Solaris and Disks

Block- vs Extent-based Allocation

Disk blocks are allocated as they are used, which means that a minimal number of filesystem blocks are allocated to a file in an attempt to conserve storage space. When a file is extended, blocks are allocated from a free block map, so that blocks are sometimes allocated in a random order. Random block allocation can be avoided by optimizing the block allocation policy so that it attempts to allocate a sequential series of blocks.

The block allocation scheme must also write information about where each new block is allocated every time the file is extended. If the file is being extended on elbock at a time, a lot of extra disk I/O will be required to write the filesystem block structure information. Filesystem block structure information is known as metadata. Filesystem metadata is always written synchronously to the storage device, which means operations that change the size of a file need to wait for each metadata operation to complete.

Extent-based filesystems allocate disk blocks in large groups at a single time, which forces sequential allocation. As a file is written, a large number of blocks are allocated, after which writes can occur in large groups or clusters of sequential blocks. Filesystem metadata is written when the file is first created. Subsequent writes within the first allocation extent of blocks do not require additional metadata writes (until the next extent is allocated).

Filesystem Size

Variable Block Sizes

ACLs

• setfacl(1)
• getfacl(1)

Logging filesystems