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A SQL Server DBA myth a day: (30/30) backup myths

(Look in the Misconceptions blog category for the rest of the month’s posts and check out the 60-page PDF with all the myths and misconceptions blog posts collected together when you join our Insider list, plus my online Myths and Misconceptions training course on Pluralsight.)

The month is finally over so time for the grand finale!

Although it’s been fun debunking all these myths, it’s been a tad stressful making sure I come up with an interesting and useful myth to debunk every day.

To round out the month, I present to you 30 myths around backups – one for each day of the month of April. Last night I sat down to write this post and was a few myths short so reached out to the fabulous SQL community on Twitter (follow me!) for help – too many people to list (you know who you are) – I thank you!

A few folks have asked if I’ll pull the month’s posts into a PDF e-book – let me know if you’d like that.

I *really* hope you’ve enjoyed the series over the last month and have had a bunch of myths and misconceptions debunked once and for all – I know quite a few of you are going to use these explanations as ammunition against 3rd-party vendors, developers, and other DBAs who insist on incorrect practices.

Ok – here we go with the last one…

Myth #30: various myths around backups…

All are FALSE!!

For a good primer on understanding backups and how they work see my TechNet Magazine article Understanding SQL Server Backups. [Edit 2016: the article has been removed – check out the backup posts at https://www.sqlskills.com/help/accidental-dba/ for more info.]

30-01) backup operations cause blocking

No, not for regular DML. Backup operations do not take locks on user objects. Backups do cause a really heavy read load on the I/O subsystem so it might *look* like the workload is being blocked, but it isn’t really. It’s just being slowed down. There’s a special case where a backup that has to pick up bulk-logged extents will take a file lock which could block a checkpoint operation – but regular DML is never blocked. There is one case of blocking – a bulk load cannot start while a log backup is running, and vice-versa.

30-02) switching from the FULL recovery model to the BULK_LOGGED recovery model and back again breaks the log backup chain

No. It just doesn’t. Switching from either FULL or BULK_LOGGED to SIMPLE *does* break the log backup chain however.

30-03) breaking the log backup chain requires a full backup to restart it

No. You can restart the log backup chain with either a full or differential backup – anything that bridges the LSN gap from the point at which the log backup chain was broken. See my blog post A SQL Server DBA myth a day: (20/30) restarting a log backup chain requires a full database backup for more details.

30-04) concurrent log backups are not possible while a full or differential backup is in progress

No, this changed in SQL Server 2005. See my blog post Search Engine Q&A #16: Concurrent log and full backups.

30-05) a full or differential backup clears the log

No. A log backup includes all the log since the last log backup – nothing can change that – no matter whether that log was also backed up by a full or differential backup. I had a famous argument on Twitter last year and wrote this blog post as proof: Misconceptions around the log and log backups: how to convince yourself. In the FULL or BULK_LOGGED recovery models, the *only* thing that clears the log is a log backup.

30-06) using the BULK_LOGGED recovery model for minimally-logged operations reduces the size of the next transaction log backup

No. A minimally-logged operation is so-named because only the page allocations are logged. A log backup needs all the information necessary to resconstitute the transaction, so a log backup following a minimally-logged operation must backup the log plus all extents changed by the minimally-logged operation. This will result in the log backup being roughly the same size as if the operation was fully logged.

30-07) full and differential backups only contain the log generated while the backup was running

No. A full or differential backup contains enough log to be able to recover the database to a transactionally-consistent view of the database at the time the data-reading portion of the backup finished (or as far back as the oldest log record that transactional replication has not yet processed – to ensure that replication works properly after a restore). Check out these two blog posts for details:

30-08) backups always test existing page checksums

No. It only does it when you use the WITH CHECKSUM option – which you should.

30-09) backups read data through the buffer pool

No. The backup subsystem opens its own channels to the database files to avoid the performance hit of having to read everything into SQL Server’s memory and back out to the backup device (and also effectively flushing the buffer pool in the process). If you ask the for page-checksum checking, it uses it’s own small portion of memory.

