Missing capacity in a RAID 5 array?

[Jeff Morrow <jmorrow@jmorrow.org>]

I've created a software RAID 5 array consisting of 8 160 GB drives.  One 
of the drives had prior data I wanted to keep, so I first created a 
7-drive array, copied the data from the original drive to the array, 
then added the 8th drive using raidreconf.  Worked like a charm (took 
about 27 hours, though!)

The OS now reports that the total capacity on /dev/md0 is about 920 GB:

/dev/md0             923029884 172872252 750157632  19% /mnt/store

Now, even if we take into account that the drives are 160 decimal GB, 
which equates to 149 binary GB, my array should still have a total 
capacity somewhere around 7 x 149 = 1043 GB.

Am I really losing 120 GB to reserved space for RAID and/or filesystem 
accounting?  I feel like I'm missing something obvious here.  Can I get 
my full terabyte somehow?

Here's my /etc/raidtab:

raiddev /dev/md0
        raid-level 5
        nr-raid-disks 8
        nr-spare-disks 0
        persistent-superblock 1
        parity-algorithm left-symmetric
        chunk-size 32
        device /dev/hdc
        raid-disk 0
        device /dev/hdd
        raid-disk 1
        device /dev/hde
        raid-disk 2
        device /dev/hdg
        raid-disk 3
        device /dev/hdh
        raid-disk 4
        device /dev/hdi
        raid-disk 5
        device /dev/hdk
        raid-disk 6
        device /dev/hdf
        raid-disk 7

And here's my /proc/mdstat:

Personalities : [raid5]
md0 : active raid5 hdf[7] hdk[6] hdi[5] hdh[4] hdg[3] hde[2] hdd[1] hdc[0]
      1094035712 blocks level 5, 32k chunk, algorithm 2 [8/8] [UUUUUUUU]

unused devices: <none>

Thanks for the help.

Jeff Morrow



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