how are files allocated on nilfs?

how are files allocated on nilfs?

[dexen deVries]

Hi,

a possibly naive question: how are files allocated on nilfs?

Asking, because I've expected a large-ish (128MB) file to be mostly 
continuous, modulo segment headers. But it doesn't seem so. A small test to 
narrow down the question:
 0) created a new, empty, nilfs, on a separate partition (about 1GB size)
 1) created a file with dd if=/dev/zero of=a.bin bs=$((128*1024*1024)) count=1
 2) ran hdparm --fibmap a.bin
output:
 byte_offset  begin_LBA    end_LBA    sectors
         0        231      16446      16216
     8302592      16503      29094      12592
    14749696      29239      32830       3592
    16588800      32895      49214      16320
    24944640      49255      58158       8904
    29503488      58327      65598       7272
    33226752      65663      81982      16320
    41582592      82007      87230       5224
    44257280      87415      98366      10952
    49864704      98431     114750      16320
    58220544     114759     116302       1544
    59011072     116503     131134      14632
    66502656     131191     145374      14184
    73764864     145519     147518       2000
    74788864     147583     163902      16320
    83144704     163951     174446      10496
    88518656     174607     180286       5680
    91426816     180351     196670      16320
    99782656     196703     203518       6816
   103272448     203695     213054       9360
   108064768     213119     229438      16320
   116420608     229455     232590       3136
   118026240     232783     245822      13040
   124702720     245887     261662      15776


while the first chunk of file fills almost whole segment, several subsequent 
chunks are much smaller than segment. Why is it so?


Regards,
-- 
dexen deVries

[[[↓][→]]]

``In other news, STFU and hack.''
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http://news.ycombinator.com/item?id=2308816
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Re: how are files allocated on nilfs?

[Ryusuke Konishi]

Hi,
On Wed, 4 May 2011 18:02:16 +0200, dexen deVries wrote:
> Hi,
> 
> a possibly naive question: how are files allocated on nilfs?
> 
> Asking, because I've expected a large-ish (128MB) file to be mostly 
> continuous, modulo segment headers. But it doesn't seem so. A small test to 
> narrow down the question:
>  0) created a new, empty, nilfs, on a separate partition (about 1GB size)
>  1) created a file with dd if=/dev/zero of=a.bin bs=$((128*1024*1024)) count=1
>  2) ran hdparm --fibmap a.bin
> output:
>  byte_offset  begin_LBA    end_LBA    sectors
>          0        231      16446      16216
>      8302592      16503      29094      12592
>     14749696      29239      32830       3592
>     16588800      32895      49214      16320
>     24944640      49255      58158       8904
>     29503488      58327      65598       7272
>     33226752      65663      81982      16320
>     41582592      82007      87230       5224
>     44257280      87415      98366      10952
>     49864704      98431     114750      16320
>     58220544     114759     116302       1544
>     59011072     116503     131134      14632
>     66502656     131191     145374      14184
>     73764864     145519     147518       2000
>     74788864     147583     163902      16320
>     83144704     163951     174446      10496
>     88518656     174607     180286       5680
>     91426816     180351     196670      16320
>     99782656     196703     203518       6816
>    103272448     203695     213054       9360
>    108064768     213119     229438      16320
>    116420608     229455     232590       3136
>    118026240     232783     245822      13040
>    124702720     245887     261662      15776
> 
> 
> while the first chunk of file fills almost whole segment, several subsequent 
> chunks are much smaller than segment. Why is it so?

You can see what's happening easily by using 'dumpseg' tool.

I guess it's because checkpoint is periodically created and some
modified blocks of meta-data are inserted for the checkpoint creation.

For your information, disk layout image of nilfs is found on the page
10 of the following slides:

  http://www.nilfs.org/papers/jls2009-nilfs.pdf


Regards,
Ryusuke Konishi
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