30-10) backups perform consistency checks (a la DBCC CHECKDB)

No. Nothing else to say.

30-11) if the backup works, the restore will too

No. Please don’t fall into this trap. You must regularly validate your backups to give yourself a high level of confidence that they will work if a disaster occurs. See Importance of validating backups for more details.

30-12) a mirrored backup will succeed if the mirror location becomes unavailable

No. If any one of the paths to a mirrored backup fails, the entire mirrored backup operation fails. I’d really like it to work the other way around – where the local backup succeeds and the remote backups fail, but it doesn’t unfortunately.

30-13) a tail-of-the-log backup is always possible

No. A tail-of-the-log backup is one that backs up all the log generated since the last log backup, in an exceptional situation. If the data files are damaged, you can still do a tail-of-the-log backup EXCEPT if the un-backed-up log contains a minimally-logged operation. That would require reading data extents – which cannot be done if the data files are damaged. For this reason, the BULK_LOGGED recovery model should not be used on databases that have 24×7 user transactions.

30-14) you can use backups instead of DBCC CHECKDB

No. See A SQL Server DBA myth a day: (27/30) use BACKUP WITH CHECKSUM to replace DBCC CHECKDB.

30-15) you can backup a database snapshot

No. It’s not implemented, but would be great if you could.

30-16) you can use database snapshots instead of log backups

No. A database snapshot is only usable while the database on which it is based is usable and online. If the source database is corrupted, the database snapshot most likely is too. If the source database goes suspect, so does the database snapshot.

Also, having multiple database snapshots (equating to multiple log backups) incurs an increasing performance drain – as every page that changes in the source database may need to be synchronously written to all existing snapshots before it can be written to the source database data files, and all existing database snapshots will grow as more pages are pushed into them.

30-17) log backups will be the size of the log

No. The log has to accommodate the space necessary to roll back active transactions, the amount of space returned by DBCC SQLPERF (LOGSPACE) on a busy system doesn’t accurately refect the amount of log records in the log. This blog spot explains: Search Engine Q&A #25: Why isn’t my log backup the same size as my log? And apart from that, a log backup is just all the log generated since the last log backup – not the whole log file usually – and if it happens to be, the first part of the explanation comes into play.

30-18) you cannot backup a corrupt database

No. In most cases you can use the WITH CONTINUE_AFTER_ERROR option to back up the corrupt database.  If that fails (maybe because of a damaged boot page or file-header page), there are no other options apart from OS-level file backups.

30-19) you cannot stop someone doing a BACKUP LOG .. WITH NO_LOG or TRUNCATE_ONLY operation

No. It’s not possible any more (yay!) and in 2005 and before, use trace flag 3231 which turns the operation into a no-op.

30-20) log backups always clear the log

No.

If there’s no concurrent data backup running, a log backup will always *try* to clear the log, and only succeed in clearing the inactive portion of the log – the log that’s only considered ‘required’ by SQL Server because it hasn’t yet been backed up. If anything else is holding the log ‘required’, it cannot be cleared, even though it has been backed up. Subsequent log backups will check again and again until the time comes when that portion of the log can be cleared. The TechNet Magazine article Understanding Logging and Recovery in SQL Server I wrote last year explains a lot more about how the log works.

Also, if there is a concurrent data backup running, the log clearing will be delayed until the data backup finishes. See the blog post in myth 30-04 for more details.

30-21) differential backups are incremental

No. Differential backups are all the data extents that have changed since the last full backup – so they are cumulative. Log backups are incremental – all log generated since the last log backup. Many people call differential backups ‘incrementals’, when they’re not really.

30-22) once a backup completes, you can safely delete the previous one

No. No. No.

If you go to restore, and you find your full backup is corrupt, what do you do? Well, if you don’t have an older full backup, you most likely start updating your resume. You need to keep a rolling-window of backups around in case a disaster occurs and you need to restore from an older set of backups.

30-23) you can back up a mirror database

No. A mirror database is not accessible except through a database snapshot. And you can’t back up that either.

