Radek Stachowiak wrote in a message to Mike Bilow:
 MB> of a pig anyway, but it is probably not fair to evaluate it on
 MB> performance when its stated design goal of journaling will have an
 MB> inherent performance penalty;
 RS> What does journaling means?
"Journal" comes from the root which means "daily."  In financial bookkeeping, 
a "journal" is a log of all transactions which take place throughout the day, 
and which are then added up and summarized to the ledger as a single daily 
transaction.
By analogy, a "journaling" filesystem or database is one in which each change 
is recorded as it is requested.  Then, every so often, the entries in the 
journal will be either "committed" or "rolled back."  The goal is to 
guarantee that, if some sort of crash occurs, the filesystem will always be 
in a stable state.  Some of the uncommitted entries might be lost, but you 
can always revert to the "last known good" point and recover the data as if 
it had never been modified.  This way, you might lose some recent changes, 
but everything else will be maintained with proper integrity, and whole 
groups of related changes will either be kept or lost together.
A common situation where this is important is in a database which consists of 
a number of distinct tables, and each table might be stored in a separate 
master file with a number of index files.  In this sort of environment, a 
single change to the database might require modifying a number of data and 
index files, and the integrity of the data depends upon the files all being 
updated together.  With a journaling filesystem, you are guaranteed that, if 
an error or a crash occurs after some but not all of the files have been 
modified, then you can return all of the files to the state they were in 
before any of them were modified.
 
-- Mike
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