How is it possible for a file larger than free space to exist?

[Randy Hagedorn]

Hi there,

I have a question of curiosity.

I am running an Oracle backup with RMAN to disk, which allocates a file that is initially equal to the file space of the datafiles being backed-up.

When I do an ll of the directory where the backup is being copied to, there is a file that is larger than the remaining free-space, as reported by bdf.

How is that possible?

# ll
total 14666692
-rw-rw---- 1 oracle dba 5388485632 Feb 10 11:02 b_6ofdo9ce_216_1
-rw-rw---- 1 oracle dba 328376320 Feb 10 11:08 b_6pfdodri_217_1
-rw-rw---- 1 oracle dba 330092544 Feb 10 11:13 b_6qfdoe5b_218_1
-rw-rw---- 1 oracle dba 330000384 Feb 10 11:17 b_6rfdoees_219_1
-rw-rw---- 1 oracle dba 322347008 Feb 10 11:22 b_6sfdoen6_220_1
-rw-rw---- 1 oracle dba 7485350912 Feb 10 15:52 b_6tfdou2i_221_1 <== file in question!


# bdf (shortened)
Filesystem kbytes used avail %used Mounted on
/dev/vg02/Group6 9252864 7433402 1762610 81% /u07

Thanks in advance,
Randy

[Rodney Hills · Accepted solution]

Two factors.

1) Some file systems reserve a portion for expansion that is not reported.

2) Some files have holes. You can define space for a file, but if it is not allocated it will not show up in bdf. An oracle DB may cause this situation.

HTH

-- Rod Hills

[Steven E. Protter]

no

7485350912/1024 to get kb

kb is what is being displayed on bdf.

Taht file is attempting to use all remaining free space.

I would expect the rman backup to hang or crash

bytes/1024=kilobytes

SEP

[A. Clay Stephenson]

Well, if I answer the question that you actually asked -- then, of course, that is possible. There was enough free space in the filesystem before the file was created.

This is the question that I think you meant to ask: Is is possible for the total of the file sizes reported by ll for an entire filesystem to exceed the size of the filesystem itself? If so, how?

Answer: Yes, absolutely. How: Sparse files.

Here is how you might create one:
fdes = open("/user",tmp1,O_CREAT | O_RDWR,0660);
char dummy = "X";
(void) write(fdes,(void *) &dummy,sizeof(dummy));
lseek(fdes,10000000L,0);
(void) write(fdes,(void *) &dummy,sizeof(dummy));
(void) close(fdes);

You created a file, write 1 byte, seeked to offset 10,000,000 and write 1 more byte.
The file's size as reported by ls -l will be 10,000,000 bytes although from the perspective of bdf only 2 blocks were used.
Sparse files are handy for preallocating large files but you do run the risk of "over subscribing" a filesystems ---- kinda like them there airlines.

[Dietmar Konermann]

Just a small example what "file that contains holes" aka "sparse file" means... on a largefile-enabled fs do this:

# echo | dd bs=1024k seek=10000 of=laaaarge

Then compare the results of "du -k laaaarge" and "ll laaaarge".


Best regards...
Dietmar.

[Sridhar Bhaskarla]

Hi Randy,

Both the above replies are true. There is a little extra space not visible to us in a filesystem under minfree area reserved in the filesystem. So, you may be able to keep a file that's more than what appears in the 'bdf''s available column.

Also bdf is reported in KBs. So, the file you have is really of 7485350912/1024 = 7309913 which is less than the available space you have in the 'bdf' output.

-Sri

[Michael Schulte zur Sur]

Hi Randy,

could it be, that you are confusing used and free space?

The first plus the last file is clearly more than the file system will yield. :-( As SEP says, it will crash.

I know Oracle for this. Once I accidently tried to resize a file to 2tb instead of 2gb on a production machine, of course the file system was full. It showed 2tb although they weren't there.

Michael