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| author | Mauro Carvalho Chehab <mchehab+samsung@kernel.org> | 2019-06-18 16:50:07 -0300 |
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| committer | Mauro Carvalho Chehab <mchehab+samsung@kernel.org> | 2019-07-15 09:20:28 -0300 |
| commit | c0b11a50aee643ac40ded5dbcd48189ee0926ee4 (patch) | |
| tree | d755f6fab44027aed7ef6ce651836b1bedfddf82 /Documentation/md | |
| parent | 19024c09c243c5107f738286459a0dd85697b089 (diff) | |
| download | net-c0b11a50aee643ac40ded5dbcd48189ee0926ee4.tar.gz | |
docs: md: move it to the driver-api book
The docs there were meant to be read by a Kernel developer.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Diffstat (limited to 'Documentation/md')
| -rw-r--r-- | Documentation/md/index.rst | 12 | ||||
| -rw-r--r-- | Documentation/md/md-cluster.rst | 385 | ||||
| -rw-r--r-- | Documentation/md/raid5-cache.rst | 111 | ||||
| -rw-r--r-- | Documentation/md/raid5-ppl.rst | 47 |
4 files changed, 0 insertions, 555 deletions
diff --git a/Documentation/md/index.rst b/Documentation/md/index.rst deleted file mode 100644 index c4db34ed327ddf..00000000000000 --- a/Documentation/md/index.rst +++ /dev/null @@ -1,12 +0,0 @@ -:orphan: - -==== -RAID -==== - -.. toctree:: - :maxdepth: 1 - - md-cluster - raid5-cache - raid5-ppl diff --git a/Documentation/md/md-cluster.rst b/Documentation/md/md-cluster.rst deleted file mode 100644 index 96eb52cec7ebc6..00000000000000 --- a/Documentation/md/md-cluster.rst +++ /dev/null @@ -1,385 +0,0 @@ -========== -MD Cluster -========== - -The cluster MD is a shared-device RAID for a cluster, it supports -two levels: raid1 and raid10 (limited support). - - -1. On-disk format -================= - -Separate write-intent-bitmaps are used for each cluster node. -The bitmaps record all writes that may have been started on that node, -and may not yet have finished. The on-disk layout is:: - - 0 4k 8k 12k - ------------------------------------------------------------------- - | idle | md super | bm super [0] + bits | - | bm bits[0, contd] | bm super[1] + bits | bm bits[1, contd] | - | bm super[2] + bits | bm bits [2, contd] | bm super[3] + bits | - | bm bits [3, contd] | | | - -During "normal" functioning we assume the filesystem ensures that only -one node writes to any given block at a time, so a write request will - - - set the appropriate bit (if not already set) - - commit the write to all mirrors - - schedule the bit to be cleared after a timeout. - -Reads are just handled normally. It is up to the filesystem to ensure -one node doesn't read from a location where another node (or the same -node) is writing. - - -2. DLM Locks for management -=========================== - -There are three groups of locks for managing the device: - -2.1 Bitmap lock resource (bm_lockres) -------------------------------------- - - The bm_lockres protects individual node bitmaps. They are named in - the form bitmap000 for node 1, bitmap001 for node 2 and so on. When a - node joins the cluster, it acquires the lock in PW mode and it stays - so during the lifetime the node is part of the cluster. The lock - resource number is based on the slot number returned by the DLM - subsystem. Since DLM starts node count from one and bitmap slots - start from zero, one is subtracted from the DLM slot number to arrive - at the bitmap slot number. - - The LVB of the bitmap lock for a particular node records the range - of sectors that are being re-synced by that node. No other - node may write to those sectors. This is used when a new nodes - joins the cluster. - -2.2 Message passing locks -------------------------- - - Each node has to communicate with other nodes when starting or ending - resync, and for metadata superblock updates. This communication is - managed through three locks: "token", "message", and "ack", together - with the Lock Value Block (LVB) of one of the "message" lock. - -2.3 new-device management -------------------------- - - A single lock: "no-new-dev" is used to co-ordinate the addition of - new devices - this must be synchronized across the array. - Normally all nodes hold a concurrent-read lock on this device. - -3. Communication -================ - - Messages can be broadcast to all nodes, and the sender waits for all - other nodes to acknowledge the message before proceeding. Only one - message can be processed at a time. - -3.1 Message Types ------------------ - - There are six types of messages which are passed: - -3.1.1 METADATA_UPDATED -^^^^^^^^^^^^^^^^^^^^^^ - - informs other nodes that the metadata has - been updated, and the node must re-read the md superblock. This is - performed synchronously. It is primarily used to signal device - failure. - -3.1.2 RESYNCING -^^^^^^^^^^^^^^^ - informs other nodes that a resync is initiated or - ended so that each node may suspend or resume the region. Each - RESYNCING message identifies a range of the devices that the - sending node is about to resync. This overrides any previous - notification from that node: only one ranged can be resynced at a - time per-node. - -3.1.3 NEWDISK -^^^^^^^^^^^^^ - - informs other nodes that a device is being added to - the array. Message contains an identifier for that device. See - below for further details. - -3.1.4 REMOVE -^^^^^^^^^^^^ - - A failed or spare device is being removed from the - array. The slot-number of the device is included in the message. - - 3.1.5 RE_ADD: - - A failed device is being re-activated - the assumption - is that it has been determined to be working again. - - 3.1.6 BITMAP_NEEDS_SYNC: - - If a node is stopped locally but the bitmap - isn't clean, then another node is informed to take the ownership of - resync. - -3.2 Communication mechanism ---------------------------- - - The DLM LVB is used to communicate within nodes of the cluster. There - are three resources used for the purpose: - -3.2.1 token -^^^^^^^^^^^ - The resource which protects the entire communication - system. The node having the token resource is allowed to - communicate. - -3.2.2 message -^^^^^^^^^^^^^ - The lock resource which carries the data to communicate. - -3.2.3 ack -^^^^^^^^^ - - The resource, acquiring which means the message has been - acknowledged by all nodes in the cluster. The BAST of the resource - is used to inform the receiving node that a node wants to - communicate. - -The algorithm is: - - 1. receive status - all nodes have concurrent-reader lock on "ack":: - - sender receiver receiver - "ack":CR "ack":CR "ack":CR - - 2. sender get EX on "token", - sender get EX on "message":: - - sender receiver receiver - "token":EX "ack":CR "ack":CR - "message":EX - "ack":CR - - Sender checks that it still needs to send a message. Messages - received or other events that happened while waiting for the - "token" may have made this message inappropriate or redundant. - - 3. sender writes LVB - - sender down-convert "message" from EX to CW - - sender try to get EX of "ack" - - :: - - [ wait until all receivers have *processed* the "message" ] - - [ triggered by bast of "ack" ] - receiver get CR on "message" - receiver read LVB - receiver processes the message - [ wait finish ] - receiver releases "ack" - receiver tries to get PR on "message" - - sender receiver receiver - "token":EX "message":CR "message":CR - "message":CW - "ack":EX - - 4. triggered by grant of EX on "ack" (indicating all receivers - have processed message) - - sender down-converts "ack" from EX to CR - - sender releases "message" - - sender releases "token" - - :: - - receiver upconvert to PR on "message" - receiver get CR of "ack" - receiver release "message" - - sender receiver receiver - "ack":CR "ack":CR "ack":CR - - -4. Handling Failures -==================== - -4.1 Node Failure ----------------- - - When a node fails, the DLM informs the cluster with the slot - number. The node starts a cluster recovery thread. The cluster - recovery thread: - - - acquires the bitmap<number> lock of the failed node - - opens the bitmap - - reads the bitmap of the failed node - - copies the set bitmap to local node - - cleans the bitmap of the failed node - - releases bitmap<number> lock of the failed node - - initiates resync of the bitmap on the current node - md_check_recovery is invoked within recover_bitmaps, - then md_check_recovery -> metadata_update_start/finish, - it will lock the communication by lock_comm. - Which means when one node is resyncing it blocks all - other nodes from writing anywhere on the array. - - The resync process is the regular md resync. However, in a clustered - environment when a resync is performed, it needs to tell other nodes - of the areas which are suspended. Before a resync starts, the node - send out RESYNCING with the (lo,hi) range of the area which needs to - be suspended. Each node maintains a suspend_list, which contains the - list of ranges which are currently suspended. On receiving RESYNCING, - the node adds the range to the suspend_list. Similarly, when the node - performing resync finishes, it sends RESYNCING with an empty range to - other nodes and other nodes remove the corresponding entry from the - suspend_list. - - A helper function, ->area_resyncing() can be used to check if a - particular I/O range should be suspended or not. - -4.2 Device Failure -================== - - Device failures are handled and communicated with the metadata update - routine. When a node detects a device failure it does not allow - any further writes to that device until the failure has been - acknowledged by all other nodes. - -5. Adding a new Device ----------------------- - - For adding a new device, it is necessary that all nodes "see" the new - device to be added. For this, the following algorithm is used: - - 1. Node 1 issues mdadm --manage /dev/mdX --add /dev/sdYY which issues - ioctl(ADD_NEW_DISK with disc.state set to MD_DISK_CLUSTER_ADD) - 2. Node 1 sends a NEWDISK message with uuid and slot number - 3. Other nodes issue kobject_uevent_env with uuid and slot number - (Steps 4,5 could be a udev rule) - 4. In userspace, the node searches for the disk, perhaps - using blkid -t SUB_UUID="" - 5. Other nodes issue either of the following depending on whether - the disk was found: - ioctl(ADD_NEW_DISK with disc.state set to MD_DISK_CANDIDATE and - disc.number set to slot number) - ioctl(CLUSTERED_DISK_NACK) - 6. Other nodes drop lock on "no-new-devs" (CR) if device is found - 7. Node 1 attempts EX lock on "no-new-dev" - 8. If node 1 gets the lock, it sends METADATA_UPDATED after - unmarking the disk as SpareLocal - 9. If not (get "no-new-dev" lock), it fails the operation and sends - METADATA_UPDATED. - 10. Other nodes get the information whether a disk is added or not - by the following METADATA_UPDATED. - -6. Module interface -=================== - - There are 17 call-backs which the md core can make to the cluster - module. Understanding these can give a good overview of the whole - process. - -6.1 join(nodes) and leave() ---------------------------- - - These are called when an array is started with a clustered bitmap, - and when the array is stopped. join() ensures the cluster is - available and initializes the various resources. - Only the first 'nodes' nodes in the cluster can use the array. - -6.2 slot_number() ------------------ - - Reports the slot number advised by the cluster infrastructure. - Range is from 0 to nodes-1. - -6.3 resync_info_update() ------------------------- - - This updates the resync range that is stored in the bitmap lock. - The starting point is updated as the resync progresses. The - end point is always the end of the array. - It does *not* send a RESYNCING message. - -6.4 resync_start(), resync_finish() ------------------------------------ - - These are called when resync/recovery/reshape starts or stops. - They update the resyncing range in the bitmap lock and also - send a RESYNCING message. resync_start reports the whole - array as resyncing, resync_finish reports none of it. - - resync_finish() also sends a BITMAP_NEEDS_SYNC message which - allows some other node to take over. - -6.5 metadata_update_start(), metadata_update_finish(), metadata_update_cancel() -------------------------------------------------------------------------------- - - metadata_update_start is used to get exclusive access to - the metadata. If a change is still needed once that access is - gained, metadata_update_finish() will send a METADATA_UPDATE - message to all other nodes, otherwise metadata_update_cancel() - can be used to release the lock. - -6.6 area_resyncing() --------------------- - - This combines two elements of functionality. - - Firstly, it will check if any node is currently resyncing - anything in a given range of sectors. If any resync is found, - then the caller will avoid writing or read-balancing in that - range. - - Secondly, while node recovery is happening it reports that - all areas are resyncing for READ requests. This avoids races - between the cluster-filesystem and the cluster-RAID handling - a node failure. - -6.7 add_new_disk_start(), add_new_disk_finish(), new_disk_ack() ---------------------------------------------------------------- - - These are used to manage the new-disk protocol described above. - When a new device is added, add_new_disk_start() is called before - it is bound to the array and, if that succeeds, add_new_disk_finish() - is called the device is fully added. - - When a device is added in acknowledgement to a previous - request, or when the device is declared "unavailable", - new_disk_ack() is called. - -6.8 remove_disk() ------------------ - - This is called when a spare or failed device is removed from - the array. It causes a REMOVE message to be send to other nodes. - -6.9 gather_bitmaps() --------------------- - - This sends a RE_ADD message to all other nodes and then - gathers bitmap information from all bitmaps. This combined - bitmap is then used to recovery the re-added device. - -6.10 lock_all_bitmaps() and unlock_all_bitmaps() ------------------------------------------------- - - These are called when change bitmap to none. If a node plans - to clear the cluster raid's bitmap, it need to make sure