Zsolt,

> We have FB 2.5.2 SS running on Win2003/x32 and Win2008/x64 servers.
> around 50 - 150 user connects to de databases .

I would recommend that you move to Classic engine, your system will perform better with that number of users.

> Every now and then some of our clients' FB server service freezes at the time
> we issue an ALTER DATABASE BEGIN BACKUP or start an nbackup session.
> After the freeze the FB service stops responding, and only hard stopping the
> process helps.

The "freeze" is the result of 2 items.

1 - The SuperServer engine is a single thread which provides pseudo-multi-threading for connections.

2 - NBackup is a very disk IO intensive.

It is very easy for NBackup to consume all available engine and server disk resource while is it running (changes have been made to try and make it "friendlier" to simultaneous operations, but there is only so much which can be done)

Given your database size and the number of remote connections, I would strongly recommend changing to Classic engine. It will allow for more consistent responses for the remote connections.

> I couldn't identify any problem at the database or environment level. Some
> 10057 and 10061 error can be found in the firebird log before the freeze, but
> these messages are quite common even the system and the nbackup
> sessions otherwise working well.

The errors are unrelated to the "freeze"

> The database sizes are around 10 - 30 GB
> and are fully backuped and restored at every weekend.

Why are you performing regular backup/restore?


Sean