SQL 101 - Understanding Transaction logs


I posted this in the microsoft.public.sqlserver.server newsgroup (response to questions about log file space):

If you build a bookshelf (physical .ldf file) and fill it up with books (transactions), its full.
If you loan 5 books to a friend (backup the t-log), there is space available on the shelf, but the shelf size didn't change, correct?

If you buy another book (DML transaction), it goes where one of the others was.
If you fill up your bookshelf and then buy 3 more books, your only choices (besides stacking) are to expand the size of the shelf (grow the physical .ldf file) or add a 2nd one (MyDB_Log2.ldf) to the wall. Or return the books (failed transaction).

Chopping off the end of the bookshelf (Shrinking the .ldf file) makes no sense, nor does making a shelf that can hold 1000 books, when you'll never have more than 100 there....wasted wall space (disk).

Hope that helps.

Kevin3NF