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.
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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% /
----------
