FIle and Block size relationships ...

[Mike Reinehr]

On Tuesday 06 June 2006 11:57 am, Ben Duncan wrote:
> Ok, I am busily writing my MVISAM package, when I noticed in some intense
> reading about the lINUX file system - file size / block size on files.
>
> I created a file with exactly 512 BYTES (It is my FILE header FWIW).
> I did a stat on it and it shows:
>
> bend at bns:mvisam -> stat test.dat
>    File: `test.dat'
>    Size: 512             Blocks: 8          IO Block: 4096   regular file
> Device: 821h/2081d      Inode: 71290       Links: 1
> Access: (0755/-rwxr-xr-x)  Uid: (  500/    bend)   Gid: (  500/  appgen)
> Access: 2006-06-06 10:20:15.000000000 -0500
> Modify: 2006-06-06 08:46:23.000000000 -0500
> Change: 2006-06-06 08:46:23.000000000 -0500
>
> Does this mean that my FILE is really using 4096 BYTES of space
> and if I go over 4096 bytes, each "extension" will cost me
> 4096 byes (i.e 4100 byes file will really occupy 8192 bytes) ?
>
> I need to know, so I can tune my buffer/cache routines ..
> Or, if anyone has any suggestion on block size / cache
> I would appreciate it ...
>
> Thanks ..

Ben,

I'm afraid so. Here's an excerpt from man mke2fs:

> -b block-size 
> Specify the size of blocks in bytes. Valid block size vales are 1024, 2048
> and 4096 bytes per block. If omitted, mke2fs block-size is heuristically
> determined by the file system size and the expected usage of the filesystem
> (see the -T option). If block-size is negative, then mke2fs will use
> heuristics to determine the appropriate block size, with the constraint that
> the block size will be at least block-size bytes. This is useful for certain
> hardware devices which require that the blocksize be a multiple of 2k.

You can use 'stat -f /dev/hd... ' to check your file system specifications.

Cheers!

cmr

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