    124455  52% /
Before unlink().
Filesystem     1M-blocks   Used Available Use% Mounted on
/dev/sda1         255875 131420    124455  52% /
After unlink().
Filesystem     1M-blocks   Used Available Use% Mounted on
/dev/sda1         255875 130396    125479  51% /
----------