Xen and storage

[Etzion Bar-Noy]

Assuming your farm is a production one, suspend to disk is rather rare, and
by design, would probably not be a day-to-day process of the system. Anyhow
- suspend to disk is an operation which is being performed on NFS SR as
well, just the same (create file which contains memory dump of the VM).

NFS has other advantages, like LUN alignment and thin provisioning (assuming
you did not purchase the "foundation for Citrix XenServer" package). Also -
for real-life production systems I have seen that network communication over
iSCSI, for about 50 VMs on 5 physical servers would not exceed the 200Mb/s
at peak times.

Of course - specific applications can (and will) stress the storage and
network, however, many common storage devices cannot maintain a high rate of
random IO (common to DBs, like Oracle, MySQL, Exchange, MSSQL, etc). The
disks would commonly be the bottleneck, and not the network/FCS transport.

Don't believe me? Check your virtual farm. See what throughput you get for
your DRBD/central storage links.

Ez

On Mon, Mar 15, 2010 at 10:58 PM, Amos Shapira <amos.shapira at gmail.com>wrote:

> 2010/3/16 Etzion Bar-Noy <ezaton at tournament.org.il>:
> >
> >
> > On Mon, Mar 15, 2010 at 10:41 AM, Amos Shapira <amos.shapira at gmail.com>
> > wrote:
> >>
> >> 2010/3/12 Hetz Ben Hamo <hetzbh at gmail.com>:
> >> > Hi,
> >> > I have taken 3 machines for a project: 2 machines will act as Xen
> >> > servers
> >> > and one machine will act as "storage".
> >> > The storage box is just a machine with few hard disks connected with a
> >> > RAID
> >> > controller.
> >> > What I would like to do is create few Xen VM's with the fastest
> possible
> >> > I/O
> >> > in terms of storage.
> >> > I have few options:
> >> > 1. I can create an LVM on the storage machine, create few Logical
> >> > Volumes
> >> > and export them as NFS to the Xen servers and configure each VM to
> some
> >> > file
> >> > images. Problem is, that file I/O with Xen is slower compared working
> >> > with
> >> > LVM's.
> >> > 2. I can create an LVM on the storage machine, create few Logical
> >> > Volumes,
> >> > and export those as iSCSI devices. I'm not sure whats the performance
> of
> >> > Xen
> >> > with iSCSI devices exported from the storage box.
> >> > 3. I can create few partitions on the storage machine, export them as
> >> > iSCSI
> >> > devices and do LVM on the Xen servers. Problem: I don't know how much
> >> > the
> >> > "penalty" doing LVM on the Xen machines.
> >> > My question: What is the best option?
> >> > Thanks,
> >> > Hetz
> >>
> >> I don't have practical experience with hosting Xen images on SAN but
> >> when I researched the market for a SAN-based configuration of our
> >> production network (currently 20 Xen hosts hosting about 10 Xen guests
> >> each, doing DRBD between pairs of Xen guests and linux-ha for HA), at
> >> least one or two of the options I checked mentioned that if I store
> >> the Xen images on the SAN then it will require much higher bandwidth
> >> to it than if I use it just for plain data.
> >
> > Why? Where does the secret IO arrive from?
>
> I haven't dug into this but I figured it was around reading the
> program files from the storage to the iSCSI client which actually runs
> the Xen image and storing the Xen guest's state if you use xen's
> "suspend to disk" stuff.
>
> --Amos
>
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