112 lines
5.7 KiB
ReStructuredText
112 lines
5.7 KiB
ReStructuredText
================
|
|
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
|
|
checksummed 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.
|