¿Por qué df muestra solo la mitad del tamaño de una matriz RAID10?

He creado una matriz RAID10 usando 4 unidades de 75G, para crear un almacenamiento de 150G.
Después de que todo terminó (incluida la sincronización inicial), todo se veía bien, excepto que la salida df -hmostró solo 73G de almacenamiento en el punto de montaje designado.

Detalles:

• La máquina es una instancia m1.large de Ubuntu 11.10 en Amazon EC2.
• Las 4 unidades son unidades EBS, cada una tiene un tamaño de 75G.
• La matriz RAID10 se creó con el siguiente script:

-

#!/bin/sh

disk1="/dev/sdh1"
disk2="/dev/sdh2"
disk3="/dev/sdh3"
disk4="/dev/sdh4"

echo "*** Verifying existence of 4 volumes $disk1, $disk2, $disk3 and $disk4"
if [ -b "$disk1" -a -b "$disk2" -a -b "$disk3" -a -b "$disk4" ]; then
    echo "# Found expected block devices."
else
    echo "!!! Did not find expected block devices.  Error."
    exit -1
fi
until read -p "??? - How big (in GB) are the disks (They should be the same size)?  " disk_size && [ $disk_size ]; do
    echo "Please enter a disk size."
done 

lv_size=$(echo "scale=2; $disk_size * 2.0" | bc)
echo "*** Assuming a per disk size of $disk_size gigs, will create a logical volume of $lv_size gigs, with $lv_size reserved for snapshots"

echo "*** Partitioning disks..."

echo "~ Partitioning $disk1"
echo ',,L' | sfdisk $disk1
echo "~ Partitioning $disk2"
echo ',,L' | sfdisk $disk2
echo "~ Partitioning $disk3"
echo ',,L' | sfdisk $disk3
echo "~ Partitioning $disk4"
echo ',,L' | sfdisk $disk4

sleep 6
echo "*** Creating /dev/md0 as a RAID 10"

/sbin/mdadm /dev/md0 --create --level=10 --raid-devices=4 $disk1 $disk2 $disk3 $disk4 

echo " ~ Allocating /dev/md0 as a physical volume."

/sbin/pvcreate /dev/md0

echo " ~ Allocating a Volume Group 'mongodb_vg'"

/sbin/vgcreate -s 64M mongodb_vg /dev/md0

echo " ~ Creating a Logical Volume 'mongodb_lv'"

num_extents=$(echo "$disk_size * 1000 / 64" | bc)

/sbin/lvcreate -l $num_extents -nmongodb_lv mongodb_vg

echo " ~ Formatting the new volume (/dev/mongodb_vg/mongodb_lv) with EXT4"

/sbin/mkfs.ext4 /dev/mongodb_vg/mongodb_lv

echo " ~ Done! Go ahead and mount the new filesystem.  Suggested FStab: "
echo " /dev/mongodb_vg/mongodb_lv /data ext4 defaults,noatime 0 0"

Este es el resultado que obtuve:

*** Verifying existence of 4 volumes /dev/xvdh1, /dev/xvdh2, /dev/xvdh3 and /dev/xvdh4
# Found expected block devices.
??? - How big (in GB) are the disks (They should be the same size)?  75
*** Assuming a per disk size of 75 gigs, will create a logical volume of 150.0 gigs, with 150.0 reserved for snapshots
*** Partitioning disks...
~ Partitioning /dev/xvdh1
Checking that no-one is using this disk right now ...
BLKRRPART: Invalid argument
OK

Disk /dev/xvdh1: 9790 cylinders, 255 heads, 63 sectors/track

sfdisk: ERROR: sector 0 does not have an msdos signature
 /dev/xvdh1: unrecognized partition table type
Old situation:   
No partitions found
New situation:   
Units = cylinders of 8225280 bytes, blocks of 1024 bytes, counting from 0

   Device Boot Start     End   #cyls    #blocks   Id  System
/dev/xvdh1p1          0+   9789    9790-  78638174+  83  Linux
/dev/xvdh1p2          0       -       0          0    0  Empty
/dev/xvdh1p3          0       -       0          0    0  Empty
/dev/xvdh1p4          0       -       0          0    0  Empty
Warning: no primary partition is marked bootable (active)
This does not matter for LILO, but the DOS MBR will not boot this disk.
Successfully wrote the new partition table

Re-reading the partition table ...
BLKRRPART: Invalid argument

If you created or changed a DOS partition, /dev/foo7, say, then use dd(1)
to zero the first 512 bytes:  dd if=/dev/zero of=/dev/foo7 bs=512 count=1
(See fdisk(8).)  
~ Partitioning /dev/xvdh2
Checking that no-one is using this disk right now ...
BLKRRPART: Invalid argument
OK

