Why not a box with a special OS (ala WAFL) that runs on what servers
recognize as a fibre hub, intercepts the SCSI calls from all attached
servers, and does all the nifty stuff needed to make the SAN(?) look like
locally attached storage? Advantage? Separate file I/O from TCP session
traffic but still allow file sharing in some form or another.
-----Original Message-----
From: Dave Hitz [mailto:hitz@netapp.com]
Sent: Saturday, March 27, 1999 1:41 PM
To: willh(a)infi.net
Cc: toasters(a)mathworks.com; barsellt(a)infi.net
Subject: Re: NAC SAN
> Tom's statement below (to a [NetApp] SAN when we build one) brings up a
> familiar argument between myself and my boss. My boss contends that
> NetApp is NOT capable of migrating to a "real SAN."
I've been delinquent in my duty of writing a NAS and SAN paper. I'm
about half way done. Whem I'm done, I'll post it here, probably in
early form, so that you guys can critique it for me.
Very briefly, I believe that NAS and SAN are different but related
technologies, and that the definitions are as follows:
NAS (Network Attached Storage)
Storage accessed over TCP/IP, using industry standard file
sharing protocols like NFS, HTTP and Windows Networking.
SAN (Storage Area Network)
Storage accessed over a Fibre Channel switching fabric,
using encapsulated SCSI.
I also think that both NAS and SAN are useful, and that -- in the long
run -- both will be critical components in any large data
infrastructure. I think there's a strong analogy to the TCP/IP
networking world. Routers and switches are very similar in some ways,
yet subtly different in others. In the mid-eighties, arguments raged
about which was "better". (Back then switches were called bridges.)
Today we understand that any large network infrastructure needs to
include both.
Some people would quibble with my definitions. Some would argue that
if you run NFS over TCP/IP over fibre channel, then that is a SAN. I
figure that we didn't invent new names for NFS over 100bT, FDDI, ATM,
or Gigabit -- what's so special about fibre channel? Others argue that
a SAN is any network dedicated entirely to storage traffic. Does this
mean a SAN turns to a NAS if you fire up a telnet on it? A year ago
these definitions were less settled, but today I think these are the
definitions that are emerging.
With these definitions, the key difference is between file system
protocols and raw disk protocols. With NAS, the filesystem runs on the
storage system itself. (For NetApp, that's WAFL.) With SAN, the
filesystem runs on the hosts attached to the storage system.
People familiar with EMC will recognize this model. With an EMC
Symetrix, you get a big box of RAID protected disks, and you can attach
multiple hosts to that one box. Thus, I view SAN as "open EMC". On
the one hand, SAN could be viewed as a threat to EMC. But on the other
hand, EMC has a solid history of products -- software and hardware --
based on this storage model, which means that they ought to have a real
head-start on SAN. I think the SAN vendors have a great opportunity if
they really can create "open EMC". After all, EMC is doing $4 Billion
a year in business with this model!
In any case, I think NAS has some serious advantages over SAN. One big
issue with running the file system on the separate hosts is that it
makes true data sharing very difficult. The on-disk byte formats are
very different between UNIX and NT, and even between different flavors
of UNIX. Given the differences between UNIT and NT filesystem
semantics, it's no surprise that the on-disk format is different. With
NAS, the filesystem is by the data, so it can convert the on-disk
format into an industry standard protocol appropriate to the host that
wants the data: NFS for UNIX, NT Shares for NT, and HTTP for web
browsers.
It is technically possible to build a "global file system" that has a
common on-disk format for multiple operating systems, thus allowing
NAS-like sharing for SAN. However, I'm skeptical of that model. First
of all, there are no such products today. And even when they become
available, sharing won't be based on industry standards. A system will
only be able to participate in sharing if it runs a special SAN file
system from a particular vendor. My believe is that the lesson of 20
years of networking is that we build heterogeneous environments by
creating open standards and allowing everyone to support them -- not by
saying that anyone who wants to participate in sharing must buy code
from one particular company.
Having said all this, though, NetApp does have plans to leverage SAN
technology. We have announced an OEM agreement with Brocade. Here are
the benefits we expect to achieve:
- Attach to lots of disks.
A fibre channel loop is limited to 127 drives, but with a 16
port fibre channel switch, the limit goes to almost 2000
drives. With additional switches, you can support an arbitrary
number of drives.
- Share tape drives between multiple servers.
I believe that backup applications are the killer application
for SAN. When people buy expensive tape jukeboxes, they want
to be able to attach them to multiple servers. With today's
SCSI tapes that's a painful job. But with fibre channel and
fibre channel switches, it's easy to connect a single tape to
any number of hosts.
- Disaster Tolerant Solutions
Fibre channel switches can do WAN tunneling to provide
connections to remote disks without any changes to the server
code. This makes it much easier to build remote mirrors for
data replication and disaster protection.
In conclusion, I believe that NAS and SAN are both important
technologies, and the any large data infrastructure in the future will
include both. I believe that NAS focuses on issues associated with
users, applications, and data sharing. I believe that SAN focuses on
issues associated with disk and tape devices, connection to large
numbers of them, and moving data back and forth between them. In the
long run, any large environment will have both sets of issues, and will
need both types of solutions. In the short run, NAS makes good sense
as a starting point, because it is here today and the standards it
relies on have been solid for over a decade.
I'll go ahead an post the whole paper when I finish, but I still
thought it might be useful to share my thoughts in this brief,
half-baked form.
Dave