CentOS virt - Jan 2016 - LVM mirror database to ramdisk

On Fri, 2016-01-22 at 14:56 -0600, NightLightHosts Admin
wrote:
> On Fri, Jan 22, 2016 at 11:02 AM, Ed Heron <Ed at heron-ent.com>
wrote:
> >   I'm still running CentOS 5 with Xen.
> >
> >   We recently replaced a virtual host system board with an Intel
> > S1400FP4, so the host went from a 4 core Xeon with 32G RAM to a 6 core
> > Xeon with 48G RAM, max 96G.  The drives are SSD.
> >
> >   I was recently asked to move an InterBase server from Windows 7 to
> > Windows Server.  The database is 30G.
> >
> >   I'm speculating that if I put the database on a 35G virtual disk
and
> > mirror it to a 35G RAM disk, the speed of database access might
improve.
> >
> >   I use local LVM for my virtual disks with DRBD on top to mirror the
> > disk to a backup server.
> >
> >   If I change grub.conf to increase RAM disk size and increase host
RAM,
> > I could create a 35G RAM disk.
> >
> >   I'd modify rc.local to add
> >         pvcreate /dev/ramdisk
> >         vgextend vg /dev/ramdisk
> >         lvconvert -m 1 --corelog vg/lv_database /dev/ramdisk
> >
> >   Even with lv_database being 35G, it doesn't take long to
activate the
> > mirror.
> >
> >   I haven't decided where to put the commands to turn off the lvm
> > mirror.
> >         lvconvert -m 0 vg/lv_database
> >         vgreduce vg /dev/ramdisk
> >         pvremove /dev/ramdisk
> >
> >   I haven't put this in real world use, yet.
> >
> >   On it's face, this might speed up database access.  Would we
expect it
> > to speed up database access in real world use?
> >
> >   Should I document the process so others could know how to do this? 
I
> > realize new documentation for CentOS 5 virtualization would be
> > considered obsolete before I wrote it but I'm expecting to test
CentOS 7
> > virtualization in the next few months and, when I am comfortable,
I'd
> > upgrade my 18 virtual hosts.  I would update the documentation, at
that
> > time, as well.
> >
> I may not understand enough to understand what you are doing, you want
> to actively mirror this with LVM or?
  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.

>  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?

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

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.