Does SQL recover databases sequentially, or in parallel?

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Does SQL recover databases sequentially, or in parallel?

jasona.work

SSC Guru

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August 13, 2012 at 11:44 am

#260685

Possibly an easy question, and one I hope I'll never find the answer to in person. If I have a problem that causes SQL to have to recover databases (say a power failure on a server with no UPS {HYPOTHETICAL!!}) will SQL recover the databases in sequence (perhaps via the database ID,) or will it recover them in parallel (work on bringing up all of the databases at once)
Presumably, the system DBs would need to come up first, so this would be after that.
I'm partly looking for this for my own personal knowledge, and partly to disaster plan for a system.
Thanks,
Jason A.
(I suppose if no one is entirely sure, I will go ahead and setup a couple test DBs on my workstation, start inserting rows in a transaction, then pull the power cord...):Whistling:

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Lowell SSC Guru Points: 323547 More actions · Answer 1

they are recovered , sequentially, master database first.; but each database seems to get it's own spid to startup, and some spids finish before others spids created after it.

if one fails, it moves on to the next database, so you might see some that do not recover at all.

if you look at the SQL Logs , you can see it for yourself;

SSMS Object Explorer:

>> Expand Server Name>>Management>>SQL Server Logs>>Current

on my server, on the last reboot, i see them in this order:

i don't see them recovering in db_id order, which i kind of expected.

I do see the spids created in database id order, though.

master

model

tempdb

[one specific userdatabase]

msdb

[other user Databases]

Lowell

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Gail Shaw SSC Guru Points: 1004514 More actions · Answer 2

SQL recovers all databases every time it starts. It's not something that happens in a disaster, it's a normal operation on startup.

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

Grant Fritchey SSC Guru Points: 399289 More actions · Answer 3

Plan on restoring them one at a time. You can open multiple windows and start multiple restores, but you'll see very poor performance. Better to know the order that you wish to recover in rather than trying to do them all at the same time. This is especially true if the backups are on shared media (all the backups in the same location).

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jasona.work SSC Guru Points: 50220 More actions · Answer 4

GilaMonster (8/13/2012)
SQL recovers all databases every time it starts. It's not something that happens in a disaster, it's a normal operation on startup.

So in terms of the user DBs, is it sequential, or serial?

Thanks,

Jason A.

Gail Shaw SSC Guru Points: 1004514 More actions · Answer 5

They recover in parallel. Multiple spids are spawned to recover each of the user databases. User databases will only get recovered after the system ones are recovered.

You can see this for yourself if you look at the SQL error log and look at the sequence of the database recovery messages.

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

jasona.work SSC Guru Points: 50220 More actions · Answer 6

GilaMonster (8/13/2012)
They recover in parallel. Multiple spids are spawned to recover each of the user databases. User databases will only get recovered after the system ones are recovered.
You can see this for yourself if you look at the SQL error log and look at the sequence of the database recovery messages.

Cool, thank you!

Jason A.

Gail Shaw SSC Guru Points: 1004514 More actions · Answer 7

Why is knowing this important for disaster recovery?

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

jasona.work SSC Guru Points: 50220 More actions · Answer 8

It was primarily to be able to give a rough estimate of how long it might take for a system to be ready to use again for all the DBs on it. I know the time to recover will depend on if there was a transaction in progress, and how big that transaction was, but with the DBs in question, there should be few, if any, long-running transactions.

Thanks again,

Jason

Gail Shaw SSC Guru Points: 1004514 More actions · Answer 9

Be careful with that assumption...

I've had a DB that only had long-running transactions overnight. During the day it was short transactions and mostly reads. After a crash at 3pm one day it took a little over 12 hours to recover.

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

jasona.work SSC Guru Points: 50220 More actions · Answer 10

Yeah, I know what they say about assumptions...

Ouch.

My hope is that I'll never need to worry about this, that the system will rarely need to be restarted and that the SQL service will never need to be restarted...

(need a fingers crossed smiley)

Jason

Gail Shaw SSC Guru Points: 1004514 More actions · Answer 11

Generally it's not the controlled restarts that are the problem. 😉

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