Volume defragmentation

[kosh2013]

Hi guys,

I am a little curious about defragmentation. Probably someone can explain defragmentation.

First let me start what I have done so far:

1. Created a DG, Volume (1GB) etc (testing system only)

2. Created about 1000 1MB files

3. Deleted every 2nd file

And now I am curious. When I ran fsadm -E /local (/local is the mounted test volume of course) I get the following output:

...

    Free Extents By Size
           1:          2            2:          2            4:          3
           8:          7           16:          6           32:          6
          64:          5          128:          5          256:          6
         512:          3         1024:        326         2048:          2

...

It shows clearly 326 free 1024K extents. From my understanding the file system is fragmentated now? Because when I create a large file now it would use all the 1024K extents and not one 128M extent, two 64M extent etc? Am I wrong/right?

And how can I rearrange the extents (if it is possible). I have tried various fsadm options like -e, -d and -C but without success.

BTW:

pkginfo -l VRTSvxfs

...
    PSTAMP:  5.1.103.000-5.1SP1RP3_x86-2012-09-16-VERITAS-FS-142635-12
...

uname -a
SunOS sol10 5.10 Generic_142910-17 i86pc i386 i86pc

Any suggestions?

[mikebounds]

See the vxfs admin guide for vxfs desribes as fragmented:

To determine the degree of fragmentation, use the following factors:

■ Percentage of free space in extents of less than 8 blocks in length

■ Percentage of free space in extents of less than 64 blocks in length

■ Percentage of free space in extents of length 64 blocks or greater

An unfragmented file system has the following characteristics:

■ Less than 1 percent of free space in extents of less than 8 blocks in length

■ Less than 5 percent of free space in extents of less than 64 blocks in length

■ More than 5 percent of the total file system size available as free extents in

   lengths of 64 or more blocks

A badly fragmented file system has one or more of the following characteristics:

■ Greater than 5 percent of free space in extents of less than 8 blocks in length

■ More than 50 percent of free space in extents of less than 64 blocks in length

■ Less than 5 percent of the total file system size available as free extents in

   lengths of 64 or more blocks

So all your free space is in extents of 1024 blocks so vxfs does not clasify this as fragmented and therefore I don't believe you can defrag it.  To make the filesystem fragmented you would need to create 32K files and then delete every 2nd file

Mike

[kosh2013]

OK, did it (after formatting the volume):

c=0;  while [[ $c -lt 29116 ]]; do let c=c+1; dd if=/dev/zero of=$c bs=32768 count=1 > /dev/null 2>&1; done
fsadm -E /local
  Extent Fragmentation Report
        Total    Average      Average     Total
        Files    File Blks    # Extents   Free Blks
        28637          32           1       54311
    blocks used for indirects: 8
    % Free blocks in extents smaller than 64 blks: 0.07
    % Free blocks in extents smaller than  8 blks: 0.01
    % blks allocated to extents 64 blks or larger: 0.00
    Free Extents By Size
           1:          1            2:          1            4:          1
           8:          0           16:          0           32:          1
          64:          0          128:          2          256:          1
         512:          1         1024:          2         2048:          1
        4096:          0         8192:          2        16384:          2

c=0;  while [[ $c -lt 29116 ]]; do let c=c+2; rm $c; done
fsadm -E /local
  Extent Fragmentation Report
        Total    Average      Average     Total
        Files    File Blks    # Extents   Free Blks
        29104          32           1      520167
    blocks used for indirects: 8
    % Free blocks in extents smaller than 64 blks: 89.54
    % Free blocks in extents smaller than  8 blks: 0.00
    % blks allocated to extents 64 blks or larger: 0.00
    Free Extents By Size
           1:          1            2:          1            4:          1
           8:          0           16:          0           32:      14555
          64:          2          128:          2          256:          1
         512:          1         1024:          2         2048:          1
        4096:          0         8192:          2        16384:          2

I have a lot of free 32K extents now. OK, the % of extents smaller than 64K has increased - pretty obvious since 32K is smaller than 64K. But what is the difference of having a lot of 1MB exntents free and a lot more 32KB extents free? And again, when can I see that my volume is fragmentated?

