[CentOS-virt] LVM mirror database to ramdisk

Fri Jan 22 22:54:53 UTC 2016
Ed Heron <Ed at Heron-ent.com>

On Fri, 2016-01-22 at 16:17 -0600, NightLightHosts Admin wrote:
> >  Ed Heron <Ed at heron-ent.com> wrote:
> >   Yes, in a test environment, I am mirroring a Logical Volume with a RAM
> > disk to increase the perceived speed of the disk.  I'm expecting to
> > convert a live guest to this type of setup, this weekend.
> >
> >   I was asking 2 questions.
> >         1. Should I expect a significant increase in speed in a real world
> > environment?  With enough RAM, a good caching system will eventually do
> > a similar function.  This is almost like pre-loading a cache.
> >         2. Should I document the process for others?  I'm using CentOS 5 now,
> > which is on it's way out, but I would update the documentation to
> > include CentOS 7 when I upgrade my servers.
> >
> See this is where I was confused.  Would not the LVM mirror have to
> sync all the time with the disk anyways?

  Yes, but it isn't that simple.  One copy of the mirror would be on a
physical disk.  The other copy of the mirror would be on RAM disk.
Since data in RAM doesn't generally survive reboot, the RAM piece would
need to be turned off before shutdown and created on startup.

> Is there something about LVM mirroring that can handle disks of
> different speeds?

  With newer LVM, there appears to be some settings that might help with
that a bit.  With this older verion, I'd be hoping that the next
available disk would handle each request.  If the physical disk takes
longer to deal with the writes, the RAM disk might be the one that is
available most of the time.

  I'd much prefer a method of pre-filling a 35G cache but I saw a
reference to creating a disk mirror in RAM and decided to explore it.