30-24) you can back up a single table

No. You can effectively back up single table if it happens to be wholly contained on a single filegroup, but there’s no way to say BACKUP TABLE.

30-25) SQL Server has to be shut down to take a backup

No. No idea how this myth started… [Edit: apparently this myth started with Oracle – and we all know how good Oracle is compared to SQL Server… :-)]

30-26) my transaction is guaranteed to be contained in the backup if it committed before the backup operation completed

No. The commit log record for the transaction has to have been written out before the data-reading portion of the backup finished. See my blog post Search Engine Q&A #6: Using fn_dblog to tell if a transaction is contained in a backup for more details.

30-27) you should shrink the database before a backup to reduce the backup size

No. Shrink just moves pages around so won’t make any difference. See my old blog post Conference Questions Pot-Pourri #10: Shrinking the database before taking a backup. And of course, shrink is evil. See A SQL Server DBA myth a day: (9/30) data file shrink does not affect performance. And what’s even worse as someone reminded me, is if you do the shrink *after* the full backup, the next differential backup may be huge, for no actual data changes!

30-28) backups are always the best way to recover from a disaster

No. Backups are usually the best way to recover with zero data-loss (as long as you have log backups up to the point of the disaster), but not necessarily the best way to recover with minimal downtime. It may be way faster to fail over, or to run repair and accept some data loss if the business requirements allow it.

30-29) you don’t need to back up master, msdb, model…

No. You should always back up the system databases. Master contains all the security info, what databases exist – msdb contains all the SSIS packages, Agent jobs, backup history – model contains the configuration for new databases. Don’t fall into the trap of only backing up user databases otherwise you’ll be in a world of hurt if you have to do a bare-metal install.

30-30) you should always plan a good backup strategy

No. Now you’re thinking ‘Huh?’…

You should plan a restore strategy. Use the business requirements and technical limitations to figure out what you need to be able to restore in what time, and then use that to figure out what backups you need to take to allow those restores to happen. See the blog posts:

The vast majority of the time people plan a backup strategy without testing or thinking about restores – and come a disaster, they can’t restore within their SLAs. Don’t let that be you.

48 thoughts on “A SQL Server DBA myth a day: (30/30) backup myths

  1. Paul,

    Thank you very much for this excellent series. I am a bit sad to see April come to an end but you are certainly entitled to a well deserved rest! Once you get that rest I think an e-book would be very well received.

    Thank you,
    Jeremy

  2. Outstanding series from a great source of information. I’ve been reading this site daily since I discovered it and have pointed quite a few people here as well. Thanks so much for the time and effort you put into it.

    I have a question regarding backups. Concurrent backups (log while full is running) are allowed as of SQL2005. What are the contents of multiple log backups that might occur during a long-running full backup?

    Scenario:
    Full backup daily
    Log Backups every 15 minutes

    The full backup takes 45 minutes. To me that looks like I’m going have at least 2 log backups start and complete during the full backup. From a restore standpoint I’m curious as to what is in those log backup files. I know I can apply them with a "WITH STOPAT" and SQL is relatively smart about it but from an academic (and perhaps restore performance) perspective, I’d like to know.

    Do both of those log backups that happened during the full backup contain some of the same information since the log wasn’t truncated after the first one?

  3. @Kent No. A log backup always contains just the log generated since the last log backup – so the two log backups will not overlap in any way. They may both overlap with what’s in the full backup though. When restoring after the full backup, you need to only restore the backup that has last-lsn-in-the-full-backup + 1. This is called finding the min lsn to restore – backup history tables or restore headeronly will give you the info.

    Cheers

  4. Perfect! Since I’m restoring to a different server than the backup was taken from (and I don’t want to set up linked servers) I don’t have access to the backup history tables. Restore headeronly was my missing piece. Thanks so much, Paul.