Thanks so far,

Marek

[mikebounds]

If you are trying to create a 5MB file then if you have 1MB sized free extents then you can write to 5 extents, but if you only have 32K extents free, then you will need to write to 160 extents - this will involve a lot more work and in particular the file will have to use indirect extents (see https://www-secure.symantec.com/connect/forums/output-fsadm).  But on saying this, if you wrote a GB file, then if you only had 1MB sized free extents, the file would need to write to over 1000 extents, so perhaps fragmentation should be a function of the size of the filesystem.  Also it would be useful if the fsadm defrag report said something like "your filesystem does/does not need defragmenting" with some easy indication of how badly it is fragmented, but it just seems to report on % of free block in extents, smaller then 8k, smaller than 64k, size of 64k and larger.

In theory if the filesystem is fragmented by the definition in the guide, then I think it should be defraged, but I did some tests and filesystems were not defraged when more than 5 percent of free space was in extents of less than 64 blocks and it didn't even always defrag when filesystem had more than 50 percent of free space in extents of less than 64 blocks (badly fragmented).  But the filesytem was defraged for filesystems where more than 1 percent of free space was in extents of less than 8 blocks (using fsadm -E -e -s /local)

Mike

[kosh2013]

It seems that fsadm does not really recognize a lot of free 32 blks extents as a problem. It must be smaller. So I formatted my volume again and created a lot of 1K files like my example before with the 32K files and deleted every second file. Then I ran fsadm -E /local:

 fsadm -E /local
  Extent Fragmentation Report
        Total    Average      Average     Total
        Files    File Blks    # Extents   Free Blks
        46799           1           1      921961
    blocks used for indirects: 8
    % Free blocks in extents smaller than 64 blks: 5.08
    % Free blocks in extents smaller than  8 blks: 5.08
    % blks allocated to extents 64 blks or larger: 0.00
    Free Extents By Size
           1:      46307            2:        245            4:          1

It shows clearly a lot of free 1 blks extents. Than I ran fsadm -e -d -s /local and fsadm -E /local to verify:

fsadm -E /local
  Extent Fragmentation Report
        Total    Average      Average     Total
        Files    File Blks    # Extents   Free Blks
        46799           1           1      922768
    blocks used for indirects: 0
    % Free blocks in extents smaller than 64 blks: 0.14
    % Free blocks in extents smaller than  8 blks: 0.01
    % blks allocated to extents 64 blks or larger: 1.67
    Free Extents By Size
           1:          2            2:          1            4:         13
           8:         23           16:         24           32:         21
          64:         22          128:        178          256:        177

Finally the extents were reorganized. So it really seems that fsadm can/will (who knows) only reorganize extents of a very small block size. Where 1 blks extent seems to be small enough to be reorginzed but not 32 blks extents.

Marek

[mikebounds · Accepted solution]

Yes, this is what I said in my last post - here were some of my tests (on RHEL 5.3, SF 5.1 - no SP):

With:

    % Free blocks in extents smaller than 64 blks: > 5%

    % Free blocks in extents smaller than  8 blks: < 1%

No defrag, but it should as % Free blocks in extents smaller than 64 blks > 5%

  Extent Fragmentation Report

        Total    Average      Average     Total

        Files    File Blks    # Extents   Free Blks

         3952          25           1       23748

    blocks used for indirects: 0

    % Free blocks in extents smaller than 64 blks: 24.81

    % Free blocks in extents smaller than  8 blks: 0.93

    % blks allocated to extents 64 blks or larger: 0.00

    Free Extents By Size

           1:          2            2:          1            4:         54   

           8:          1           16:          0           32:        177   

          64:         87          128:          2          256:          1   

 Pass 1 Statistics

             Extents    Reallocations   Ioctls                Errors

             Searched     Attempted     Issued     FileBusy   NoSpace     Total

 total           3962           0           0          0          0          0

  Extent Fragmentation Report

        Total    Average      Average     Total

        Files    File Blks    # Extents   Free Blks

         3952          25           1       23748

    blocks used for indirects: 0

    % Free blocks in extents smaller than 64 blks: 24.81

    % Free blocks in extents smaller than  8 blks: 0.93

    % blks allocated to extents 64 blks or larger: 0.00

    Free Extents By Size

           1:          2            2:          1            4:         54   

           8:          1           16:          0           32:        177   

          64:         87          128:          2          256:          1   

With:

    % Free blocks in extents smaller than 64 blks: > 5%

    % Free blocks in extents smaller than  8 blks: > 1%

fsadm now defrags:

