Diskless Blades boot to SAN [pros/cons]

[Jackson Houndmugger]

Hi,

Could someone briefly talk about the pros/cons of having all blade
servers boot to a SAN as opposed to containing their own hard drives?

I imagine the scenario would look something like this: A SAN contains 14
drives. One huge RAID 5 partition is created. Somehow, logical drives are
created in the partition, one logical drive per blade. Each blade boots to
its own logical drive. Do I have the right idea conceptually?

This seems somewhat dangerous to me since if the SAN controller fails,
ALL of your blades go down.... On the other hand, people much smarter than I
are using this technique so there must be some advantages. What do you
think?

Jackson

 

[Arno Wagner]

Hi,

Could someone briefly talk about the pros/cons of having all blade
servers boot to a SAN as opposed to containing their own hard drives?

I imagine the scenario would look something like this: A SAN contains 14
drives. One huge RAID 5 partition is created. Somehow, logical drives are
created in the partition, one logical drive per blade. Each blade boots to
its own logical drive. Do I have the right idea conceptually?

This seems somewhat dangerous to me since if the SAN controller fails,
ALL of your blades go down.... On the other hand, people much smarter than I
are using this technique so there must be some advantages. What do you
think?

I have something roughly similar (22 Standard PCs in a rack with
one boot server).

Pro: You can configure boot-parameters centrally without hassle.
You have only one storage device to monitor (and repair).
You need to do only one backup.

Con: Your blades are useless if the SAN fails.
You have a _major_ bottleneck for disk-intensive applications.
Your backup is huge.


In the end I decided to have both, local disks for the OS and data as
well as Kernels deliverd from fileserver and more data partitions
there. Backup is only from the fileserver and the installation on the
individual PCs can be reconstucted in minutes with FAI (Fully
Automatic Installation), which makes recovery from a disk-replacement
easy. Also users are told thet there is no backup of node-local
disks. (There is no backup of data on the fileservers either, but
that is because a 3TB backup would just take far too long and be
too expensive. We have all raw data on tape anyways.)

I think the bottleneck argument is the most important. Personally
I found that as little as 3 PCs can saturate a RAID5 array on
64 Bit PCI. A SAN may be able to take more (I use Linux software
RAID), but not that many more. And local storage scales linearly.

Arno

 

[AWS]

There are good reasons either way based on the business needs of the
application. Your concerns are well founded, but the addtional risk
associated with booting off the SAN can be mitigated with the correct
architecture-- using multiple HBAs across multiple fabrics (or
direct-attached) to multiple ports on your disk array.

I'm not sure of the specification of your bladecenter, but the blades
that I've seen, only suppported a single IDE laptop-style drive.

This produces a couple of disadvantages:
1. The speed of ATA drives, especially for typical random workloads is
very poor in comparison to SCSI/FC drives on most SAN-attached disk
arrays.

2. In order to provide redundancy, you would need to mirror between
the SAN and the internal disk. This would deliver the poor performance
of your IDE disk, and you would be unable to use parity RAID (instead
of mirroring) to reduce disk usage.

Some advantages of having your OS on the SAN include:
1. The ability to move disk between blades easily. Keep in mind, that
since your blades are going to have nearly identical hardware, the
installed OS would be very portable.

2. With some disk arrays, disk space can be added on the fly.

3. As a blade server becomes higher profile, additional features like
point-in-time copies, remote-mirroring, and remote replication are
available.

HTH
Aaron