===============================================
LFSCK is an online tool to scan, check and repair a Lustre file system that can
-be used with a file system that is mounted and in use. It checks for a large
-variety of inconsistencies between meta data targets (MDTs) and object storage
-targets (OSTs) and provides automatic correction where possible.
+be used with a file system that is mounted and in use. It contains three main
+LFSCK components: OI Scrub (primarily of use for ldiskfs-based backend), Layout
+LFSCK, and Namespace LFSCK. Each component identifies different types of Lustre
+inconsistencies.
-LFSCK does not check consistency of the on-disk format and assumes that it is
-consistent. For ldiskfs, e2fsck from e2fsprogs should be used to ensure the on
+LFSCK does not verify the on-disk format and assumes that it is consistent. For
+ldiskfs-based backend, e2fsck from e2fsprogs should be used to ensure the on
disk format is consistent. ZFS is designed to always have a valid on-disk
structure and as a result, no 'fsck' is necessary.
+* OI Scrub
+
+OI scrub is of primary use for ldiskfs-based targets. It maintains the ldiskfs
+special OI mapping consistency, reconstructs the OI mapping after the target
+is restored from file-level backup, and upgrades (if necessary) the OI mapping
+when target (MDT/OST) is upgraded from a previous release.
+
+* Layout LFSCK
+
+Layout LFSCK is concerned with consistency between metadata targets (MDTs) and
+object storage targets (OSTs). It automatically corrects inconsistencies where
+possible.
+
+* Namespace LFSCK
+
+Namespace LFSCK is concerned with consistency across the Lustre namespace.
+Namespace LFSCK works transparently across single and multiple MDTs.
Quick usage instructions
===============================================
-- start a standard scan
+* Start LFSCK
+
+If you only want OI scrub on a given MDT or OST, use this command on the given
+MDT or OST:
+# lctl lfsck_start -t scrub -M ${FSNAME}-${TARGETNAME}
+
+(FSNAME: the specified file system name created during format, e.g. "testfs".
+ TARGETNAME: the target name in the system, e.g. "MDT0000" or "OST0001".)
+
+If you want Layout LFSCK or Namespace LFSCK on a given MDT(s) and OST(s), use
+this command on the specified MDT:
+
+# lctl lfsck_start -t namespace -M ${FSNAME}-${MDTNAME}
+or
+# lctl lfsck_start -t layout -M ${FSNAME}-${MDTNAME}
+
+(MDTNAME: the MDT name in the system, e.g. "MDT0000", "MDT0001".)
-LFSCK only runs on an MDS, and starts scanning automatically if an
-inconsistency is detected when the MDT service is started. The scan can be
-started manually on a running MDT using the command:
+You can trigger multiple LFSCK components via single LFSCK command:
+# lctl lfsck_start -t namespace -t layout -M ${FSNAME}-${MDTNAME}
-# lctl lfsck_start --type namespace --type layout -M testfs-MDT0000
+For more usage, please run:
+# lctl lfsck_start -h
-- reviewing the status of lfsck
+* review the status of LFSCK
-lfsck only provides status from a MDS.
+Each LFSCK component has its own status interface on a given target.
+For example, the Namespace LFSCK status on the MDT:
+# lctl get_param -n mdd.${FSNAME}-${MDTNAME}.lfsck_namespace
-# lctl get_param -n mdd.lustre-MDT0000.lfsck_namespace
+Or the Layout LFSCK status on the OST:
+# lctl get_param -n obdfilter.${FSNAME}-${OSTNAME}.lfsck_layout
+NOTE: Layout LFSCK also works on a OST.
-- stop a lfsck scan
+(OSTNAME: the OST name in the system, e.g. "OST0000", "OST0001".)
-# lctl lfsck_stop -M lustre-MDT0000
+Or the OI Scrub status on the MDT/OST:
+# lctl get_param -n osd-ldiskfs.${FSNAME}-${TARGETNAME}.oi_scrub
+
+* stop the LFSCK
+
+Run the command on the given MDT/OST:
+# lctl lfsck_stop -M ${FSNAME}-${MDTNAME}
+
+To stop all LFSCK across the system:
+# lctl lfsck_stop -M ${FSNAME} -A
Features
* online scanning.
