what does restore log do after the log is restored?

  • i1888

    SSCommitted

    Points: 1806

    When you see the title, don't get me wrong:)

    I am not sure if anyone see when execute the restore log command, in the messages window it shows how many seconds the restore takes, at the meantime, on the status bar, it also shows the seconds the command takes.

    Two values are different and could be very different, please see below examples , restoring takes 1.8 seconds, but in total the command takes 4 seconds to complete, the other one is 8.1 seconds and 12 seconds.

    What does SQL Server or Windows do after the restoring?

    pic a:

    pic b:

    I did a xperf, I can see after the restoring is completed, sql server did garbage collect and log write, which just run very quickly, but storage is busy on reading the log file for nearly 2.2 seconds( 4-1.8), and 4 seconds ( 12-8.1) .

    pic 1:

    pic 2:

    please see pic 1 above, from 13 to 17, the restore operation is finished, but the storage jump to 100% active to do some reads, only reads no writes. zoom that period shows pic 2, it read 4096 (I don't know the unit size) for about 4 seconds, what does this do?

    Data file, log file, backup file are no different drives, but all local drive, the interesting point is the read jumped after restoring, I tested it on different server, same result...

  • Gail Shaw

    SSC Guru

    Points: 1004474

    Runs the recovery process, same as when you restore a database, there's extra time after the data copying portion completes.

    Gail Shaw
    Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
    SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

    We walk in the dark places no others will enter
    We stand on the bridge and no one may pass

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