FlashCache System Administration Guide笔记
Choice of Caching Modes :
=========================Writethrough - safest, all writes are cached to ssd but also written to disk
immediately. If your ssd has slower write performance than your disk (likely
for early generation SSDs purchased in 2008-2010), this may limit your system
write performance. All disk reads are cached (tunable).
Writearound - again, very safe, writes are not written to ssd but directly todisk. Disk blocks will only be cached after they are read. All disk reads
are cached (tunable).
Writeback - fastest but less safe. Writes only go to the ssd initially, andbased on various policies are written to disk later. All disk reads are
cached (tunable).
Writeonly - variant of writeback caching. In this mode, only incoming writesare cached. No reads are ever cached.
write around，应用在读负载很大，随机读，但同时伴随有大量的顺序写的情况，如HBASE。
Cache creation and loading using the flashcache utilities :
=========================================================
Included are 3 utilities - flashcache_create, flashcache_load and
flashcache_destroy. These utilities use dmsetup internally, presenting
a simpler interface to create, load and destroy flashcache volumes.
It is expected that the majority of users can use these utilities
instead of using dmsetup.
flashcache_create : Create a new flashcache volume.
flashcache_create [-v] -p back|around|thru [-s cache size] [-w] [-b block size] cachedevname ssd_devname disk_devname
-v : verbose.
-p : cache mode (writeback/writethrough/writearound).
-s : cache size. Optional. If this is not specified, the entire ssd device
is used as cache. The default units is sectors. But you can specify
k/m/g as units as well.
-b : block size. Optional. Defaults to 4KB. Must be a power of 2.
The default units is sectors. But you can specify k as units as well.
(A 4KB blocksize is the correct choice for the vast majority of
applications. But see the section "Cache Blocksize selection" below).
-f : force create. by pass checks (eg for ssd sectorsize).
-w : write cache mode. Only writes are cached, not reads
-d : disk associativity, within each cache set, we store several contigous
disk extents. Defaults to off.
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 ssd
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".
flashcache_load : Load an existing writeback cache volume.
flashcache_load ssd_devname [cachedev_name]
Example :
flashcache_load /dev/sd
Load the existing writeback cache on /dev/sdc, using the virtual
cachedev_name from when the device was created. If you're upgrading from
an older flashcache device format that didn't store the cachedev name
internally, or you want to change the cachedev name use, you can specify
it as an optional second argument to flashcache_load.
For writethrough and writearound caches flashcache_load is not needed; flashcache_create
should be used each time.
flashcache_destroy : Destroy an existing writeback flashcache. All data will be lost !!!
flashcache_destroy ssd_devname
Example :
flashcache_destroy /dev/sdc
Destroy the existing cache on /dev/sdc. All data is lost !!!
For writethrough and writearound caches this is not necessary.
Removing a flashcache volume :
============================
Use dmsetup remove to remove a flashcache volume. For writeback
cache mode, the default behavior on a remove is to clean all dirty
cache blocks to disk. The remove will not return until all blocks
are cleaned. Progress on disk cleaning is reported on the console
(also see the "fast_remove" flashcache sysctl).
A reboot of the node will also result in all dirty cache blocks being
cleaned synchronously (again see the note about "fast_remove" in the
sysctls section).
For writethrough and writearound caches, the device removal or reboot
results in the cache being destroyed. However, there is no harm is
doing a 'dmsetup remove' to tidy up before boot, and indeed
this will be needed if you ever need to unload the flashcache kernel
module (for example to load an new version into a running system).
Example:
dmsetup remove cachedev
This removes the flashcache volume name cachedev. Cleaning
all blocks prior to removal.
Cache Stats :
===========
Use 'dmsetup status' for cache statistics.
'dmsetup table' also dumps a number of cache related statistics.
Examples :
dmsetup status cachedev
dmsetup table cachedev
Flashcache errors are reported in
/proc/flashcache/<cache name>/flashcache_errors
Flashcache stats are also reported in
/proc/flashcache/<cache name>/flashcache_stats
for easier parseability.
Using Flashcache sysVinit script (Redhat based systems):
=======================================================
Kindly note that, this sections only applies to the Redhat based systems. Use
'utils/flashcache' from the repository as the sysvinit script.
This script is to load, unload and get statistics of an existing flashcache
writeback cache volume. It helps in loading the already created cachedev during
system boot and removes the flashcache volume before system halt happens.
This script is necessary, because, when a flashcache volume is not removed
before the system halt, kernel panic occurs.
Configuring the script using chkconfig:
1. Copy 'utils/flashcache' from the repo to '/etc/init.d/flashcache'
2. Make sure this file has execute permissions,
'sudo chmod +x /etc/init.d/flashcache'.
3. Edit this file and specify the values for the following variables
SSD_DISK, BACKEND_DISK, CACHEDEV_NAME, MOUNTPOINT, FLASHCACHE_NAME
4. Modify the headers in the file if necessary.
By default, it starts in runlevel 3, with start-stop priority 90-10
5. Register this file using chkconfig
'chkconfig --add /etc/init.d/flashcache'