# xtrabackup 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.

## Tune 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:

```text
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:

```text
posix_fadvise(file, 0, 0, POSIX_FADV_SEQUENTIAL)
```

## Copy data files

When copying the data files to the target directory, *xtrabackup* reads and
writes 1 MB 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 *Percona 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.