none
DPM Consistency Check and SQL Express Full Backup

    Question

  • Team,

    Thanks in advance, I've gone through multiple questions on the forums related to CC but still I'm little confused with CC and Express Full Backup of a SQL Database. So first let me share my thought what Consistency check does.

    After the protection group has been created an initial replica job is triggered which will create a replica of the protected SQL DB on the DPM Storage pool. After the initial replica is created, my 15 minutes cycle of Incremental sync gets triggered which will bring only the log files of the SQL and will save it on the storage pool ( (Q1) I'm not sure if it is saved separately than the replica)

    As the 15 minutes cycle only brings the changed block from the replica, DPM doesn't know if the blocks which DPM isn't bring has changed or not hence we've to perform the consistency check so that the Database replica should be similar on the DPM Storage pool.

    (Q2 ) So if CC job does that then what does Daily Express Full Backup Does? It brings the changed block of the SQL DB/ MDF and then make the replica on the storage pool of DPM the same as the protected DB. Am i correct? or is there anything extra done by Express Full Backup. If Express Full Backup does only this then why I need to automatically check for Database consistency check? I will only use the first option of CC which says that run the CC once DPM finds any inconsistencies.

    Monday, February 18, 2019 5:31 PM

Answers

  • Yes sorry for the confusion, so:

    A synchronization will only bring over changed blocked between sync times. The Consistency Check checks every file in a two-step process.

    1. The first one is a lightweight comparison of file metadata to see what files may be different.

    2. The second one is a heavyweight comparison of the files data to correct differences.

    --------------------------------------------------------------------------------------

    When inconsistencies occur, they usually occur for the following two reasons:

    1. The filter driver on the protected server loses track of block level changes (it could be for example a system crash, or unexpected cluster failover).

    2. A synchronization that has failed in the middle and we could not update the replica.

    The first (1) consistency check option will run an immediate consistency check when we detect the replica is inconsistent.

    The second (2) option will check to see if a replica is inconsistent at a certain time, and only run a consistency check if it is inconsistent.

    For some reason all of these information regarding every feature in DPM is not as well documented in the newer DPM documentation, but you can find some over here:

    How does DPM work?
    https://docs.microsoft.com/en-us/system-center/dpm/how-dpm-protects-data?view=sc-dpm-1807


    Blog: https://thesystemcenterblog.com LinkedIn:

    • Marked as answer by Qureshi Aquib Tuesday, February 19, 2019 10:57 AM
    Tuesday, February 19, 2019 9:41 AM

All replies

  • Hello Qureshi,

    This is indeed very confusing for many, let me try to answer this in a simple and short way:

    (Q1) I'm not sure if it is saved separately than the replica

    Answer:
    Each incremental synchronization and express full backup creates a recovery point.

    (Q2 ) So if CC job does that then what does Daily Express Full Backup Does?

    Answer:
    The Consistency Check (CC) / Synchronization will only copy the log files, while the Express Full backup applies the block level changes to the database files.

    Best regards,
    Leon


    Blog: https://thesystemcenterblog.com LinkedIn:

    Monday, February 18, 2019 7:12 PM
  • Hi Leon,

    Are you sure that the CC only copies the log files? On every article which i saw they mentioned that CC job will make sure that DPM replica is upto date with the backup target. And you mentioned that CC will copy the transaction log files.

    Monday, February 18, 2019 7:18 PM
  • Yes it only brings over the log files.

    Blog: https://thesystemcenterblog.com LinkedIn:

    Monday, February 18, 2019 7:35 PM
  • Hi Leon,

    Thanks for the help, I'm trying to find the consistency check document which states that it only brings the log files. But I'm unable to find it over the web. Do you have any relevant article for this.

    Tuesday, February 19, 2019 4:26 AM
  • thanks Leon but that is of Express Full Backup and not include much details about the consistency check and it transferring only the log file.
    Tuesday, February 19, 2019 8:20 AM
  • Here's an older documentation of the consistency check: What Is a Consistency Check?


    Blog: https://thesystemcenterblog.com LinkedIn:

    Tuesday, February 19, 2019 8:27 AM
  • Thanks Leon, I was looking for this. So here they've mentioned that CC "As part of the synchronization process, a consistency check performs block-by-block verification to ensure that all the data on the replica is consistent with the protected data"

    So i believe express full backup in SQL will bring the changed block from the last initial replication or last successful express full backup(changes to the mdf file). Incremental sync will bring only the transaction log file.  And if express full backup or incremental synchronization has failed then CC will trigger to make the replica on the DPM server same as the protected computer.

    Tuesday, February 19, 2019 8:54 AM
  • Yes sorry for the confusion, so:

    A synchronization will only bring over changed blocked between sync times. The Consistency Check checks every file in a two-step process.

    1. The first one is a lightweight comparison of file metadata to see what files may be different.

    2. The second one is a heavyweight comparison of the files data to correct differences.

    --------------------------------------------------------------------------------------

    When inconsistencies occur, they usually occur for the following two reasons:

    1. The filter driver on the protected server loses track of block level changes (it could be for example a system crash, or unexpected cluster failover).

    2. A synchronization that has failed in the middle and we could not update the replica.

    The first (1) consistency check option will run an immediate consistency check when we detect the replica is inconsistent.

    The second (2) option will check to see if a replica is inconsistent at a certain time, and only run a consistency check if it is inconsistent.

    For some reason all of these information regarding every feature in DPM is not as well documented in the newer DPM documentation, but you can find some over here:

    How does DPM work?
    https://docs.microsoft.com/en-us/system-center/dpm/how-dpm-protects-data?view=sc-dpm-1807


    Blog: https://thesystemcenterblog.com LinkedIn:

    • Marked as answer by Qureshi Aquib Tuesday, February 19, 2019 10:57 AM
    Tuesday, February 19, 2019 9:41 AM
  • thanks leon for your reply.
    Tuesday, February 19, 2019 10:57 AM
  • You’re most welcome!

    Blog: https://thesystemcenterblog.com LinkedIn:

    Tuesday, February 19, 2019 11:09 AM