mkfs oddity

[Christopher Browne]

On 2009-03-02, Lennart Sorensen <lsorense-1wCw9BSqJbv44Nm34jS7GywD8/FfD2ys at public.gmane.org> wrote:
> 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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