кеширование LVM (LVM caching)
Описание (Description)
lvm(8) includes two kinds of caching that can be used to improve
the performance of a Logical Volume (LV). When caching, varying
subsets of an LV's data are temporarily stored on a smaller,
faster device (e.g. an SSD) to improve the performance of the LV.
To do this with lvm, a new special LV is first created from the
faster device. This LV will hold the cache. Then, the new fast LV
is attached to the main LV by way of an lvconvert command.
lvconvert inserts one of the device mapper caching targets into
the main LV's i/o path. The device mapper target combines the
main LV and fast LV into a hybrid device that looks like the main
LV, but has better performance. While the main LV is being used,
portions of its data will be temporarily and transparently stored
on the special fast LV.
The two kinds of caching are:
• A read and write hot-spot cache, using the dm-cache kernel
module. This cache tracks access patterns and adjusts its
content deliberately so that commonly used parts of the main LV
are likely to be found on the fast storage. LVM refers to this
using the LV type cache.
• A write cache, using the dm-writecache kernel module. This
cache can be used with SSD or PMEM devices to speed up all
writes to the main LV. Data read from the main LV is not stored
in the cache, only newly written data. LVM refers to this
using the LV type writecache.
