On Monday, 17 March 2014 06:40:02 UTC+11, cap wrote:

> An advantage of snapshots is with active filesystems such as those used by 
> a database.  For a consist at database backup you of course need to stop 
> the program then backup then restart ( or use some database tool if 
> available) .  The time to create a snapshot is essentially zero so the 
> above start - stop is actually practical.  Then you use your backup 
> software of choice on the snapshot not the active file system.
> This is only fine if your database is read-only or you have control on the 
update workload.
Most database systems use a combination of no-force+steal buffering and WAL 
logging (e.g. MySQL InnoDB or PostgreSQl and basically all commercial 
Taking a file-system level snapshot underneath does not guarantee that you 
get a consistent snapshot of the database log and data pages.
Together with high update rates, this can be dangerous. Better use the 
database system's snapshot facility too before you take the ZFS snapshot. 
Granted, open source systems are a bit weak in that regard...


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