Um, because the distinctive characteristic of SAN is that
clients can write directly to the storage devices bypassing
any server? This is possible only if a storage-level (e.g.
block-level) protocol like SCSI runs on the wire. (Actually,
I'm lying here. There are two ways of enabling this. One is
to use a really simple protocol like SCSI. The other is to
implement an industry-standard clustered filesystem with the
disk drives/storage devices as an intelligent component of
said filesystem. A very complex protocol whose viability
and practicality is an issue in its own right. The rest of this
mail assumes a block-level SAN protocol.)
As folks have pointed out, SAN has its advantages and disadvantages.
It eliminates the backplane of server (unless you have a RAID controller
in front of the drives :-) from the I/O path, eliminating a possible
I/O bottleneck. But there is no server to protect the integrity
of the data and even if there is, the protocol is so low-level in
a SAN, the server has no effective way of detecting when a client
has gone insane (maliciously or not) and is scribbling gibberish
on the drives. So trust becomes a definite issue.
Personally, I see data sharing using SANS as making sense
in small workgroup environments and what I call "machine room
clusters" -- a SAN in a machine room supporting a dedicated
cluster. Typically you'd do the latter because scaling via
a cluster is cheaper (and more available) than scaling via MP.
But both are situations where you are willing to trust all the
machines in the cluster because they are all firmly controlled by
the group whose storage/data is being shared.
Ray Chen
rcc(a)netapp.com
> -----Original Message-----
> From: Adams, Christian [mailto:Adams_Christian@emc.com]
> Sent: Tuesday, March 30, 1999 10:14 AM
> To: 'Dave Hitz'; willh(a)infi.net
> Cc: toasters(a)mathworks.com; barsellt(a)infi.net
> Subject: RE: NAC SAN
>
>
> Hey Dave Hitz -
>
> Why do you focus on existing protocols in your discussion?
> This indicates
> that the introduction of Fibrechanel with either encapsulated SCSI or
> Ethernet is the critical event in the creation of SAN or NAS.
> Further in
> your discussion, you draw a parallel to switches and routers
> which indicate
> that you may be thinking in terms of shared versus segmented
> data flows.
> And yet in all of your discussion, you do not address the issues of
> management, integration, or value.
>
> Loved your EMC bit,
> /Christian Adams
> EMC
>
> > -----Original Message-----
> > From: Dave Hitz [SMTP: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
>
