Wednesday, 13 March 2013

Cannot reduce filesystem size

Cannot reduce filesystem size

Cannot reduce fs size

I have little issue reducing the fs on AIX .5.3. Here is what I get

root@dccccc-svc/etc>df -g /data/edw/init_stg

Filesystem GB blocks Free %Used Iused %Iused Mounted on

/dev/init_stg_lv01 793.00 425.25 47% 1081 1%


root@dccccc-svc/etc>oslevel -s



root@dccccc-svc/etc>chfs -a size=-10G /data/edw/init_stg

chfs: There is not enough free space to shrink the file system.

System Model: IBM,9131-52A

Machine Serial Number: 0xxxxxxxxxx

Processor Type: PowerPC_POWER5

Processor Implementation Mode: POWER 5

Processor Version: PV_5_3

Number Of Processors: 4

Processor Clock Speed: 1648 MHz

CPU Type: 64-bit

Kernel Type: 64-bit

LPAR Info: 3 dccccc-normal

Memory Size: 32000 MB

Good Memory Size: 32000 MB

Platform Firmware level: SF240_332

Firmware Version: IBM,SF240_332

That happens when you try to reduce a big chunk of data (in this case

10G) that may not be contiguous in the filesystem because you have files

scattered everywhere.

1. Try to defrag the FS

#defragfs -s /data/edw/init_stg

2. If you still can't reduce it after this. Try reducing the FS in

smaller chunks.

Instead of 10G at a time, try reducing 1 or 2 gigs. Then, repeat the


3. Try looking for files large using the find cmd and move them out

temporarily, just to see if we can shrink the fs without them:

#find /<filesystem> -xdev -size +2048 -ls|sort -r +6|pg

4. Sometimes processes open big files and use lots of temporary space in

those filesystem.

You could check processes/applications running against the filesystem

and stop them temporarily, if you can.

#fuser -cu[x] <filsystem>

Please, let me know if this works.


Explanations to the behavior of shrinkfs:
In the beginning of the JFS2 filesystem, there is the superblock, the

superblock backup, and then the data and metadata of the filesystem. At

the end is the inline log (if there is one), and the fsck working area.

The way the filesystem shrink works is this: When chfs is run and a

size is given (either -NUM or an absolute NUM size) AIX calculates where

that exists within the filesystem. This marker is known as "the fence".

The system then calculates how much data is left outside the fence, that

must be moved inside it (since we don't want to lose data). It

calculates the free space available, and subtracts a minimal amount for

the fsck working area and inline log (if any) that must go at the tail

end of the filesystem.

What chfs has to do is some complex calculating: in the area outside the

fence, is there any data to be saved and moved inside? In the area

inside the fence, how much data is there? Is it contiguous? How much

free space is there we have to play with? Is there enough space to move

the data from outside the fence inside it to save it? And lastly, is

there enough space to move the fsck working area and inline logs inside

also along with these?

It does not try to reorganize the data in any way. If a large file

outside the fence is make up of contiguous extents, then AIX looks for

an equivalent contiguous free space area inside the fence to move the

file to. If it can't find one, either due to a lack of space or free

space fragmentation, it fails this operation and won't shrink the

filesystem. The chfs shrink will also not purposely fragment a file to

force it to fit within fragmented free space.

In some cases running defragfs on the filesystem to defragment the files

will help, but many times it doesn't. The reason is because the purpose

of defragfs is to coalesce files into more contiguous extents, but not

to coalesce the free space in between them.

If non-contiguous free space is the issue, the only way to get them to

coalesce into large enough regions is to back up the data, remove it,

and restore it. Then the filesystem shrink may find enough contiguous

free space when chfs is run to move the data outside the fence into.

There's a limit to how much chfs can shrink a filesystem. This is

because chfs has to take into account not only the data you are

moving around, but it tries to keep the contiguous blocks of data in

files still contiguous. So if you have a filesystem with a lot of

space that is broken up into small areas, but you are moving around

large files it may fail even though it looks like you have a lot of

space left to shrink.

The free space reported by the df command is not necessary the space

that can be truncated by a shrinkFS request due to filesystem

fragmentation. A fragmented filesystem may not be shrunk if it does

not have enough free space for an object to be moved out of the region

to be truncated, and shrinkFS does not perform filesystem

defragmentation. In this case, the chfs command should fail with the

returned message: chfs: There is not enough free space to shrink the

file system - return code 28 (ENOSPC).

One of the common areas we see that limits customers is the

inclusion of large, unfragmented files in a filesystem, such as binary

database files. If a filesystem consists of a few, but extremely large

files, depending on how these are laid out the chfs may fail to find

enough space to move the data from outside the fence into it if it were

to attempt to shrink the filesystem.

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