* control of scanning rate.
* automatic checkpoint recovery of an interrupted scan.
-* reconstruciton of the FID-to-inode mapping after a file level restore or 1.8
- upgrade
-* fixing FID-in-Dirent name entry to be consistent with the FID in the inode
- LMA.
-* detection and repair including:
- * MDT-OST inconsistencies, including:
- * dangling references.
- * unreferenced OST objects.
- * mismatched references.
- * multiple references.
* monitoring using proc and lctl interfaces.
+* maintain OI mapping for ldiskfs-based backend.
+ * reconstruction of the OI mapping after the target (MDT/OST) restored from
+ file-level backup.
+ * generate OI mapping for the MDT upgraded from 1.8.
+ * rebuild OI mapping if some of them lost or crashed.
+* check and repair kinds of namespace related inconsistent issues:
+ * the FID-in-dirent should be consistent with the FID-in-LMA.
+ * the linkEA should be consistent with related name entries.
+ * Dangling name entry: the name entry exists, but related MDT-object does
+ not exist.
+ * Orphan MDT-object: the MDT-object exists, but there is no name entry to
+ reference it.
+ * Multiple-referenced name entry: more than one MDT-objects point back to
+ the same name entry, but the name entry only references one of them.
+ * Unmatched name entry and MDT-object pairs: the name entry references the
+ MDT-object that has no linkEA for back-reference or points back to another
+ name entry that does not exist or does not reference the MDT-object.
+ * Unmatched object types: the file type in the name entry does not match the
+ type that is claimed by the MDT-object.
+ * Invalid nlink count: the MDT-object's nlink count does not match the number
+ of name entries that reference such MDT-object.
+ * Invalid name hash for striped directory: the name hash for the name entry
+ on a shard of a striped directory does not match the index stored in the
+ shard's LMV xattr.
+* verify layout consistency between MDT and OST:
+ * MDT-object with dangling reference: the MDT-object1 claims that the
+ OST-object1 is its child stripe, but on the OST, the OST-object1 does not
+ exist, or it is not materialized (so does not recognize the MDT-object1 as
+ its parent).
+ * Unmatched referenced MDT-object/OST-object pairs: the MDT-object1 claims
+ that the OST-object1 is its child stripe, but the OST-object1 claims that
+ its parent is the MDT-object2 rather than the MDT-object1. On the MDT,
+ the MDT-object2 does not exist, or not recognize the OST-object1 as its
+ child stripe. An additional case exists where the child index stored in
+ the parent layout information does not match the index information stored
+ in the child itself.
+ * Multiple referenced OST-object: the MDT-object1 claims that the OST-object1
+ is its child stripe, but the OST-object1 claims that its parent is the
+ MDT-object2 rather than the MDT-object1. On the other hand, the MDT-object2
+ recognizes the OST-object1 as its child stripe.
+ * Unreferenced (orphan) OST-object: the OST-object1 claims that the
+ MDT-object1 is its parent, but on the MDT, the MDT-object1 does not exist,
+ or it does not recognize the OST-object1 as its child.
/proc entries
===============================================
-Information about lfsck can be found in
-/proc/fs/lustre/mdd/<fsname>-<mdt>/lfsck_{namespace,layout}
+Information about LFSCK can be found in:
+/proc/fs/lustre/mdd/${FSNAME}-${MDTNAME}/lfsck_{namespace,layout}
+/proc/fs/lustre/obdfilter/${FSNAME}-${OSTNAME}/lfsck_layout
+/proc/fs/lustre/osd-ldiskfs/${FSNAME}-${TARGETNAME}/oi_scrub
LFSCK master slave design
===============================================
-The LFSCK master engine resides on the MDT, and is implemented as a kernel
+* master engine
+
+The LFSCK master engine resides on each MDT, and is implemented as a kernel
thread in the LFSCK layer. The master engine is responsible for scanning on the
-MDT and also controls slave engines on OSTs. Scanning on both MDTs and OSTs
-occurs in two stages. These stages are firstly consistency check and repair and
-secondly orphan identification and processing.
