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| -rw-r--r-- | Documentation/admin-guide/md.rst | 5 | ||||
| -rw-r--r-- | Documentation/md/raid5-cache.txt | 109 |
2 files changed, 114 insertions, 0 deletions
diff --git a/Documentation/admin-guide/md.rst b/Documentation/admin-guide/md.rst index e449fb5..1e61bf5 100644 --- a/Documentation/admin-guide/md.rst +++ b/Documentation/admin-guide/md.rst @@ -725,3 +725,8 @@ These currently include: to 1. Setting this to 0 disables bypass accounting and requires preread stripes to wait until all full-width stripe- writes are complete. Valid values are 0 to stripe_cache_size. + + journal_mode (currently raid5 only) + The cache mode for raid5. raid5 could include an extra disk for + caching. The mode can be "write-throuth" and "write-back". The + default is "write-through". diff --git a/Documentation/md/raid5-cache.txt b/Documentation/md/raid5-cache.txt new file mode 100644 index 0000000..2b210f2 --- /dev/null +++ b/Documentation/md/raid5-cache.txt @@ -0,0 +1,109 @@ +RAID5 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. |
