It is continuous range of blocks large enough to contain sb.s_inodes_per_group * sb.s_inode_size bytes.Īs for the ordering of items in a block group, it is generally established that the super block and the group descriptor table, if present, will be at the beginning of the block group. The location of the inode table is given by grp.bg_inode_table_*. By default, a filesystem is allowed to increase in size by a factor of 1024x over the original filesystem size. Note also that when the filesystem is freshly formatted, mkfs will allocate "reserve GDT block" space after the block group descriptors and before the start of the block bitmaps to allow for future expansion of the filesystem.
If the group does not have a redundant copy, the block group begins with the data block bitmap. Redundant copies of the superblock and group descriptors are written to some of the block groups across the disk in case the beginning of the disk gets trashed, though not all block groups necessarily host a redundant copy (see following paragraph for more details).
#Ext2 volume manager disappear driver
The ext4 driver primarily works with the superblock and the group descriptors that are found in block group 0.
For all other block groups, there is no padding. However, if for some reason the block size = 1024, then block 0 is marked in use and the superblock goes in block 1. The superblock will start at offset 1024 bytes, whichever block that happens to be (usually 0). The layout of a standard block group is approximately as follows (each of these fields is discussed in a separate section below):įor the special case of block group 0, the first 1024 bytes are unused, to allow for the installation of x86 boot sectors and other oddities. It's not clear what happens with larger filesystems. Files with extents must be placed within the first 2^48 blocks of a filesystem. files using block maps) must be placed within the first 2^32 blocks of a filesystem. By default a filesystem can contain 2^32 blocks if the '64bit' feature is enabled, then a filesystem can have 2^64 blocks.Ĥ,398,314,962,956 (really 2^32 due to field size limitations) 64KiB blocks on a i386 which only has 4KiB memory pages). You may experience mounting problems if block size is greater than page size (i.e. Block size is specified at mkfs time and typically is 4KiB. Blocks are in turn grouped into larger units called block groups. A block is a group of sectors between 1KiB and 64KiB, and the number of sectors must be an integral power of 2. HOWEVER, all fields in jbd2 (the journal) are written to disk in big-endian order.Įxt4 allocates storage space in units of "blocks".
The number of block groups is the size of the device divided by the size of a block group.Īll fields in ext4 are written to disk in little-endian order. With the default block size of 4KiB, each group will contain 32,768 blocks, for a length of 128MiB. The size of a block group is specified in sb.s_blocks_per_group blocks, though it can also calculated as 8 * block_size_in_bytes. To reduce performance difficulties due to fragmentation, the block allocator tries very hard to keep each file's blocks within the same group, thereby reducing seek times.
#Ext2 volume manager disappear series
When referenced in preformatted text blocks, sb refers to fields in the super block, and inode refers to fields in an inode table entry.Īn ext4 file system is split into a series of block groups. For the sake of convenience, the logical block size will be referred to as $block_size throughout the rest of the document. Throughout this document, disk locations are given in terms of these logical blocks, not raw LBAs, and not 1024-byte blocks.
Generally, the block size will be 4KiB (the same size as pages on x86 and the block layer's default block size), though the actual size is calculated as 2 ^ (10 + sb.s_log_block_size) bytes. Ext4 divides a storage device into an array of logical blocks both to reduce bookkeeping overhead and to increase throughput by forcing larger transfer sizes.
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