Re: DB question
Re: DB question
- Subject: Re: DB question
- From: Jeff Schmitz <email@hidden>
- Date: Tue, 17 Mar 2009 11:29:33 -0500
Thanks Miguel,
Very informative. I'm using Frontbase. I cleaned out a lot of unneeded rows in several tables in hopes that it would speed by pre-fetching. Sounds like most of the extra info produced by this will remain on the disc. Is there a size of the database where you may want to "vacuum"? I'm setting at about 200 Meg saved for a uncompressed backup using the Frontbase "live" backup method.
Jeff
On Tuesday, March 17, 2009, at 08:04AM, "Miguel Arroz" <email@hidden> wrote:
>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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