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    Why File system is showing Less space than the actual disk



    Query:  Why /storage FS is showing Only 4 TB of Storage, where From storage 5 TB space is allocated.

    /dev/md0              4.0T  2.7T  1.4T  67% /storage




    1.   Following disks are assigned to the Server with 2.5 TB Each.
    Disk /dev/sda: 2498.0 GB, 2498090418176 bytes
    Disk /dev/sdb: 2497.6 GB, 2497603174400 bytes
    2.   However the Disks are formatted with fdisk, which supports maximum size of 2.2 TB.
    Disk /dev/sda1: 2199.0 GB, 2199020350464 bytes
    Disk /dev/sdb1: 2199.0 GB, 2199020350464 bytes
    3.   We have RAID(md0) on this 2 disks, so Logically we should have 4.4 TB Space for File system.
    /dev/md0:
            Version : 0.90
      Creation Time : Wed Jan 14 12:05:53 2009
         Raid Level : raid0
         Array Size : 4294961152 (4095.99 GiB 4398.04 GB)
       Raid Devices : 2
      Total Devices : 2
        Update Time : Wed Jan 14 12:05:53 2009
              State : clean
    Active Devices : 2
    Working Devices : 2
    Chunk Size : 256K
    UUID : a70ea626:0a5c99d8:8fe9e169:8540d711
     Events : 0.1
    Number   Major   Minor   RaidDevice State
      0       8        1        0      active sync   /dev/sda1
      1       8       17        1      active sync   /dev/sdb1

    4.   However Metadata version 0.90 has following Limitations, so we cannot use  beyond 4 TB Storage space.
    Version 0.90 Metadata Limitations:

        28 devices maximum in one array
        each device may be a maximum of 2 TiB in size
        No support for bad-block-management


    Solution/Recommendations For using Full capacity:
    1.   We need to back up the data.
    2.   Delete the RAID.
    3.   Format the disk with parted.
    4.   Update the OS to Kernel Version 2.6.27 or Higher     
    And/OR
    5.   Create the RAID with updated metadata version 1 or 3 (It is latest)
    6.   Then create the File system.
    7.   Restore the Data.
     

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