  Extent Fragmentation Report

        Total    Average      Average     Total

        Files    File Blks    # Extents   Free Blks

         3917          25           1       23788

    blocks used for indirects: 0

    % Free blocks in extents smaller than 64 blks: 24.94

    % Free blocks in extents smaller than  8 blks: 1.09

    % blks allocated to extents 64 blks or larger: 0.00

    Free Extents By Size

           1:          2            2:          1            4:         64   

           8:          1           16:          0           32:        177   

          64:         87          128:          2          256:          1   

 Pass 1 Statistics

             Extents    Reallocations   Ioctls                Errors

             Searched     Attempted     Issued     FileBusy   NoSpace     Total

 total           3927         194         194          0          0          0

 Pass 2 Statistics

             Extents    Reallocations   Ioctls                Errors

             Searched     Attempted     Issued     FileBusy   NoSpace     Total

 total           3927          67          67          0          0          0

 Pass 3 Statistics

             Extents    Reallocations   Ioctls                Errors

             Searched     Attempted     Issued     FileBusy   NoSpace     Total

 total           3927           0           0          0          0          0

  Extent Fragmentation Report

        Total    Average      Average     Total

        Files    File Blks    # Extents   Free Blks

         4141          24           1       23784

    blocks used for indirects: 0

    % Free blocks in extents smaller than 64 blks: 14.16

    % Free blocks in extents smaller than  8 blks: 0.30

    % blks allocated to extents 64 blks or larger: 0.00

    Free Extents By Size

           1:          2            2:          1            4:         17   

           8:          2           16:          1           32:        102   

          64:         75          128:          4          256:          3   

BUT with:

    % Free blocks in extents smaller than 64 blks: > 5%

    % Free blocks in extents smaller than  8 blks: < 1%

sometimes fsadm does do a half-hearted attempt at defrag as note it only does 2 reallocations per pass, so only reduces Free blocks in extents smaller than 64 blks from 35.42% to 35.16% !

  Extent Fragmentation Report

        Total    Average      Average     Total

        Files    File Blks    # Extents   Free Blks

         3587          32           1       27552

    blocks used for indirects: 0

    % Free blocks in extents smaller than 64 blks: 35.42

    % Free blocks in extents smaller than  8 blks: 0.06

    % blks allocated to extents 64 blks or larger: 0.00

    Free Extents By Size

           1:          2            2:          1            4:          3   

           8:          0           16:          1           32:        304   

          64:         86          128:          2          256:          1   

 Pass 1 Statistics

             Extents    Reallocations   Ioctls                Errors

             Searched     Attempted     Issued     FileBusy   NoSpace     Total

 total           3600           2           1          0          0          0

 Pass 2 Statistics

             Extents    Reallocations   Ioctls                Errors

             Searched     Attempted     Issued     FileBusy   NoSpace     Total

 total           3600           2           1          0          0          0

 Pass 3 Statistics

             Extents    Reallocations   Ioctls                Errors

             Searched     Attempted     Issued     FileBusy   NoSpace     Total

 total           3600           2           1          0          0          0

 Pass 4 Statistics

             Extents    Reallocations   Ioctls                Errors

             Searched     Attempted     Issued     FileBusy   NoSpace     Total

 total           3600           2           1          0          0          0

 Pass 5 Statistics

             Extents    Reallocations   Ioctls                Errors

             Searched     Attempted     Issued     FileBusy   NoSpace     Total

 total           3600           2           1          0          0          0

  Extent Fragmentation Report

        Total    Average      Average     Total

        Files    File Blks    # Extents   Free Blks

         3587          32           1       27537

    blocks used for indirects: 0

    % Free blocks in extents smaller than 64 blks: 35.16

    % Free blocks in extents smaller than  8 blks: 0.06

    % blks allocated to extents 64 blks or larger: 0.00

    Free Extents By Size

           1:          3            2:          1            4:          3   

           8:          0           16:          0           32:        302   

          64:         87          128:          2          256:          1   

But other times I had % Free blocks in extents smaller than 64 blks at 75% and even 100% and fsadm did not do ANY defrag.

Perhaps someone from Symantec can comment why fsadm does not defrag when by vxfs manual definition the filesystem is fragmented (% Free blocks in extents smaller than 64 blks > 5%) and still doesn't when badly fragmented (% Free blocks in extents smaller than 64 blks > 50%).

Mike