Disk /dev/xvdh2: 9790 cylinders, 255 heads, 63 sectors/track

sfdisk: ERROR: sector 0 does not have an msdos signature
 /dev/xvdh2: unrecognized partition table type
   Old situation:   
No partitions found
New situation:   
Units = cylinders of 8225280 bytes, blocks of 1024 bytes, counting from 0

   Device Boot Start     End   #cyls    #blocks   Id  System
/dev/xvdh2p1          0+   9789    9790-  78638174+  83  Linux
/dev/xvdh2p2          0       -       0          0    0  Empty
/dev/xvdh2p3          0       -       0          0    0  Empty
/dev/xvdh2p4          0       -       0          0    0  Empty
Warning: no primary partition is marked bootable (active)
This does not matter for LILO, but the DOS MBR will not boot this disk.
Successfully wrote the new partition table

Re-reading the partition table ...
BLKRRPART: Invalid argument

If you created or changed a DOS partition, /dev/foo7, say, then use dd(1)
to zero the first 512 bytes:  dd if=/dev/zero of=/dev/foo7 bs=512 count=1
(See fdisk(8).)  
~ Partitioning /dev/xvdh3
Checking that no-one is using this disk right now ...
BLKRRPART: Invalid argument
OK

Disk /dev/xvdh3: 9790 cylinders, 255 heads, 63 sectors/track

sfdisk: ERROR: sector 0 does not have an msdos signature
 /dev/xvdh3: unrecognized partition table type
Old situation:   
No partitions found
New situation:   
Units = cylinders of 8225280 bytes, blocks of 1024 bytes, counting from 0

   Device Boot Start     End   #cyls    #blocks   Id  System
/dev/xvdh3p1          0+   9789    9790-  78638174+  83  Linux
/dev/xvdh3p2          0       -       0          0    0  Empty
/dev/xvdh3p3          0       -       0          0    0  Empty
/dev/xvdh3p4          0       -       0          0    0  Empty
Warning: no primary partition is marked bootable (active)
This does not matter for LILO, but the DOS MBR will not boot this disk.
Successfully wrote the new partition table

Re-reading the partition table ...
BLKRRPART: Invalid argument
If you created or changed a DOS partition, /dev/foo7, say, then use dd(1)
to zero the first 512 bytes:  dd if=/dev/zero of=/dev/foo7 bs=512 count=1
(See fdisk(8).)  
~ Partitioning /dev/xvdh4
Checking that no-one is using this disk right now ...
BLKRRPART: Invalid argument
OK

Disk /dev/xvdh4: 9790 cylinders, 255 heads, 63 sectors/track

sfdisk: ERROR: sector 0 does not have an msdos signature
 /dev/xvdh4: unrecognized partition table type
Old situation:   
No partitions found
New situation:   
Units = cylinders of 8225280 bytes, blocks of 1024 bytes, counting from 0

   Device Boot Start     End   #cyls    #blocks   Id  System
/dev/xvdh4p1          0+   9789    9790-  78638174+  83  Linux
/dev/xvdh4p2          0       -       0          0    0  Empty
/dev/xvdh4p3          0       -       0          0    0  Empty
/dev/xvdh4p4          0       -       0          0    0  Empty
Warning: no primary partition is marked bootable (active)
This does not matter for LILO, but the DOS MBR will not boot this disk.
Successfully wrote the new partition table

Re-reading the partition table ...
BLKRRPART: Invalid argument

If you created or changed a DOS partition, /dev/foo7, say, then use dd(1)
to zero the first 512 bytes:  dd if=/dev/zero of=/dev/foo7 bs=512 count=1
(See fdisk(8).)  
*** Creating /dev/md0 as a RAID 10
mdadm: partition table exists on /dev/xvdh1 but will be lost or
       meaningless after creating array
mdadm: partition table exists on /dev/xvdh2 but will be lost or
       meaningless after creating array
mdadm: partition table exists on /dev/xvdh3 but will be lost or
       meaningless after creating array
mdadm: partition table exists on /dev/xvdh4 but will be lost or
       meaningless after creating array
Continue creating array? y
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.
 ~ Allocating /dev/md0 as a physical volume.

