The Curious Case of… emergency-mode repair

(The Curious Case of… used to be part of our bi-weekly newsletter but we decided to make it a regular blog post instead so it can sometimes be more frequent. It covers something interesting one of us encountered when working with a client, doing some testing, or were asked in a random question from the community.)

I had a blog comment question a few days ago that asked why emergency-mode repair requires the database to be in EMERGENCY mode as well as SINGLE_USER mode.

All repair operations that DBCC CHECKDB (and related commands) performs require the database to be in single-user mode so there’s a guarantee that nothing can be changing while the checks and repairs are done. But that doesn’t change the behavior of what repair does – that needs emergency mode too.

Emergency mode is internally known as ‘bypass recovery’ mode, and is used when a database hasn’t had crash recovery run on it, because of corruption issues or a missing or damaged log file. It allows access to the database in the unrecovered state, which may mean the database has inconsistencies in its structures and/or data because of in-flight transactions that have not been rolled back (as recovery couldn’t run).

When a database is in emergency mode, it’s not possible to create a database snapshot (as that involves running recovery) and so a DBCC CHECKDB without a repair option simply treats the database as if it’s read-only, and runs the consistency checks. When a repair option *is* specified (and only REPAIR_ALLOW_DATA_LOSS is permitted) in emergency mode, that tells DBCC CHECKDB that the drastic, last-resort emergency mode repair should be performed. This will:

  • Run as much crash recovery as possible, skipping errors, unlike real crash recovery which will fail when it encounters an error
  • Delete the transaction log
  • Create a new transaction log
  • Run a regular REPAIR_ALLOW_DATA_LOSS check and repair
  • Bring the database online if possible, albeit maybe with corruptions still in the database

So emergency mode isn’t single-user mode, and vice versa – they’re both required for an emergency-mode repair to run.

You can read more about emergency-mode repair in my blog post EMERGENCY-mode repair: the very, very last resort.

6 thoughts on “The Curious Case of… emergency-mode repair

  1. Marvelous post Indeed !

    Sir, I upgraded 1 database from sql 2012 to sql 2017 and it restored successfully. When I ran dbcc checkdb, it failed with PFS corruption issue as mentioned in your 1 blog for sql 2014 PFS Corruption.

    DBCC recommended repair allow data loss.

    Here I dint use emergency mode SO will I always need Emergency mode for Repair plz?
    PFS corruption and subsequently SGAM issue is still prevalent in sql 2017 also?

      1. Respected Sir,

        On Source database, multiple dbcc runs came clean and no issue reported in years. Still we need to be worried.

        Thank you always

        1. We also received the same error where no issue was reported on source server but threw sorts of these errors.

          source server -sql 2012
          destination server- 2017

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