Innodb always keeps it's files in a consistent state on disk. Power
failure and snapshots of the system won't result in a corrupt data
file. You can find more details on how innodb achieves this here:
http://www.mysqlperformanceblog.com/2006/08/04/innodb-double-write/
More replies inline.
On Tue, Apr 22, 2008 at 5:22 PM, Augusto Bott <augusto@stripped> wrote:
> Hi Mark, Eric!
>
> The last comments on this thread got me thinking about all those
> backup chalenges... since a FLUSH TABLES won't stop internal InnoDB
> threads from working and changing the datafiles in the background, how
> can we be absolutely sure that a filesystem snapshot will be actually
> consistent when it's taken? What if when we're taking that snapshot in
> the precise moment when InnoDB threads are working and modifying data?
> How can we be sure that the snapshot will not be taken on the precise
> moment there's a write in place?
>
> I must acknowledge under this scenario, taking a 'instant' snapshot
> from the filesystem will greatly reduce the probability of corruption
> but... any ideas on how to get it 100% safe? (hm... perhaps we should
> discuss this on internals@ ?)
See the notes above on why the snapshot is guaranteed not to be corrupt.
>
> On the other hand, we're talking about warm backups on a slave, right?
> So... what if we stop replication, issued a flush tables, waited for a
> moment until IO operations stop, and then start copying the datafiles
> or taking that snapshot?
You may as well just shutdown the slave. It's safer than trying to
determine if innodb isn't going to write something.
>
> Anyway... I don't recall having experienced corruption when restoring
> from a warm backup (ever), but that may be that I just "got lucky" or
> didn't look into the data deep enough to actually find any problems
> :-)
>
> Cheers!
>
> --
> Augusto Bott
>
>
>
>
>
> On Mon, Apr 21, 2008 at 8:27 PM, Mark Callaghan <mcallaghan@stripped>
> wrote:
> >
> >
> >
> > On Mon, Apr 21, 2008 at 5:14 PM, Augusto Bott <augusto@stripped>
> wrote:
> > > Hi Rick, Eric, Marcus!
> > >
> > > I must correct some of the things that have been said on this thread.
> > > I'm not commenting on rumors, just the facts from the documentation
> > > (and a few years of experience with MySQL) :-)
> > >
> > > If you're running your MySQL server with
> > > innodb_flush_log_at_trx_commit=1 (that's the default), issuing FLUSH
> > > TABLES WITH READ LOCK will indeed prevent modification to the
> > > datafiles while the global read lock is held since all committed
> > > transactions (so far) will be for sure on disk. The global read lock
> > > stays in place until UNLOCK TABLES is issued or until that connection
> > > is closed/timed out. This makes it safe to copy the datafiles for a
> > > backup - the result is indeed a consistent backup.
> >
> > FLUSH TABLES WITH READ LOCK should allow for a snapshot (LVM, ZFS) to be
> > used to copy all files -- as long as they are on the same filesystem. But I
> > do not think it is safe to copy InnoDB files without a snapshot as the
> > background IO threads (purge, insert buffer, page writer) can do IO during
> > FLUSH TABLES WITH READ LOCK. I have not seen any code that makes the InnoDB
> > background IO threads wait when there is a global read lock.
> >
> > >
> > >
> > >
> > >
> > >
> > > This operation can be called a 'warm backup'. A hot backup is what
> > > "InnoDB Hot Backup" does: it does not lock anything while it's running
> > > and transactions can start and commit while it's running (the tool
> > > requires a brief global read lock at the beginning, when the backup is
> > > starting) and takes a full consistent backup (you must license it to
> > > use it). A cold backup would be shutting down MySQL, copying the
> > > datafiles and firing it up again (also a safe move).
> > >
> > > The "caveat" when you make a warm backup is that the datafiles will be
> > > marked 'dirty' and on the next startup, the logs will be replayed to
> > > bring the database to a consistent point, since it "wasn't closed
> > > properly" (to the very same point when you issued the FLUSH TABLES
> > > WITH READ LOCK statement).
> > >
> > > --
> > > Augusto Bott
> > >
> > >
> > >
> > >
> > >
> > > On Sun, Apr 20, 2008 at 5:46 PM, Eric Bergen
> <ebergen@stripped>
> > wrote:
> > > > Hi,
> > > >
> > > > The flush tables with read lock issue has to do with copying the
> files
> > > > off the disk, not with using mysqldump's sql. The issue is that
> flush
> > > > tables with read lock is enough to prevent clients from modifying
> > > > innodb but it doesn't make innodb "hold still" . It still has
> > > > background threads modifying the data files. If you did a flush
> tables
> > > > with read lock; then copied the data files they will almost
> certainly
> > > > be corrupted because different parts of the files are copied at
> > > > different times.
> > > >
> > > > If you use mysqldump --master-data it will turn on
> --lock-all-tables
> > > > automatically giving you a consistent snapshot across all storage
> > > > engines. The downside of this is that mysqldump has to hold a read
> > > > lock the entire time it's dumping data. If you're only using innodb
> > > > you can specify --master-data --single-transaction mysqldump will
> > > > issue a flush tables with read lock long enough to copy the master
> > > > data and start a transaction. With a transaction started innodb
> will
> > > > only return rows that were committed before the transaction was
> > > > started. Other transactions can proceed and innodb won't see the
> rows.
