Lines Matching +full:back +full:- +full:to +full:- +full:back

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
10 refer to mdadm manual for details. By default (RAID array starts), the cache is
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
19 In both modes, all writes to the array will hit cache disk first. This means
22 write-through mode
26 shutdown can cause data in some stripes to not be in consistent state, eg, data
30 tries to resync the array to bring it back to normal state. But before the
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
42 exposed to 'write hole' again.
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
62 filesystems) right after the data hits cache disk. The data is flushed to raid
66 In write-back mode, MD also caches data in memory. The memory cache includes
74 mode depending on the workloads. It's recommended to use a cache disk with at
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
86 worry about the order. MD superblock has a field pointed to the valid meta data
90 order in which MD writes data to cache disk and RAID disks. Specifically, in
91 write-through mode, MD calculates parity for IO data, writes both IO data and
92 parity to the log, writes the data and parity to RAID disks after the data and
96 In write-back mode, MD writes IO data to the log and reports IO completion. The
98 memory cache. If some conditions are met, MD will flush the data to RAID disks.
102 stripe write, free cache disk space is low or free in-kernel memory cache space
108 raid4/5 and 2 for raid6), MD will write the data and parities to RAID disks. If
110 they are discarded too. MD then loads valid data and writes them to RAID disks