shells and bells

[niall at kst.com (Niall Kavanagh)]

On Wed, 3 May 2000, Derek Martin wrote:

> While I can't speak to Mike's assertions about ODBC or locking, the only
> alternative I know of to DBMS-supported on-line back up is to shut down
> the database and dump it to text files, which can then be backed up.  
> Otherwise, as Mike said, you have no guarantee that the data on the disk
> is in a form which is useful to back up, and it's rather likely that it
> won't be.
> 
> As I understand the problem (and I'll admit up front that my understanding
> may be flawed), this is due to the fact that a database write, while
> "atomic" from the database's perspective, is not an atomic operation from
> the kernel's perspective, unless the whole operation can be done with
> exactly one system call (system calls are guaranteed to be atomic, IIRC).
> Otherwise, the dbms may finish its time slice while a record update is in
> progress, at which time the back-up program may read the database's files,
> yeilding spaghetti data on your backup tape.
>

This is why during a backup procedure you READ LOCK the mysql database.
Clients will still be able to query the database,. The better way would be
to do a local lock, which would also allow INSERTS during the process, and
then dump the SQL data from the database (which is not what you folks are
talking about).
 
> > 
> > Mysql uses a locking method they call "atomic operations" (I'm not
> 
> The term "atomic operation" is certainly not a MySQL-ism... it refers to
> an operation that must complete before any other operation can begin.  It
> is certainly possible for MySQL to consider its record updates atomic,
> while in the framework of the OS they are not.  Wether or not that is
> actually the case, I can't say...
> 

I knew I should have double-double quoted it. ;)

I know what an atomic operation is, I just didn't know if it was only
mysql applying the description to the way their DBMS works or if other
DBMS vendors use the term.

--
Niall Kavanagh, niall at kst.com
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