NAME¶
flashcache_create - create a new flashcache volume
DESCRIPTION¶
flashcache_create initializes a new flashcache volume from unformated
block devices. It creates flashcache meta data and provides new volumes though
their volume mappings.
SYNOPSIS¶
flashcache_create -p
back|
around|
thru [-s
cache
size] [-b
block size] [-v]
cachedevname cache_devname
disk_devname
OPTIONS¶
- -v
- verbose
- -p
- cache mode. Required argument. Specify any supported option:
(write-) back, (write-) thru or (write-)around. Their
respective implications are denoted below.
- -s
- cache size. Optional argument. By default, flashcache will use the
auto-detected full cache device size. When present, the given cache size
is used instead. The expected units are sectors, however any value can be
suffixed by "k", "m" or "g" to interpret the
argument in kilo-, mega- or gigabytes respectively.
- -b
- block size. Optional argument. Defaults to 4KB. Must be a power of
2. The default units is sectors. However, k can be specified as
unit type as well. (A 4KB blocksize is the correct choice for the vast
majority of applications.
- -f
- force create. Bypass all sanity checks (for example for the sector
size).
Use with care.
CACHING MODES¶
The following caching modes are supported:
Writethrough (
thru) - safest mode, all writes are cached to the
cache device but are also being written to disk immediately. If the used cache
device has a lower write performance than the backend disk (many early
generation SSD drives manufactured between 2008-2010 are known for such a poor
write performance) enabling the writethrough mode may decrease the system
write performance. All disk reads are cached (tunable through flashcache's
/proc interface).
Writearound (
ardound) - again, very safe, writes are not written
to the cache device, but directly to the backend disk instead. Disk blocks
will only be cached after they are read. All disk reads are cached (tunable
through flashcache's
/proc interface).
Writeback (
back) - fastest mode but less safe. Writes only go to
the cache device initially, and are being written to the backend disk later,
depending on configured system policie. All disk reads are cached (tunable
through flashcache's
/proc interface).
CACHE PERSISTENCE¶
Writethru and Writearound caches are not persistent across a device removal or a
reboot. Only Writeback caches are persistent across device removals and
reboots. This reinforces 'writeback is fastest', 'writethrough is safest'.
EXAMPLES¶
- flashcache_create -p back -s 1g -b 4k cachedev /dev/sdc
/dev/sdb
- Creates a 1GB writeback cache volume with a 4KB block size on the cache
device /dev/sdc to cache the disk volume /dev/sdb. The name of the device
created is "cachedev".
- flashcache_create -p thru -s 2097152 -b 8 cachedev /dev/sdc
/dev/sdb
- Same as above but creates a write through cache with units specified in
sectors instead. The name of the device created is
"cachedev".
SEE ALSO¶
flashcache_load(8),
flashcache_destroy(8)
README and other documents in
/usr/share/doc/flashcache-utils. In
particular, see
flashcache-sa-guide.txt for configuring the flashcache
kernel module through its
/proc interface.
The flashcache source code and all documentation may be downloaded from
<
https://github.com/facebook/flashcache/>.
AUTHORS¶
Flashcache is developed by Mohan Srinivasan <mohan@fb.com>
This man page was written by Dmitry Smirnov <onlyjob@member.fsf.org> for
Debian GNU/Linux (but may be used by others).