Xen and storage

[Amos Shapira]

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