Question on Log Backup and NORECOVERY

Dear experts,
I am confused. I always thought that using NORECOVERY was essential when
restoring multiple transactional log backups in a row and I always thought
this was due to the fact that without NORECOVERY the transactional log
backups wouldn't "fit" to each other, because of potential rollbacks
happening in recovery. E.g. I have a tlb (transactional log backup, for
brevity's sake) ending at LSN 124. I restore with recovery and transaction
106 isn't commited before LSN 124, so it gets rolled back. Now I try to
restore the tlb beginning at LSN 125 and lo and behold, it doesn't work
because transaction 106 is commited at LSN 145 after all! I thought this is
why you have to use NORECOVERY.
BUT: There is a sentence in Solid Quality Learning's fine publication "SQL
Server 2005 Implementation and Maintenance" that made me think otherwise
(pg. 419): "A log backup backs up the active log. It starts at the Log
Sequence Number (LSN) at which the previous log backup completed. SQL Server
then backs up all subsequent transactions UNTIL THE BACKUP ENCOUNTERS AN
OPEN TRANSACTION." (emphasis mine)
Now if this is so, why is there any need for a rollback after restoring a
tlb with recovery anyway? All transactions included in the tlb are not open
(i.e. commited or rolled backed), so the worst thing that could happen is
the need for a rollforward, in case the db isn't consistent with the tlb. So
why do we need NORECOVERY?
Somewhat related bonus question: What portion of the log is exactly backed
up when I do a FULL backup? It's only the part after the oldest open
transaction, right?
Thank you a lot
Nils LoeberHi Nils
This is not correct, but I appreciate your recognition of the quality of the
document as a whole, even though there are some errors.
Books Online is correct in stating that during a log backup:
the log is backed up from the last successfully executed log backup to
the current end of the log.
I have no idea where the comment about open transaction came from.
When you do a full backup, SQL Server records the current LSN when the full
backup starts. When the full backup is over, the new current LSN is records.
All the log records between the two recorded LSNs are then backed up. So it
basically captures all other changes that were going on while the backup was
taking place. (It may start much later than the oldest open transaction.)
--
HTH
Kalen Delaney, SQL Server MVP
"Nils Loeber" <nils@.NOSPAMFORMEPLEASEnils-loeber.de> wrote in message
news:eIYWsFmuGHA.736@.TK2MSFTNGP02.phx.gbl...
> Dear experts,
> I am confused. I always thought that using NORECOVERY was essential when
> restoring multiple transactional log backups in a row and I always thought
> this was due to the fact that without NORECOVERY the transactional log
> backups wouldn't "fit" to each other, because of potential rollbacks
> happening in recovery. E.g. I have a tlb (transactional log backup, for
> brevity's sake) ending at LSN 124. I restore with recovery and transaction
> 106 isn't commited before LSN 124, so it gets rolled back. Now I try to
> restore the tlb beginning at LSN 125 and lo and behold, it doesn't work
> because transaction 106 is commited at LSN 145 after all! I thought this
> is why you have to use NORECOVERY.
> BUT: There is a sentence in Solid Quality Learning's fine publication "SQL
> Server 2005 Implementation and Maintenance" that made me think otherwise
> (pg. 419): "A log backup backs up the active log. It starts at the Log
> Sequence Number (LSN) at which the previous log backup completed. SQL
> Server then backs up all subsequent transactions UNTIL THE BACKUP
> ENCOUNTERS AN OPEN TRANSACTION." (emphasis mine)
> Now if this is so, why is there any need for a rollback after restoring a
> tlb with recovery anyway? All transactions included in the tlb are not
> open (i.e. commited or rolled backed), so the worst thing that could
> happen is the need for a rollforward, in case the db isn't consistent with
> the tlb. So why do we need NORECOVERY?
> Somewhat related bonus question: What portion of the log is exactly backed
> up when I do a FULL backup? It's only the part after the oldest open
> transaction, right?
>
> Thank you a lot
> Nils Loeber
>|||Hi Kalen,
this is the kind of helpful answer I had hoped for. Thank you very much.
Best regards
Nils Loeber
"Kalen Delaney" <replies@.public_newsgroups.com> schrieb im Newsbeitrag
news:OYJ2dQmuGHA.2260@.TK2MSFTNGP03.phx.gbl...
> Hi Nils
> This is not correct, but I appreciate your recognition of the quality of
> the document as a whole, even though there are some errors.
> Books Online is correct in stating that during a log backup:
> the log is backed up from the last successfully executed log backup to
> the current end of the log.
> I have no idea where the comment about open transaction came from.
> When you do a full backup, SQL Server records the current LSN when the
> full backup starts. When the full backup is over, the new current LSN is
> records. All the log records between the two recorded LSNs are then backed
> up. So it basically captures all other changes that were going on while
> the backup was taking place. (It may start much later than the oldest open
> transaction.)
> --
> HTH
> Kalen Delaney, SQL Server MVP
>
> "Nils Loeber" <nils@.NOSPAMFORMEPLEASEnils-loeber.de> wrote in message
> news:eIYWsFmuGHA.736@.TK2MSFTNGP02.phx.gbl...
>> Dear experts,
>> I am confused. I always thought that using NORECOVERY was essential when
>> restoring multiple transactional log backups in a row and I always
>> thought this was due to the fact that without NORECOVERY the
>> transactional log backups wouldn't "fit" to each other, because of
>> potential rollbacks happening in recovery. E.g. I have a tlb
>> (transactional log backup, for brevity's sake) ending at LSN 124. I
>> restore with recovery and transaction 106 isn't commited before LSN 124,
>> so it gets rolled back. Now I try to restore the tlb beginning at LSN 125
>> and lo and behold, it doesn't work because transaction 106 is commited at
>> LSN 145 after all! I thought this is why you have to use NORECOVERY.
>> BUT: There is a sentence in Solid Quality Learning's fine publication
>> "SQL Server 2005 Implementation and Maintenance" that made me think
>> otherwise (pg. 419): "A log backup backs up the active log. It starts at
>> the Log Sequence Number (LSN) at which the previous log backup completed.
>> SQL Server then backs up all subsequent transactions UNTIL THE BACKUP
>> ENCOUNTERS AN OPEN TRANSACTION." (emphasis mine)
>> Now if this is so, why is there any need for a rollback after restoring a
>> tlb with recovery anyway? All transactions included in the tlb are not
>> open (i.e. commited or rolled backed), so the worst thing that could
>> happen is the need for a rollforward, in case the db isn't consistent
>> with the tlb. So why do we need NORECOVERY?
>> Somewhat related bonus question: What portion of the log is exactly
>> backed up when I do a FULL backup? It's only the part after the oldest
>> open transaction, right?
>>
>> Thank you a lot
>> Nils Loeber
>