Add a section about journal checkpointing, including information about the ioctl EXT4_IOC_CHECKPOINT which can be used to trigger a journal checkpoint from userspace. Also, update the journal allocation information to reflect that up to 1GB is used for the journal and that the journal is not necessarily contiguous. Signed-off-by: Leah Rumancik <leah.rumancik@xxxxxxxxx> --- Documentation/filesystems/ext4/journal.rst | 26 +++++++++++++++++----- 1 file changed, 20 insertions(+), 6 deletions(-) diff --git a/Documentation/filesystems/ext4/journal.rst b/Documentation/filesystems/ext4/journal.rst index cdbfec473167..0404e99f9988 100644 --- a/Documentation/filesystems/ext4/journal.rst +++ b/Documentation/filesystems/ext4/journal.rst @@ -4,12 +4,11 @@ Journal (jbd2) -------------- Introduced in ext3, the ext4 filesystem employs a journal to protect the -filesystem against corruption in the case of a system crash. A small -continuous region of disk (default 128MiB) is reserved inside the -filesystem as a place to land “important” data writes on-disk as quickly -as possible. Once the important data transaction is fully written to the -disk and flushed from the disk write cache, a record of the data being -committed is also written to the journal. At some later point in time, +filesystem against corruption in the case of a system crash. Up to 1GB is +reserved inside the filesystem as a place to land “important” data writes +on-disk as quickly as possible. Once the important data transaction is fully +written to the disk and flushed from the disk write cache, a record of the data +being committed is also written to the journal. At some later point in time, the journal code writes the transactions to their final locations on disk (this could involve a lot of seeking or a lot of small read-write-erases) before erasing the commit record. Should the system @@ -731,3 +730,18 @@ point, the refcount for inode 11 is not reliable, but that gets fixed by the replay of last inode 11 tag. Thus, by converting a non-idempotent procedure into a series of idempotent outcomes, fast commits ensured idempotence during the replay. + +Journal Checkpoint +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Checkpointing the journal ensures all transactions and their associated buffers +are submitted to the disk. This is used internally during critical updates to +the filesystem including journal recovery, filesystem resizing, and freeing +of the journal_t structure. + +A journal checkpoint can be triggered from userspace via the ioctl +EXT4_IOC_CHECKPOINT. This ioctl takes a single, u64 argument for flags. +Currently, the only flag supported is EXT4_IOC_CHECKPOINT_FLAG_DISCARD. When +this flag is set, the journal blocks are discarded after the journal checkpoint +is complete. The ioctl may be useful when snapshotting a system or for complying +with content deletion SLOs (when discard is supported and the discard flag is set). -- 2.31.1.527.g47e6f16901-goog