no other - nodes are using the raid which is achieved by lock all bitmap - locks within the cluster, and also those locks are unlocked - accordingly. - -7. Unsupported features -======================= - -There are somethings which are not supported by cluster MD yet. - -- change array_sectors. diff --git a/Documentation/md/raid5-cache.rst b/Documentation/md/raid5-cache.rst deleted file mode 100644 index d7a15f44a7c3aa..00000000000000 --- a/Documentation/md/raid5-cache.rst +++ /dev/null @@ -1,111 +0,0 @@ -================ -RAID 4/5/6 cache -================ - -Raid 4/5/6 could include an extra disk for data cache besides normal RAID -disks. The role of RAID disks isn't changed with the cache disk. The cache disk -caches data to the RAID disks. The cache can be in write-through (supported -since 4.4) or write-back mode (supported since 4.10). mdadm (supported since -3.4) has a new option '--write-journal' to create array with cache. Please -refer to mdadm manual for details. By default (RAID array starts), the cache is -in write-through mode. A user can switch it to write-back mode by:: - - echo "write-back" > /sys/block/md0/md/journal_mode - -And switch it back to write-through mode by:: - - echo "write-through" > /sys/block/md0/md/journal_mode - -In both modes, all writes to the array will hit cache disk first. This means -the cache disk must be fast and sustainable. - -write-through mode -================== - -This mode mainly fixes the 'write hole' issue. For RAID 4/5/6 array, an unclean -shutdown can cause data in some stripes to not be in consistent state, eg, data -and parity don't match. The reason is that a stripe write involves several RAID -disks and it's possible the writes don't hit all RAID disks yet before the -unclean shutdown. We call an array degraded if it has inconsistent data. MD -tries to resync the array to bring it back to normal state. But before the -resync completes, any system crash will expose the chance of real data -corruption in the RAID array. This problem is called 'write hole'. - -The write-through cache will cache all data on cache disk first. After the data -is safe on the cache disk, the data will be flushed onto RAID disks. The -two-step write will guarantee MD can recover correct data after unclean -shutdown even the array is degraded. Thus the cache can close the 'write hole'. - -In write-through mode, MD reports IO completion to upper layer (usually -filesystems) after the data is safe on RAID disks, so cache disk failure -doesn't cause data loss. Of course cache disk failure means the array is -exposed to 'write hole' again. - -In write-through mode, the cache disk isn't required to be big. Several -hundreds megabytes are enough. - -write-back mode -=============== - -write-back mode fixes the 'write hole' issue too, since all write data is -cached on cache disk. But the main goal of 'write-back' cache is to speed up -write. If a write crosses all RAID disks of a stripe, we call it full-stripe -write. For non-full-stripe writes, MD must read old data before the new parity -can be calculated. These synchronous reads hurt write throughput. Some writes -which are sequential but not dispatched in the same time will suffer from this -overhead too. Write-back cache will aggregate the data and flush the data to -RAID disks only after the data becomes a full stripe write. This will -completely avoid the overhead, so it's very helpful for some workloads. A -typical workload which does sequential write followed by fsync is an example. - -In write-back mode, MD reports IO completion to upper layer (usually -filesystems) right after the data hits cache disk. The data is flushed to raid -disks later after specific conditions met. So cache disk failure will cause -data loss. - -In write-back mode, MD also caches data in memory. The memory cache includes -the same data stored on cache disk, so a power loss doesn't cause data loss. -The memory cache size has performance impact for the array. It's recommended -the size is big. A user can configure the size by:: - - echo "2048" > /sys/block/md0/md/stripe_cache_size - -Too small cache disk will make the write aggregation less efficient in this -mode depending on the workloads. It's recommended to use a cache disk with at -least several gigabytes size in write-back mode. - -The implementation -================== - -The write-through and write-back cache use the same disk format. The cache disk -is organized as a simple write log. The log consists of 'meta data' and 'data' -pairs. The meta data describes the data. It also includes checksum and sequence -ID for recovery identification. Data can be IO data and parity data. Data is -checksumed too. The checksum is stored in the