Implementation Details¶
This page contains notes on various internal aspects of the xtrabackup tool’s operation.
File Permissions¶
xtrabackup opens the source data files in read-write mode, although it does not modify the files. This means that you must run xtrabackup as a user who has permission to write the data files. The reason for opening the files in read-write mode is that xtrabackup uses the embedded InnoDB libraries to open and read the files, and InnoDB opens them in read-write mode because it normally assumes it is going to write to them.
Tuning the OS Buffers¶
Because xtrabackup reads large amounts of data from the filesystem, it uses
posix_fadvise() where possible, to instruct the operating system not to try
to cache the blocks it reads from disk. Without this hint, the operating system
would prefer to cache the blocks, assuming that xtrabackup is likely to need
them again, which is not the case. Caching such large files can place pressure
on the operating system’s virtual memory and cause other processes, such as the
database server, to be swapped out. The xtrabackup tool avoids this with the
following hint on both the source and destination files:
posix_fadvise(file, 0, 0, POSIX_FADV_DONTNEED)
In addition, xtrabackup asks the operating system to perform more aggressive read-ahead optimizations on the source files:
posix_fadvise(file, 0, 0, POSIX_FADV_SEQUENTIAL)
Copying Data Files¶
When copying the data files to the target directory, xtrabackup reads and
writes 1MB of data at a time. This is not configurable. When copying the log
file, xtrabackup reads and writes 512 bytes at a time. This is also not
possible to configure, and matches InnoDB’s behavior (workaround exists in
Percona Server for MySQL because it has an option to tune
innodb_log_block_size for XtraDB, and in that case XtraBackup will
match the tuning).
After reading from the files, xtrabackup iterates over the 1MB buffer a page
at a time, and checks for page corruption on each page with InnoDB’s
buf_page_is_corrupted() function. If the page is corrupt, it re-reads and
retries up to 10 times for each page. It skips this check on the doublewrite
buffer.