+MDTs and also controls slave engines on OSTs. Scanning on both MDTs and OSTs
+occurs in two stages. First-stage scanning will identify and resolve most of
+inconsistencies. In the second stage, information from the first stage will be
+used to resolve a remaining set of inconsistencies that had uncertain
+resolution after only one scan.
+
1. The master engine is started either by the user space command or an
-excessive number of MDT-OST inconsistency events are detected. On starting, the
-master engine sends RPCs to related OSTs to start the slave engines.
+excessive number of inconsistency events are detected (defined by
+osd-ldiskfs.<fsname>-<targetname>.full_scrub_threshold_rate). On starting, the
+master engine sends RPCs to other MDTs (when necessary) to start other master
+engines and to related OSTs to start the slave engines.
-2. The master engine on the MDS scans the MDT device using namespace iteration
-(described below). For each striped file, it calls the registered LFSCK process
-handlers to perform the relevant system consistency check/repair, which is are
-enumerated in the 'features' section. All objects on OSTs that are never
-referenced during this scan (because, for example, they are orphans) are
-recorded in an OST orphan object index on each OST.
+2. The master engine on the MDT scans the MDT local device. Each object is
+checked for the consistency criteria enumerated in the 'features' section of
+this document.
3. After the MDT completes first-stage system scanning, the master engine sends
-RPCs to OSTs that have relations to the MDT, to make the OST begin scanning.
-The master engine waits for the slave engines to complete the first-stage
-system scan and is signaled in turn by an RPC from each OST.
+RPCs to related LFSCK engines on other targets to notify that the first-stage
+scanning is complete on this MDT. The MDT waits until related targets have
+completed the first-stage scanning. At this point, the first stage scanning is
+complete and the second-stage scanning begins.
+* slave engine
-The LFSCK slave engine resides on each OST, and is implemented as a kernel
+The LFSCK slave engine resides on each OST and is implemented as a kernel
thread in the LFSCK layer. This kernel thread drives the first-stage system
scan on the OST.
1. When the slave engine is triggered by the RPC from the master engine in the
-first phase, the OST scans the local OST deviceto generate the in-memory OST
-orphan object index.
+first-stage scanning, the OST scans the local OST device to generate the
+in-memory OST orphan object index.
-2. When the first-stage system scan (for both MDTs and OSTs) is complete a list
-of non-referenced OST-objects is available. Only objects that are not accessed
-during the first stage scan are regarded as potential orphans.
+2. When the first-stage scanning (for both MDTs and OSTs) is complete a list of
+non-referenced OST-objects has been accumulated. Only objects that are not
+accessed during the first stage scan are regarded as potential orphans.
-3. In the second stage, the OSTs scan to resolve orphan objects in the file
-system. The OST orphan object index is used as input to the second stage. For
-each item in the index, the presence of a parent MDT object is verified. Orphan
-objects will either be relinked to an existing file if found - or moved into a
-new file in .lustre/lost+found.
+3. In the second-stage scanning, the OSTs work to resolve orphan objects in the
+file system. The OST orphan object index is used as input to the second stage.
+For each item in the index, the presence of a parent MDT object is verified.
+Orphan objects will either be relinked to an existing file if found - or moved
+into a new file in .lustre/lost+found.
If multiple MDTs are present, MDTs will check/repair MDT-OST consistency in
parallel. To avoid scans of the OST device the slave engine will not begin
Object traversal design reference
===============================================
-Objects are traversed by LFSCK with two methods. inode traversal and namespace
-traversal. For all types, the OST iterates through objects with inode
-traversal. The MDT will choose the iteration appropriate to the scaning type
-requested. Layout uses inode traversal, namespace use namespace traversal.
+Objects are traversed by LFSCK with two methods: object-table based iteration
+and namespace based directory traversal.
-* inode traversal
+* object-table based iteration
-Two kernel threads are employed to maximize the performance of this operation.
