driver-api/md/raid5-cache.rst

7 caches data to the RAID disks. The cache can be in write-through (supported
8 since 4.4) or write-back mode (supported since 4.10). mdadm (supported since
9 3.4) has a new option '--write-journal' to create array with cache. Please
11 in write-through mode. A user can switch it to write-back mode by::
13 	echo "write-back" > /sys/block/md0/md/journal_mode
15 And switch it back to write-through mode by::
17 	echo "write-through" > /sys/block/md0/md/journal_mode
20 the cache disk must be fast and sustainable.
22 write-through mode
34 The write-through cache will cache all data on cache disk first. After the data
36 two-step write will guarantee MD can recover correct data after unclean
39 In write-through mode, MD reports IO completion to upper layer (usually
44 In write-through mode, the cache disk isn't required to be big. Several
47 write-back mode
50 write-back mode fixes the 'write hole' issue too, since all write data is
51 cached on cache disk. But the main goal of 'write-back' cache is to speed up
52 write. If a write crosses all RAID disks of a stripe, we call it full-stripe
53 write. For non-full-stripe writes, MD must read old data before the new parity
56 overhead too. Write-back cache will aggregate the data and flush the data to
61 In write-back mode, MD reports IO completion to upper layer (usually
66 In write-back mode, MD also caches data in memory. The memory cache includes
67 the same data stored on cache disk, so a power loss doesn't cause data loss.
75 least several gigabytes size in write-back mode.
80 The write-through and write-back cache use the same disk format. The cache disk
91 write-through mode, MD calculates parity for IO data, writes both IO data and
96 In write-back mode, MD writes IO data to the log and reports IO completion. The
102 stripe write, free cache disk space is low or free in-kernel memory cache space