  5. “30-25) SQL Server has to be shut down to take a backup – No. No idea how this myth started…”
    …I know the “administrators” who made ​​a backup from copying DB data files ;o) So they needed ShotDown SQLServer (ones made copy without shotdown, copiinq “live” DB data files. He was lucky that nobody not need to restore them ;o))

  6. Yes, almost two years post-series, it is still getting reads. Really great material, and it gets my vote for an e-book as well. As always, thanks, Paul.

  7. fantastic. Now, do you have anything that says how to restore from a master failure?
    30-29) you don’t need to back up master, msdb, model…

  8. “Many people call differential backups ‘incrementals’, when they’re not really.”..funny I wonder how they came up with the types in msdb.dbo.backupset.

    D = Database

    I = Differential database

    L = Log

    F = File or filegroup

    G =Differential file

    P = Partial

    Q = Differential partial

    Can be NULL.

  9. In SQL Server 2012 ,log backup acuquires lock in primary . it doesn’t lock any objects but lock the log file to prevent backups at the same time. am I correct.

  10. I think this is on topic…

    Does switching between Simple to Full and vice versa cause any locking or blocking issues or do things just slow down? The reason I ask is that we have a database (production) log file that is larger than the database itself and want to shrink it. The following was suggested to me:

    1. Backup the TLOG
    2. Change DB from Full to Simple
    3. Change the initial size of the TLOG to something much smaller
    4. Perform the log shrink
    5. Change from Simple to Full
    6. Perform full backup of DB to start log chain

    Is this proper? If not, what would your process be to fix this matter?

  11. Thanks for a great site, Paul.
    And sorry if you have already answered this elsewhere.

    Does a running backup of the model database prevent an exclusive lock on that database from a “create database […]” statement?
    I have recently seen a few errors like this one:
    “[SQLSTATE 01000] (Message 50000) Could not obtain exclusive lock on database ‘model’. Retry the operation later. [SQLSTATE 42000] (Error 1807) CREATE DATABASE failed. Some file names listed could not be created”

  12. Hi Paul,

    Thanks for the nice article. A question on backups dont cause blocking. In the special case of bulk operation pages getting backed up – You said it will lock the file. So, what if another bulk operation wants to occur when the file is locked. It does want to write to BCM pages as well. Wont this cause some kind of internal blocking ?

    1. It doesn’t lock the file – it take a bulk operation lock. Concurrent bulk operations can occur, as they acquire the lock in a mode that’s compatible, but not compatible with the lock mode that the backup takes.

  13. Great reference, Paul. Quick question on 30-09, inspired by this DBA.SE question: https://dba.stackexchange.com/questions/173438/

    You said above: “The backup subsystem opens its own channels to the database files to avoid the performance hit of having to read everything into SQL Server’s memory and back out to the backup device.”

    How does this work with TDE?

    Does the backup process just dump raw encrypted extents to disk without decrypting them?

    Or does it decrypt them in memory (because it has to, or so it can validate checksums, or for some other reason), then re-encrypt them before dumping to disk?

    1. It works exactly the same way. The buffer pool does encryption then adds a page checksum before writing a page to disk. Backups *never* read through the buffer pool. So yes, a backup of TDE database has the encryption still in it. Page checksums are validated, but by backup code, not buffer pool code.

  14. Hi Paul,
    Like pure sunlight! My eyes are now wide open, but my brain still needs to catch up. I will re-read several times. In the meantime, and this is basic I am sure, but I am a newbie: Is it correct or best practice to backup the Full, the Differentials, and the Trans Logs to the same backupset and Mediaset? I have been searching for an answer all evening and have yet to find one.
    Thanks!

  15. Hi Paul,

    Thank you for these excellent videos.There were a ton of light bulb moments for me after going through most of your courses. In your plural sight course “Understanding and Performing Backups->Module 5->Introduction and Minimal Logging” you state that the “A log backup will block a minimally logged operation from starting and Vice versa”.

    However in this blog post for “30-01” you state that DML is never blocked. So how does it affect “INSERT INTO SELECT WITH TABLOCK”. Is this considered a minimally logged DML operation? Could you please clarify.

    Thanks,
    Vijay

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