-One Object Storage Device (OSD) thread performs the inode table iteration,
-which scans MDT inode table and submits inode read requests asynchronously to
-drive disk I/O efficiently. The second thread is the OI Scrub thread which
-searches the OI table and updates related mapping entries. The two threads run
-concurrently and iterate inodes in a pipeline.
+The Object Storage Device (OSD) is the abstract layer above a concrete backend
+file system (i.e. ldiskfs, ZFS, Btrfs, etc.). Each OSD implementation differs
+internally to support concrete file systems. The object-table based iteration
+is implemented inside the OSD. It uses the backend special efficient scanning
+method, such as linear scanning for ldiskfs backend, to scan the local device.
+Such iteration is presented via the OSD API as a virtual index that contains
+all the objects that reside on this target.
-The Object Storage Device (OSD) is the abstract layer above a concrete back-end
-file system (i.e. ext4, ZFS, Btrfs, etc.). Each OSD implementation differs
-internally to support concrete file systems. In order to support OI Scrub the
-inode iterator is presented via the OSD API as a virtual index that contains
-all the inodes in the file system. Common interface calls are created to
-implement inode table based iteration to enable support for additional concrete
-file system in the future.
+* namespace based directory traversal
-* namespace traversal
-
-In addition to inode traversal, there are directory based items that
-need scanning for namespace consistency. For example, FID-in-Dirent and LinkEA
-are directory based features.
+In addition to object-table based iteration, there are directory based items
+that need scanning for namespace consistency. For example, FID-in-dirent and
+LinkEA are directory based features.
A naive approach to namespace traversal would be to descend recursively from
the file system root. However, this approach will typically generate random IO,
currently being scanned. For these reasons a hybrid approach to scanning is
employed.
-1. LFSCK begins inode traversal.
+1. LFSCK begins object-table based iteration.
2. If a directory is discovered then namespace traversal begins. LFSCK does not
descend into sub-directories. LFSCK ignores rename operations during the
-directory traversal because the subsequent inode traversal will guarantee
-processing of renamed objects. Reading directory blocks is a small fraction of
-the data needed for the inodes they reference. In addition, entries in the
-directory are typically allocated following the directory inode on the disk so
-for many directories the children inodes will already be available because of
-pre-fetch.
-
-3. Process each entry in the directory checking the FID-in-Dirent and the FID
+directory traversal because the subsequent object-table based iteration will
+guarantee processing of renamed objects. Reading directory blocks is a small
+fraction of the data needed for the objects they reference. In addition, entries
+in the directory are typically allocated following the directory object on the
+disk so for many directories the children objects will already be available
+because of pre-fetch.
+
+3. Process each entry in the directory checking the FID-in-dirent and the FID
in the object LMA are consistent. Repair if not. Check also that the linkEA
points back to the parent object. Check also that '.' and '..' entries are
consistent.
-4. Once all directory entries are exhausted, return to inode traversal.
+4. Once all directory entries are exhausted, return to object-table based
+iteration.
References
===============================================
OSD - Object storage device. A generic term for a storage device with an
- interface that extends beyond a block-orientated device interface.
-
-OI - Object Index. A table that maps FIDs to inodes. This table must be
- regenerated if a file level restore is performed as inodes will change.
+ interface that extends beyond a block-oriented device interface.
FID - File IDentifier. A Lustre file system identifies every file and object
with a unique 128-bit ID.
-FID-in-Dirent - FID in Directory Entry. To enhance the performance of readdir,
- the FID (and name) of a file are recorded in the current directory entry.
+OI - Object Index. A table that maps the FID to the object's backend identifier.
+ For ldiskfs-based backend, this table must be regenerated if restored from
+ file-level backup.
+
+FID-in-dirent - FID in Directory Entry. To enhance the performance of readdir,
+ the FID of a file is recorded in its directory name entry.
LMA - Lustre Metadata Attributes. A record of Lustre specific attributes, for
- example HSM state.
+ example HSM state, self-FID, and so on.
linkEA - Link Extended Attributes. When a file is created or hard-linked the
parent directory name and FID are recorded as extended attributes to the file.
-
git://git.whamcloud.com / fs / lustre-release.git / blobdiff