Hi!
Jeff, this depends on the DB engine you are using.
On Postgresql, you have two sets of files. The base files, which
contains the "stable" data, and the write-ahead logs (WALs). Every
time you do an operation, whatever that is (including delete), the
operation is written to the WALs. Data in WALs is never overwritten,
only appended.
From time to time (namely, when a WAL file is full - usually 16 MB,
and when no active transaction is using that file) the data on the
WAL
files is copied back to the base files, and the WALs are discarded.
Even so, this might not release unused space. To do that, you have
to run VACUUM queries. Note that running VACUUM on a table (or the
entire DB) is a very heavy process, specially for the disks, so it
will kill your DB performance while it's being done. Anyway, you
don't
really need to, because PostgreSQL will be able to use the unused
space in a smart way, and it automatically vacuums some tables from
time to time.
Note also that each DB has it's own procedure for making a correct
backup (ie, a backup you are 100% sure it's consistent and will work
if recovery is needed). Here's what you should do for PostgreSQL:
http://www.postgresql.org/docs/8.1/interactive/backup.html
. Note that, on most DBs (if not all), simply copying the DB files
for another drive is NOT a safe way to backup, your backup will most
probably be corrupt. You have to make the backups in accordance with
the DB engine itself.
Yours
Miguel Arroz
On 2009/03/17, at 09:02, Jeff Schmitz wrote:
This may be a dumb question, but why after deleting a bunch of EOs
is my database backup larger than before? Looking at the data in
the database, the data corresponding to the EOs does seem to be gone
now.
Jeff
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