> > > >
> > > >
> > > >
> > > > On Fri, Apr 18, 2008 at 4:14 PM, Rick James
> <rjames@stripped>
> > wrote:
> > > > > Rumor has it that InnoDB used to still have some things in RAM
> after
> > the "FLUSH TABLES WITH READ LOCK", thereby disallowing it as a safe way to
> > dump InnoDB tables. Another rumor has it that that has been fixed. [Sorry
> > for lack of details; hope someone will respond with details.]
> > > > >
> > > > > I prefer to stop mysql, copy the necessary directories, then
> restart
> > mysql. An improvement on that is to use Snapshots (eg LVM on Linux). They
> > make that extremely fast, hence very practical. (The copy can be done from
> > the snapshot _after_ restarting mysql.)
> > > > >
> > > > >
> > > > >
> > > > > > -----Original Message-----
> > > > > > From: Marcus Bointon [mailto:marcus@stripped]
> > > > > > Sent: Friday, April 18, 2008 2:42 AM
> > > > > > To: replication@stripped
> > > > > > Subject: Re: innodb replication
> > > > > >
> > > > > > On 18 Apr 2008, at 10:27, Ed W wrote:
> > > > > >
> > > > > > >> I´m going to set up a innodb database
> replication, and I
> > > > > > have some
> > > > > > >> doubts
> > > > > > >> about the backing up the master database step.
> > > > > > >>
> > > > > > >> Normally, I do a "FLUSH TABLES WITH READ LOCK",
> backup the data
> > > > > > >> files (or
> > > > > > >> use mysqldump), get the current log coordinates
> with "SHOW
> > MASTER
> > > > > > >> STATUS"
> > > > > > >> and unlock the tables. Using exactly the same
> steps work for
> > innodb
> > > > > > >> databases?
> > > > > > >>
> > > > > > >> According to some texts, it seems not, but I
> haven´t found
> > > > > > a clear
> > > > > > >> cookbook.
> > > > > > >>
> > > > > > >>
> > > > > > >> Just doing a "mysqldump --master-data
> --single-transaction" is
> > > > > > >> enough to
> > > > > > >> replace the above steps?
> > > > > > >>
> > > > > > >>
> > > > > > >
> > > > > > > How would you put these two things together in the
> case of
> > > > > > a server
> > > > > > > using mixed database table types for a clean backup
> that
> > > > > > can be used
> > > > > > > to restart synchronisation?
> > > > > >
> > > > > >
> > > > > > You can't. Only InnoDB supports single-transaction, so
> though
> > > > > > it would
> > > > > > still generate a dump file for other table types, you'd
> have no
> > > > > > assurance that they are clean because writes could be
> > > > > > happening as you
> > > > > > do it. This is one of the advantages of using innodb.
> > > > > >
> > > > > > You can of course do separate dumps for other DBs, and
> because
> > that
> > > > > > wouldn't need to include the innodb tables, it would at
> least
> > reduce
> > > > > > the time that your tables are locked.
> > > > > >
> > > > > > You could also look at maatkit's parallel dump/load
> scripts.
> > > > > >
> > > > > > Marcus
> > > > > > --
> > > > > > Marcus Bointon
> > > > > > Synchromedia Limited: Creators of
> http://www.smartmessages.net/
> > > > > > UK resellers of info@hand CRM solutions
> > > > > > marcus@stripped |
> http://www.synchromedia.co.uk/
> > > > > >
> > > > > >
> > > > > >
> > > > > > --
> > > > > > MySQL Replication Mailing List
> > > > > > For list archives: http://lists.mysql.com/replication
> > > > > > To unsubscribe:
> > > > > > http://lists.mysql.com/replication?unsub=1
> > > > >
> > > > >
> > > > > >
> > > > > >
> > > > >
> > > > > --
> > > > > MySQL Replication Mailing List
> > > > > For list archives: http://lists.mysql.com/replication
> > > > > To unsubscribe:
> > http://lists.mysql.com/replication?unsub=1
> > > > >
> > > > >
> > > >
> > > >
> > > >
> > > > --
> > > > Eric Bergen
> > > > eric.bergen@stripped
> > > > http://www.provenscaling.com
> > > >
> > > >
> > > >
> > > > --
> > > > MySQL Replication Mailing List
> > > > For list archives: http://lists.mysql.com/replication
> > > > To unsubscribe:
> > http://lists.mysql.com/replication?unsub=1
> > >
> > >
> > >
> > >
> > >
> > >
> > > >
> > > >
> > >
> > > --
> > > MySQL Replication Mailing List
> > > For list archives: http://lists.mysql.com/replication
> > > To unsubscribe:
> > http://lists.mysql.com/replication?unsub=1
> > >
> > >
> >
> >
> >
> > --
> > Mark Callaghan
> > mcallaghan@stripped
>
> --
>
> MySQL Replication Mailing List
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> To unsubscribe: http://lists.mysql.com/replication?unsub=1
>
>
--
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