From: Darrick J. Wong <djwong@xxxxxxxxxx> Start the fourth chapter of the online fsck design documentation, which discusses the user interface and the background scrubbing service. Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx> --- .../filesystems/xfs-online-fsck-design.rst | 114 ++++++++++++++++++++ 1 file changed, 114 insertions(+) diff --git a/Documentation/filesystems/xfs-online-fsck-design.rst b/Documentation/filesystems/xfs-online-fsck-design.rst index d630b6bdbe4a..42e82971e036 100644 --- a/Documentation/filesystems/xfs-online-fsck-design.rst +++ b/Documentation/filesystems/xfs-online-fsck-design.rst @@ -750,3 +750,117 @@ Proposed patchsets include `general stress testing <https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfstests-dev.git/log/?h=race-scrub-and-mount-state-changes>`_ and the `evolution of existing per-function stress testing <https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfstests-dev.git/log/?h=refactor-scrub-stress>`_. + +4. User Interface +================= + +The primary user of online fsck is the system administrator, just like offline +repair. +Online fsck presents two modes of operation to administrators: +A foreground CLI process for online fsck on demand, and a background service +that performs autonomous checking and repair. + +Checking on Demand +------------------ + +For administrators who want the absolute freshest information about the +metadata in a filesystem, ``xfs_scrub`` can be run as a foreground process on +a command line. +The program checks every piece of metadata in the filesystem while the +administrator waits for the results to be reported, just like the existing +``xfs_repair`` tool. +Both tools share a ``-n`` option to perform a read-only scan, and a ``-v`` +option to increase the verbosity of the information reported. + +A new feature of ``xfs_scrub`` is the ``-x`` option, which employs the error +correction capabilities of the hardware to check data file contents. +The media scan is not enabled by default because it may dramatically increase +program runtime and consume a lot of bandwidth on older storage hardware. + +The output of a foreground invocation is captured in the system log. + +The ``xfs_scrub_all`` program walks the list of mounted filesystems and +initiates ``xfs_scrub`` for each of them in parallel. +It serializes scans for any filesystems that resolve to the same top level +kernel block device to prevent resource overconsumption. + +Background Service +------------------ + +To reduce the workload of system administrators, the ``xfs_scrub`` package +provides a suite of `systemd <https://systemd.io/>`_ timers and services that +run online fsck automatically on weekends. +The background service configures scrub to run with as little privilege as +possible, the lowest CPU and IO priority, and in a CPU-constrained single +threaded mode. +It is hoped that this minimizes the amount of load generated on the system and +avoids starving regular workloads. + +The output of the background service is also captured in the system log. +If desired, reports of failures (either due to inconsistencies or mere runtime +errors) can be emailed automatically by setting the ``EMAIL_ADDR`` environment +variable in the following service files: + +* ``xfs_scrub_fail@.service`` +* ``xfs_scrub_media_fail@.service`` +* ``xfs_scrub_all_fail.service`` + +The decision to enable the background scan is left to the system administrator. +This can be done by enabling either of the following services: + +* ``xfs_scrub_all.timer`` on systemd systems +* ``xfs_scrub_all.cron`` on non-systemd systems + +This automatic weekly scan is configured out of the box to perform an +additional media scan of all file data once per month. +This is less foolproof than, say, storing file data block checksums, but much +more performant if application software provides its own integrity checking, +redundancy can be provided elsewhere above the filesystem, or the storage +device's integrity guarantees are deemed sufficient. + +The systemd unit file definitions have been subjected to a security audit +(as of systemd 249) to ensure that the xfs_scrub processes have as little +access to the rest of the system as possible. +This was performed via ``systemd-analyze security``, after which privileges +were restricted to the minimum required, sandboxing was set up to the maximal +extent possible with sandboxing and system call filtering; and access to the +filesystem tree was restricted to the minimum needed to start the program and +access the filesystem being scanned. +The service definition files restrict CPU usage to 80% of one CPU core, and +apply as nice of a priority to IO and CPU scheduling as possible. +This measure was taken to minimize delays in the rest of the filesystem. +No such hardening has been performed for the cron job. + +Proposed patchset: +`Enabling the xfs_scrub background service +<https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfsprogs-dev.git/log/?h=scrub-media-scan-service>`_. + +Health Reporting +---------------- + +XFS caches a summary of each filesystem's health status in memory. +The information is updated whenever ``xfs_scrub`` is run, or whenever +inconsistencies are detected in the filesystem metadata during regular +operations. +System administrators should use the ``health`` command of ``xfs_spaceman`` to +download this information into a human-readable format. +If problems have been observed, the administrator can schedule a reduced +service window to run the online repair tool to correct the problem. +Failing that, the administrator can decide to schedule a maintenance window to +run the traditional offline repair tool to correct the problem. + +**Question**: Should the health reporting integrate with the new inotify fs +error notification system? + +**Question**: Would it be helpful for sysadmins to have a daemon to listen for +corruption notifications and initiate a repair? + +*Answer*: These questions remain unanswered, but should be a part of the +conversation with early adopters and potential downstream users of XFS. + +Proposed patchsets include +`wiring up health reports to correction returns +<https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git/log/?h=corruption-health-reports>`_ +and +`preservation of sickness info during memory reclaim +<https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git/log/?h=indirect-health-reporting>`_.
