SQL Azure备份：事务一致意味着什么？

Question

I'm using redgate's sql azure backup tool: http://www.red-gate.com/products/dba/sql-azure-backup/

It looks like if you check "Make Backup Transactionally Consistent" you get charged a full day's use for sql server. I'm wondering if I need to check this.

I do daily backups to blob storage and I backup the database to my local machine to work with every 3 days or so.

If I don't check the Transactionally Consistent box, am I going to run into any problems?

Answer 1

Well as the person who wrote SQL Azure Backup at Red Gate I can say that the only way to create a guaranteed transactionally consistent backup in Azure currently is indeed to use CREATE DATABASE ... AS COPY OF. This copy only exists for the duration of us taking the backup and is then dropped immediately afterwards.

If you don't check the box you'll only hit problems if there is a risk of transactions being in an inconsistent state when reading the data from each table in turn. CREATE COPY OF can take a very long time and also may cost money for the copy too.

If you're backing up to a BLOB you're using the Microsoft Import Export service rather than SQL Compare and SQL Data Compare technology but that also reads data from the tables to could be inconsistent too.

Hope this helps Richard

Answer 2

AFAIK transactionally consistent means that you get a snapshot of the database at a point in time (which presumably means SQL Azure locks the db while (quickly we hope) it makes a copy of the entire database = your one day charge for a db that exists for only a few minutes).

This is better illustrated by non-transactionally consistent backup where begin by copying table X. While you are doing that someone amends (as it's a live database) table Y, which later gets copied to the backup. The foreign keys between X and Y might now not match 'cos X is from an earlier time period than Y.

I have used Sql Azure Backup and I did go for transactional consistency because the backups are for an emergency and the last thing I want in that scenario is inconsistencies in the data.

edit: now I think about it, Redgate should really state that if you backup every day you are effectively paying twice the rate for your database. I've been waiting for the sync framework which I think is there now...

Answer 3

To answer the question in the title: a SQL Azure database copy (the 'backup') is a SQL Azure database that is copied (fully online) from the source database and contains no uncommitted transactions (ie. is transactionally consistent ). This is achieved the same way database snapshots or backup restores achieve consistency on the standalone SQL Server product: all pending transactions at the moment of 'separation' are rolled back.

As to why or how RedGate's product utilizes this, I don't know. I would venture a guess that in order to achieve a 'transitionally consistent backup' they are doing a CREATE DATABASE ... AS COPY OF ... (which creates the desired transactionally consistence) and then they use the technology from SQL Compare and Data Compare to copy out the schema and data.