Subject RE: [Firebird-Architect] WAL and JOURNAL
Author Leyne, Sean
> How does this differ from mirroring?

I believe you meant to say "How does this differ from shadowing?"

Most significantly, it differs as follows:

- Shadowing works by applying page changes, in sync with the live
database, which means that shadow I/O performance, can significantly
affect the live database performance, due to operation latency. This
means that shadowing to any other computer than the master server is
'problematic' with respect to performance.

Mirroring works by building a list of page which have changed since a
point in time and then build an incremental update package to be applied
to the slave database, accordingly normal database performance is not
significantly/measurably affected.

- Shadowing requires full-time/online access to the slave database. In
fact, if a shadowing connection is lost, the shadow 'set' is broken.

Mirroring the slave database can be updated in a 'batch' mode, without a
full-time connection to the master database. Further, mirroring will
support multiple slave databases, each with its own database 'state'