meta data ahead of the data. The -checksum is an optimization because MD can write meta and data freely without -worry about the order. MD superblock has a field pointed to the valid meta data -of log head. - -The log implementation is pretty straightforward. The difficult part is the -order in which MD writes data to cache disk and RAID disks. Specifically, in -write-through mode, MD calculates parity for IO data, writes both IO data and -parity to the log, writes the data and parity to RAID disks after the data and -parity is settled down in log and finally the IO is finished. Read just reads -from raid disks as usual. - -In write-back mode, MD writes IO data to the log and reports IO completion. The -data is also fully cached in memory at that time, which means read must query -memory cache. If some conditions are met, MD will flush the data to RAID disks. -MD will calculate parity for the data and write parity into the log. After this -is finished, MD will write both data and parity into RAID disks, then MD can -release the memory cache. The flush conditions could be stripe becomes a full -stripe write, free cache disk space is low or free in-kernel memory cache space -is low. - -After an unclean shutdown, MD does recovery. MD reads all meta data and data -from the log. The sequence ID and checksum will help us detect corrupted meta -data and data. If MD finds a stripe with data and valid parities (1 parity for -raid4/5 and 2 for raid6), MD will write the data and parities to RAID disks. If -parities are incompleted, they are discarded. If part of data is corrupted, -they are discarded too. MD then loads valid data and writes them to RAID disks -in normal way. diff --git a/Documentation/md/raid5-ppl.rst b/Documentation/md/raid5-ppl.rst deleted file mode 100644 index 357e5515bc5550..00000000000000 --- a/Documentation/md/raid5-ppl.rst +++ /dev/null @@ -1,47 +0,0 @@ -================== -Partial Parity Log -================== - -Partial Parity Log (PPL) is a feature available for RAID5 arrays. The issue -addressed by PPL is that after a dirty shutdown, parity of a particular stripe -may become inconsistent with data on other member disks. If the array is also -in degraded state, there is no way to recalculate parity, because one of the -disks is missing. This can lead to silent data corruption when rebuilding the -array or using it is as degraded - data calculated from parity for array blocks -that have not been touched by a write request during the unclean shutdown can -be incorrect. Such condition is known as the RAID5 Write Hole. Because of -this, md by default does not allow starting a dirty degraded array. - -Partial parity for a write operation is the XOR of stripe data chunks not -modified by this write. It is just enough data needed for recovering from the -write hole. XORing partial parity with the modified chunks produces parity for -the stripe, consistent with its state before the write operation, regardless of -which chunk writes have completed. If one of the not modified data disks of -this stripe is missing, this updated parity can be used to recover its -contents. PPL recovery is also performed when starting an array after an -unclean shutdown and all disks are available, eliminating the need to resync -the array. Because of this, using write-intent bitmap and PPL together is not -supported. - -When handling a write request PPL writes partial parity before new data and -parity are dispatched to disks. PPL is a distributed log - it is stored on -array member drives in the metadata area, on the parity drive of a particular -stripe. It does not require a dedicated journaling drive. Write performance is -reduced by up to 30%-40% but it scales with the number of drives in the array -and the journaling drive does not become a bottleneck or a single point of -failure. - -Unlike raid5-cache, the other solution in md for closing the write hole, PPL is -not a true journal. It does not protect from losing in-flight data, only from -silent data corruption. If a dirty disk of a stripe is lost, no PPL recovery is -performed for this stripe (parity is not updated). So it is possible to have -arbitrary data in the written part of a stripe if that disk is lost. In such -case the behavior is the same as in plain raid5. - -PPL is available for md version-1 metadata and external (specifically IMSM) -metadata arrays. It can be enabled using mdadm option --consistency-policy=ppl. - -There is a limitation of maximum 64 disks in the array for PPL. It allows to -keep data structures and implementation simple. RAID5 arrays with so many disks -are not likely due to high risk of multiple disks failure. Such restriction -should not be a real life limitation. |