I have no long running transactions. I believe the deadlock to have been caused by a high concurrency of transactions.
As soon as the backup started, mysqldump got the global lock, and seemed to hang during the flush tables (while hundreds of other clients try to execute queries). Should be easy enough to reproduce, but unfortunately I cannot as this is my production DB. -----Original Message----- From: Devananda [mailto:[EMAIL PROTECTED] Sent: Tuesday, July 11, 2006 10:59 AM To: Mark Steele Cc: mysql@lists.mysql.com Subject: Re: mysqldump with single-transaction with high-concurrency DB Argh! I should stop writing emails this early in the morning - I always miss something. I just noticed that, at the end of your first email, you did say the backup process was stuck on "flush tables". The only way that I can think of for this to happen is if another thread were holding a table open for a long transaction. As soon as that thread were to COMMIT or close, the table should be flushed. Every client that connected after the backup process /should/ have to wait for the FLUSH TABLES to complete, but /that/ has to wait for all previous connections. Does that seem to fit what happened? Regards, Devananda Mark Steele wrote: > Dan: The options I specified are correct (according to the > documentation) to get a consistent non-blocking snapshot. > (--single-transaction disables --lock-tables, --opt is the default > behavior for mysqldump). > > My question was more in the nature of will these options work in high > concurrency situations or will they cause a deadlock. (or am I missing > something here) > > The documentation states that --single-transaction will get a global > lock 'for a short period of time', which I thought to mean that it'll be > short enough to not disturb normal operations (which is what is implied > in the documentation). > > If this isn't the case in high-concurrency situations, anyone have > another method to get a consistent snapshot? > > Cheers, > > Mark > > -----Original Message----- > From: Dan Buettner [mailto:[EMAIL PROTECTED] > Sent: Monday, July 10, 2006 3:21 PM > To: Mark Steele > Cc: mysql@lists.mysql.com > Subject: Re: mysqldump with single-transaction with high-concurrency DB > > Sorry Mark - I thought your question was more of a "does this seem > right" and "how do I" than a "something's wrong here" post. > > I think your problem is coming in with the use of --opt. The article > you reference, where it says "This is an online, non-blocking backup", > makes no mention of --opt, which as you note includes --lock-tables. > >From mysqldump man page: > > --lock-tables, -l > > Lock all tables before starting the dump. The tables are > locked with > READ LOCAL to allow concurrent inserts in the case of MyISAM > tables. > For transactional tables such as InnoDB and BDB, > --single-transaction is a much better option, because it does > not > need to lock the tables at all. > > Please note that when dumping multiple databases, > --lock-tables > locks tables for each database separately. So, this option > does not > guarantee that the tables in the dump file are logically > consistent > between databases. Tables in different databases may be dumped > in > completely different states. > > Try running without --opt, possibly specifying the included options > you need individually, and see if that works better for you. > > I understand what you're saying about MySQL replication; hence the > need for monitoring the replication to ensure good backups. > > Dan > > > > > On 7/10/06, Mark Steele <[EMAIL PROTECTED]> wrote: > >> Hi Dan, >> >> >> --single-transaction >> Creates a consistent snapshot by dumping all tables in a >> single transaction. Works ONLY for tables stored in >> storage engines which support multiversioning (currently >> only InnoDB does); the dump is NOT guaranteed to be >> consistent for other storage engines. Option >> automatically turns off --lock-tables. >> --opt >> Same as --add-drop-table, --add-locks, --create-options, >> --quick, --extended-insert, --lock-tables, --set-charset, >> and --disable-keys. Enabled by default, disable with >> --skip-opt. >> >> See http://dev.mysql.com/doc/refman/5.0/en/backup-policy.html >> >> These options should produce a non-blocking consistent database >> snapshot. >> >> I can already accomplish this on a slave server, however MySQL >> replication can lead to slave drift as it is statement based (as >> > opposed > >> to row-based replication). The only safe way to guarantee a real >> > backup > >> in a MySQL replication setup is via snapshots on the master. >> >> -----Original Message----- >> From: Dan Buettner [mailto:[EMAIL PROTECTED] >> Sent: Monday, July 10, 2006 2:42 PM >> To: Mark Steele >> Cc: mysql@lists.mysql.com >> Subject: Re: mysqldump with single-transaction with high-concurrency >> > DB > >> Mark, that's the expected behavior of mysqldump with --opt and >> --single-transaction; it locks all databases and all tables for the >> duration of the dump, ensuring a consistent snapshot. >> >> With a database this size (100 GB), it's an area where throwing >> hardware at the problem may be your best bet. I suggest one of two >> approaches as possible solutions: >> >> 1) Buy a *really fast* disk array and set it up as striped on a >> superfast connection, like Ultra320 SCSI or fibre. This will lower >> the amount of time required to write the mysqldump output (which will >> likely exceed 100 GB data size due to overhead within the file). You >> might even look at 2 disk arrays on 2 channels, striping across both >> the disks in the array and across the arrays. Pros: fairly easy to >> do, not terribly expensive. Cons: You still lock up your main >> database server for backups, though possibly for less time than you do >> now. >> >> 2) Buy a second physical server for MySQL and set up replication. >> Then use the replication server to do your backups - provided you >> never let people connect directly to it, no one will notice when it >> locks up for a few hours dumping data. Once it's done dumping, >> replication will catch up on its own. This doesn't even have to be a >> very fast box, depending on your needs. If it falls behind from time >> to time that may be acceptable - depends on your needs. Pros: >> possibly less expensive than superfast arrays, no lockups of your main >> server, backup server in case of primary failure. Cons: requires >> monitoring of replication, and still requires a one-time consistent >> dump as a starting point for replication. >> >> HTH, >> Dan >> >> On 7/10/06, Mark Steele <[EMAIL PROTECTED]> wrote: >> >>> Hi folks, >>> >>> >>> >>> I've recently tried to do a database backup on a database server >>> > that > >>> has a fairly high concurrency rate (1000+ queries/sec) and have >>> >> noticed >> >>> that the backup process seemed to deadlock the machine and I had to >>> resort to extreme measures to get the database back up (killed the >>> process and had to restart it in recovery mode). >>> >>> >>> >>> The command: >>> >>> mysqldump --all-databases --opt --single-transaction --master-data=1 >>> >>>> dump.txt >>>> >>> All my tables use InnoDB, and the database is about 100 gigabytes in >>> size. >>> >>> >>> >>> Does anyone have any suggestions for getting consistent database >>> snapshots? >>> >>> >>> >>> I tried the InnoDB binary backup tool in the past, but that lead to >>> > a > >>> corrupted database, and I'm not sure that it'll lead to a different >>> outcome as both single-transaction and the binary backup tool use >>> > the > >>> same mechanism (versionnning). The documentation describes the >>> single-transaction as taking a short global lock, which is the root >>> cause of the deadlock I saw I believe. >>> >>> >>> >>> When the server was deadlocked, all the connections were 'waiting on >>> table', and the backup process was apparently stuck on 'flushing >>> tables'. >>> >>> >>> >>> Cheers, >>> >>> >>> >>> Mark Steele >>> Information Systems Manager >>> >>> Zango >>> >>> E: [EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]> >>> P: 514.787.4681 | F: 514.787.4707 >>> >>> www.zango.com <BLOCKED::http://www.zango.com> >>> >>> Read our blog at http://blog.zango.com >>> >> <BLOCKED::http://blog.zango.com> >> >>> >>> >>> > > -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED]
