Re: LVM mirror database to ramdisk

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On Fri, 2016-01-22 at 14:56 -0600, NightLightHosts Admin wrote:
> On Fri, Jan 22, 2016 at 11:02 AM, Ed Heron <Ed@xxxxxxxxxxxxx> 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.


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