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2TB OS limit

Discussion in 'Storage & Backup' started by rowan194, Sep 13, 2009.

  1. rowan194

    rowan194 Member

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    I've seen a few articles recently that suggest HD capacity growth may slow a little due to the 2TB "limit." Is this a Windows only thing or some fundamental barrier related to partitions, integer bits etc?
     
  2. terrastrife

    terrastrife Member

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    its a limitation of MFT, tis why we use GPT.
     
  3. oupimiquo

    oupimiquo Member

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    MBR, not MFT :p
     
  4. username_taken

    username_taken Member

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    MFT not MBR

    ( Master File Table not Master Boot Record )
     
  5. oupimiquo

    oupimiquo Member

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    No, it really is the MBR that limits you to 2 TB. The MFT is a NTFS-specific thing and has nothing to do with the maximum size of the partition (or disk, or NTFS volume size).
     
  6. terrastrife

    terrastrife Member

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    why would the boot section of a hdd have anything to do with it? thats for initally booting your windows install.
     
  7. dakiller

    dakiller (Oscillating & Impeding)

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    It is hardly a problem
    [​IMG]
     
  8. oupimiquo

    oupimiquo Member

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    The MBR also contains the partition information for non-GPT partitions. The start and end of each partition is stored in a 32 bit value, in units of sectors. So the last sector of an non-GPT partition must be at or below sector 2^31 - 1, which is 2 TB (in binary prefixes, 2.2 TB in HDD-manufacturer-speak).

    edit: Slight mistake above. The values stored in the MBR (and extended boot records) are the start sector (LBA) and the number of sectors. So with only a MBR, you can actually have just shy of 4 TB, made up by two partitions of 2 TB each. However, with extended partitons the start LBA of the partition is relative to the end LBA of the previous partition, so you can have as many 2 TB extended partitions as needed to use up the whole disk.
     
    Last edited: Sep 14, 2009
  9. terrastrife

    terrastrife Member

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    that applies to 4k allocation unit size. a larger unit size with MFT will allow a larger partition, but most of us dont do that :)

    same existing problem that we had with fat. bigger cluster, bigger partition :)
     
  10. oupimiquo

    oupimiquo Member

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  11. terrastrife

    terrastrife Member

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    how about a quick summary, i hate wiki, not to mention its still too early for me and long sections of black on white give me a headace :)

    the disk format is obviously a lower level than the partition format, but what youre saying is that mbr is limited to 2TB, but what im saying is that MBR can support over 2TB if the paritition unit wize is greater than 4k with NTFS (or exFAT).

    Im getting tired :(
     
    Last edited: Sep 14, 2009
  12. oupimiquo

    oupimiquo Member

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    MBR:
    [] Defines up to 4 primary partitions on the disk. Start and size of each primary partition is specified in (512-byte) sectors, hence the 2 TB partition limit for MBR partitions.
    [] The MBR can also have "extended" partitions, which use the same structure (more or less) so are also limited to 2 TB each. However, you can have as many as you want.

    MFT:
    [] Contains the metadata (filename, first cluster, etc) for each file in the partition. Is in fact just a file itself (called "$MFT"), so can grow and shrink as required as files are added/deleted. Has nothing to do with partitioning, and imposes no limitations on the size of a partition. The only limit it imposes is on the number of files (since each file entry takes up space, you eventually will run out of room on the partition).

    NTFS:
    [] Has no realistic limitations on volume sizes, number of files, or indivisual file sizes (all in the order of 2^64). Pre-SP1 XP can't handle NTFS volumes greater than 2^32 clusters though (16 TB if using the default 4 KB clusters). Post-SP1 XP can handle anything up to 2^64 clusters though.

    GPT:
    [] Like the MFT, but better :) Uses 64 bit quantities for the starting LBA and sizes, so has no real limits. Plus allows for more parititions, GUIDs, etc.

    Partition sizes are always in units of 512 bytes (the smallest read/write unit for (S)ATA disks). Filesystem cluster sizes are a completely different thing.
     
    Last edited: Sep 14, 2009
  13. terrastrife

    terrastrife Member

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    welcome to my morning, i knew something didnt feel right :p

    haha (i start work at 1pm)
     
  14. wilsontc

    wilsontc Member

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    On Windows XP you need to use greater than 4k blocks.

    Boot volumes probably have certain restrictions, but on systems > Windows XP (Vista, Windows Server, Unix systems etc) I am not aware of any additional setting that is required.

    Make sure your disk controller is 48 bit or higher though! Cheap controllers may only be 32 bit, and won't create arrays past a certain size, regardless of file system limitations.
     
  15. oupimiquo

    oupimiquo Member

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    I know the feeling :)
     
  16. jboles

    jboles Member

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    MBR uses a 32-bit number to store the partition size. The biggest partition that can be encoded in the MBR using a 32-bit number and the standard hard disk sector size of 512 bytes is 2tb.

    2^32 (# of sectors) * 2^9 (bytes per sector)
    = 2^41 bytes
    = 2 * 2^40 bytes
    = 2 tb

    Can't move from MBR to GPT (and therefore can't have boot volumes bigger than 2TB) until there is widespread migration from BIOS to EFI. BIOS's are historically hardcoded to read an MBR and boot that. That is not going to change either because of backward compatibility -- it would break all current OS's that put their boot sector at that fixed position at the start of the drive so that the BIOS will pick it up.

    incorrect, none of these limitations have anything to do with MFT, block size, NTFS or Windows. The partition size is stored in the MBR as the # of 512-byte hard disk sectors because that's the only measure that is file system independent. The MBR has no information about how file systems are laid out within the partitions defined in it, nor what block sizes they are using (or even if they are using blocks).

    It's theoretically possible that the MBR could be used with parititons bigger than 2tb by changing the sector size upwards from 512 bytes (hard disks support this IIRC) but 512 bytes has been the standard 'forever' and it would break so much software it wouldn't be funny.
     
    Last edited: Sep 18, 2009

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