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Diffstat (limited to 'Documentation/filesystems/journalling.rst')
| -rw-r--r-- | Documentation/filesystems/journalling.rst | 33 |
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diff --git a/Documentation/filesystems/journalling.rst b/Documentation/filesystems/journalling.rst index 7e2be2faf653..5a5f70b4063e 100644 --- a/Documentation/filesystems/journalling.rst +++ b/Documentation/filesystems/journalling.rst @@ -132,6 +132,39 @@ The opportunities for abuse and DOS attacks with this should be obvious, if you allow unprivileged userspace to trigger codepaths containing these calls. +Fast commits +~~~~~~~~~~~~ + +JBD2 to also allows you to perform file-system specific delta commits known as +fast commits. In order to use fast commits, you first need to call +:c:func:`jbd2_fc_init` and tell how many blocks at the end of journal +area should be reserved for fast commits. Along with that, you will also need +to set following callbacks that perform correspodning work: + +`journal->j_fc_cleanup_cb`: Cleanup function called after every full commit and +fast commit. + +`journal->j_fc_replay_cb`: Replay function called for replay of fast commit +blocks. + +File system is free to perform fast commits as and when it wants as long as it +gets permission from JBD2 to do so by calling the function +:c:func:`jbd2_fc_begin_commit()`. Once a fast commit is done, the client +file system should tell JBD2 about it by calling +:c:func:`jbd2_fc_end_commit()`. If file system wants JBD2 to perform a full +commit immediately after stopping the fast commit it can do so by calling +:c:func:`jbd2_fc_end_commit_fallback()`. This is useful if fast commit operation +fails for some reason and the only way to guarantee consistency is for JBD2 to +perform the full traditional commit. + +JBD2 helper functions to manage fast commit buffers. File system can use +:c:func:`jbd2_fc_get_buf()` and :c:func:`jbd2_fc_wait_bufs()` to allocate +and wait on IO completion of fast commit buffers. + +Currently, only Ext4 implements fast commits. For details of its implementation +of fast commits, please refer to the top level comments in +fs/ext4/fast_commit.c. + Summary ~~~~~~~ |