mkfs oddity

[Lennart Sorensen]

On Mon, Mar 02, 2009 at 04:57:53PM -0500, D. Hugh Redelmeier wrote:
> I've got a really big disk for MythTV (1.5T!).  Two, actually.  Mind
> you, the reports of problems with these Seagate drives make me regret
> buying them.  But that's not why I'm writing.
> 
> I'm creating a really big partition and I decided that since Myth
> files are really big, the file system needn't have a lot of inodes.
> 
> As you can see, I asked for 100000 inodes (probably 40 times more than
> I need, but a lot fewer than the default of almost 90 million).
> 
> So why did it choose to give me 174864 inodes?
> 
> (Ubuntu 8.10 + MythUbuntu packages + updates)
> 
>     $ sudo mkfs.ext3 -N 100000 /dev/sdf6
>     mke2fs 1.41.3 (12-Oct-2008)
>     Filesystem label=
>     OS type: Linux
>     Block size=4096 (log=2)
>     Fragment size=4096 (log=2)
>     174864 inodes, 358090850 blocks
>     17904542 blocks (5.00%) reserved for the super user
>     First data block=0
>     Maximum filesystem blocks=4294967296
>     10929 block groups
Note this ^

>     32768 blocks per group, 32768 fragments per group
>     16 inodes per group
Note this ^

>     Superblock backups stored on blocks: 
> 	    32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208, 
> 	    4096000, 7962624, 11239424, 20480000, 23887872, 71663616, 78675968, 
> 	    102400000, 214990848
> 
>     Writing inode tables: done                            
>     Creating journal (32768 blocks): done
>     Writing superblocks and filesystem accounting information: done
> 
>     This filesystem will be automatically checked every 33 mounts or
>     180 days, whichever comes first.  Use tune2fs -c or -i to override.
>     $

10929*16=174864

The number of inodes per group probably has to be a power of 2, so 16
was the closest higher value to what you requested (which would have
resulted in about 10 or 11 inodes per group).

-- 
Len Sorensen
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