You partition a 4 TB disk with an MBR partition table and find that, however you arrange the partitions, about half the disk cannot be addressed. Which property of the MBR scheme causes that ceiling?
LPIC-1 Exam 101-500, objective 104. Devices, Linux filesystems, filesystem hierarchy standard hard
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The options
Correct Each MBR partition entry stores its start and length as 32-bit LBA sector counts, which with 512-byte sectors tops out at 2 TiB.
Correct. The 32-bit fields can count at most 2^32 sectors, and 2^32 x 512 bytes is 2 TiB, so no MBR partition can begin or end beyond that point.
Not correct An MBR table holds only four primary partitions, so a large disk runs out of entries before it runs out of space.
Wrong as the cause. The four-entry limit is real, and extended plus logical partitions work around it, but the number of partitions has nothing to do with the capacity ceiling.
Not correct The ext4 filesystem cannot exceed 2 TiB, so the remaining space cannot be formatted.
Wrong. ext4 supports filesystems far larger than 2 TiB; the limit here belongs to the partition table, not to any filesystem placed inside a partition.
Not correct The boot sector is only 512 bytes, so it cannot describe more than 2 TiB of disk.
Wrong reasoning. The 512-byte sector is why the partition table has room for just four entries, but the capacity limit comes from the width of the LBA fields inside those entries.
Why
An MBR partition entry is 16 bytes and records the first LBA and the sector count as 32-bit values, which caps any addressable location at 2^32 sectors. On a conventional 512-byte-sector disk that is 2 TiB; disks with 4096-byte sectors push the same arithmetic to 16 TiB. GPT exists precisely to remove this limit: it uses 64-bit LBAs and, by default on Linux tools, 128 partition entries, which is why any disk larger than 2 TiB is partitioned with GPT.
Where this comes from
- Cited
- manual page fdisk(8)
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