Installation guide

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Glossary
This glossary defines common terms relating to file systems and storage used throughout the Storage
Administration Guide.
Defragmentation The act of reorganizing a file's data blocks so that they are more
physically contiguous on disk.
Delayed Allocation An allocator behavior in which disk locations are chosen when data is
flushed to disk, rather than when the write occurs. This can generally
lead to more efficient allocation because the allocator is called less
often and with larger requests.
Extended Attributes Name/Value metadata pairs which may be associated with a file.
Extent A unit of file allocation, stored in the file's metadata as an offset,
length pair. A single extent record can describe many contiguous
blocks in a file.
File System Repair (fsck) A method of verifying and repairing consistency of a file system's
metadata. May be needed post-crash for non-journalling file systems,
or after a hardware failure or kernel bug.
Fragmentation The condition in which a file's data blocks are not allocated in
contiguous physical (disk) locations for contiguous logical offsets
within the file. File fragmentation can lead to poor performance in
some situations, due to disk seek time.
Metadata Journaling A method used to ensure that a file system's metadata is consistent
even after a system crash. Metadata journalling can take different
forms, but in each case a journal or log can be replayed after a crash,
writing only consistent transactional changes to the disk.
Persistent Preallocation A type of file allocation which chooses locations on disk, and marks
these blocks as used regardless of when or if they are written. Until
data is written into these blocks, reads will return 0s. Preallocation is
performed with the fallocate() glibc function.
POSIX Access Control Lists
(ACLs)
Metadata attached to a file which permits more fine-grained access
controls. ACLS are often implemented as a special type of extended
attribute.
Quota A limit on block or inode usage of individual users and groups in a file
system, set by the administrator.
Stripe Unit Also sometimes referred to as stride or chunk-size. The stripe unit is
the amount of data written to one component of striped storage before
moving on to the next. Specified in byte or file system block units.
Stripe Width The number of individual data stripe units in striped storage
(excluding parity). Depending on the administrative tool used, may be
specified in byte or file system block units, or in multiples of the stripe
unit.
Stripe-aware allocation An allocator behavior in which allocations and I/O are well-aligned to
underlying striped storage. This depends on stripe information being