"There has been much hype about journaled filesystems
for GNU/Linux in recent months, and Debian Planet has hosted its
fair share of articles concerning various alternative filesystems.
In this series of articles we will take you through these
filesystems, giving background information, history and tips.
Furthermore in each article we will install and use the filesystem
covered and provide benchmarks (which should only used in
comparison with those disclosed within other articles within this
But first a bit about filesystems in general. Your files and
data are stored in sections of disk known as blocks. The drive head
on the disk knows how to find and read these blocks and return them
to the CPU for processing. However, these blocks are not
particularly useful to read and write to at a raw level, so there
is a filesystem which provides the user with a way of managing his
or her files. The filesystem presents a hierarchical structure to
the user. When the user attempts to read a file, the filesystem
reads the blocks from the drive which the filesystem maps to.
Journalled filesystems are designed to help protect the data
stored on them. In general with a traditional filesystem, when you
perform write actions on a device, the data to be written is stored
in buffers and then only written when the filesystem is unmounted,
when the system load is suitable or when a user uses sync. However
if a power failure occurs before this data has been written to disk
or the drive was not unmounted, the filesystem maybe in an
incomplete state. The usual solution to this problem is to run a
filesystem checker such as fsck on these filesystems to remove any
inconsistencies. However as drive capacities become larger, this
lengthy operation is becoming annoying and costly , particularly to
those who need high availability. The solution is journalled
filesystems. Journalled filesystems keep a log of all pending
writes to be carried out on the drive. As these writes are carried
out the journal is flushed, keeping the disk up to date. If a power
failure occurs or if the drive is not unmounted cleanly, the
journal is read and the actions listed in it are "replayed", this
is much quicker than doing a filesystem check each time."
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