Concerns and What Does Not Work in XtraDB Backup
Jun 30, 2009 / By Sheeri Cabral
A short time ago I posted how I was Using XtraDB Backup to backup InnoDB. Overall, the blog post was positive, but experiences that others have had (and commented to on that blog post) have made me want to put out another short article about using XtraDB backup.
The first few points remain the same — the backup process is stable, we were able to use the binaries without compiling, and using Innobackupex as the wrapper script, analogous to Innobackup.pl.
However, we did figure out why Xtrabackup had to be run as the mysql user:
Xtrabackup writes to the data dictionary file (ibdata1, for example). We have not examined if it also writes to the data and index files (still ibdata1 by default, or the .ibd files when using innodb_file_per_table). [EDIT: The authors of Xtrabackup have commented below as to why the write occurs:
xtrabackup is kind of very small and restricted instance of InnoDB, and reuses a lot of InnoDB code.
InnoDB by default requires O_RDWR option on ibdata1 files at start, and xtrabackup therefore also did that. In the latest push to trunk it was fixed, now xtrabackup opens file with O_RDONLY flag.
When the new version is released, we will be sure to test it so that we can run the backup as a read-only user, and report back.]
On the one hand, Xtrabackup is a free tool. On the other hand, modifying InnoDB’s underlying files risks corrupting all the InnoDB tables in the system. Which is a tricky situation when it is your backup tool that might cause corruption that is beyond repair, as you do not know if you can trust your backups.
Regarding the complaint Dan R commented on the previous post that innobackupex could stream the backup to gzip, and helpfully gave the syntax. Shlomi Noach also pointed out streaming as a feature of Xtrabackup that ibbackup does not have. However, Gerry Narvaja, a co-worker noted (and commented):
I’ve been trying to install xtrabackup/innobackupex for a customer and I’m finding a few glitches, especially w/ streaming:
We use innoback(ex) wrapped in our own scripts to allow for rotation and other operations. We also use ZRM for some installations, so this would apply to integration with this tool as well. These are the glitches I found:
1. Using streaming by piping it into ‘gzip’ masks the return code from innobackupex.pl. Since gzip will almost always return 0, you can’t rely on it to determine backup success.
2. The next alternative would be to review the innobackupex.pl’s output for the OK at the end. But since it redirects the output to ‘stderr’ to allow for streaming, you need to add “2> innobackupex.log” before piping and grep for the “OK” at the end.
and noted that there were some limitations:
innobackupex script is limited in the type of options you can specify compared to what the xtrabackup executable supports. I find this annoying since it limits the directories where you can have the backups, data directories and logs.
Xtrabackup doesn’t work for MySQL v4.1. In the Percona forums there was a suggestion that the 5.0 patch should work. This is true, but xtrabackup.c has other dependencies on 5.x definitions and structures I didn’t have time to review. Baron Schwartz correctly suggested that these dependencies might be trivial in a tweet directed to myself. I’ll post my findings to the Percona forums and hopefully we can soon have a patched version.
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