MD: add doc for raid5-cache

      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".

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.