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I'm starting document of the raid5-cache feature. Please note this is a kernel doc instead of a mdadm manual, so I don't add the details about how to use the feature in mdadm side. Cc: NeilBrown <neilb@suse.com> Reviewed-by: Song Liu <songliubraving@fb.com> Signed-off-by: Shaohua Li <shli@fb.com>
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RAID5 cache | ||
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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: | ||
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echo "write-back" > /sys/block/md0/md/journal_mode | ||
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And switch it back to write-through mode by: | ||
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echo "write-through" > /sys/block/md0/md/journal_mode | ||
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In both modes, all writes to the array will hit cache disk first. This means | ||
the cache disk must be fast and sustainable. | ||
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------------------------------------- | ||
write-through mode: | ||
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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'. | ||
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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'. | ||
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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. | ||
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In write-through mode, the cache disk isn't required to be big. Several | ||
hundreds megabytes are enough. | ||
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-------------------------------------- | ||
write-back mode: | ||
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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. | ||
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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. | ||
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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: | ||
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echo "2048" > /sys/block/md0/md/stripe_cache_size | ||
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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. | ||
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-------------------------------------- | ||
The implementation: | ||
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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. | ||
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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. | ||
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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. | ||
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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. |