  Physical volume "/dev/md0" successfully created
 ~ Allocating a Volume Group 'mongodb_vg'
  Volume group "mongodb_vg" successfully created
 ~ Creating a Logical Volume 'mongodb_lv'
  Logical volume "mongodb_lv" created
 ~ Formatting the new volume (/dev/mongodb_vg/mongodb_lv) with EXT4
mke2fs 1.41.14 (22-Dec-2010)
Filesystem label=
OS type: Linux   
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=128 blocks, Stripe width=256 blocks
4800512 inodes, 19185664 blocks
959283 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=4294967296
586 block groups 
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
        32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
        4096000, 7962624, 11239424

Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 36 mounts or
180 days, whichever comes first.  Use tune2fs -c or -i to override.
 ~ Done! Go ahead and mount the new filesystem.  Suggested FStab:
 /dev/mongodb_vg/mongodb_lv /data ext4 defaults,noatime 0 0

Este es el resultado relevante de df -h:

Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/mongodb_vg-mongodb_lv
                       73G  180M   69G   1% /ebsRaid

Esta es la salida de mdadm --detail /dev/md0

/dev/md0:
        Version : 1.2
  Creation Time : Wed Feb 29 10:14:39 2012
     Raid Level : raid10
     Array Size : 157283328 (150.00 GiB 161.06 GB)
  Used Dev Size : 78641664 (75.00 GiB 80.53 GB)
   Raid Devices : 4
  Total Devices : 4
    Persistence : Superblock is persistent

    Update Time : Wed Feb 29 13:21:49 2012
          State : clean
 Active Devices : 4
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 0

         Layout : near=2
     Chunk Size : 512K

           Name : my.site.com:0  (local to host my.site.com)
           UUID : CENSORED
         Events : 19

    Number   Major   Minor   RaidDevice State
       0     202      113        0      active sync   /dev/xvdh1
       1     202      114        1      active sync   /dev/xvdh2
       2     202      115        2      active sync   /dev/xvdh3
       3     202      116        3      active sync   /dev/xvdh4

Esta es la salida de cat /proc/mdstat:

Personalities : [raid10] 
md0 : active raid10 xvdh4[3] xvdh3[2] xvdh2[1] xvdh1[0]
      157283328 blocks super 1.2 512K chunks 2 near-copies [4/4] [UUUU]

unused devices: <none>

EDITAR1:

Esta es la salida de lvdisplay -m:

  --- Logical volume ---
  LV Name                /dev/mongodb_vg/mongodb_lv
  VG Name                mongodb_vg
  LV UUID                SEpGth-cXd3-ZFhy-XLHo-T5pV-gEd1-Tgancs
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                73.19 GiB
  Current LE             1171
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     4096
  Block device           252:0

  --- Segments ---
  Logical extent 0 to 1170:
    Type                linear
    Physical volume     /dev/md0
    Physical extents    0 to 1170

EDITAR 2:

Esta es la salida de vgdisplay:

  --- Volume group ---
  VG Name               mongodb_vg
  System ID             
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  2
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                1
  Open LV               1
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               149.94 GiB
  PE Size               64.00 MiB
  Total PE              2399
  Alloc PE / Size       1171 / 73.19 GiB
  Free  PE / Size       1228 / 76.75 GiB
  VG UUID               CENSORED

Su grupo de volúmenes no está utilizando la totalidad de las extensiones creadas para él:

VG Size               149.94 GiB
PE Size               64.00 MiB
Total PE              2399
Alloc PE / Size       1171 / 73.19 GiB
Free  PE / Size       1228 / 76.75 GiB

Puede agregar más extensiones con el siguiente comando:

lvextend -l +100%FREE /dev/mongodb_vg/mongodb_lv /dev/md0

LEA LA PÁGINA HOMBRE ANTES DE ESCRIBIR ESTO

Este comando extendería el grupo de volúmenes para usar todas las extensiones GRATUITAS que quedan (también puede seleccionar menos si desea mantener algunas extensiones libres). Utilizará las extensiones en md0 para hacer esto.

Luego puede cambiar el tamaño de la partición en línea usando:

resize2fs  /dev/mongodb_vg/mongodb_lv

Y debería decir que está cambiando de tamaño en línea. Creo que esto resolverá su problema, pero lea las páginas de manual y comprenda lo que están haciendo antes de intentarlo. No soy responsable de que destroces tus discos.

Por otro lado, RAID en un EBS y luego lvm en la parte superior parece un nivel innecesario de virtualización de los discos. No mejorará el rendimiento ni la seguridad de los datos al agregar RAID adicional. LVM ya duplica o raya si recuerdo correctamente. Aunque técnicamente puede ejecutar LVM en RAID en LVM en RAID hasta el infinito, no estoy seguro de que esté ganando mucho al hacerlo (aunque estoy más que feliz de que me señalen que está equivocado en esto).