Hi Chris
Comments below.
tl
-----Original Message-----
From: Chris Madden [mailto:maddenca AT MYREALBOX DOT COM]
Sent: Tuesday, April 08, 2003 12:18 PM
To: NETWORKER AT LISTMAIL.TEMPLE DOT EDU
Subject: [Networker] SAN Tape Drive / Storage / Host Connectivity
I have a SAN in production for disk access and we are looking at adding
fibre tape drives to the setup, initially driving these devices from a
single storage node and thereafter doing LAN-free backups (i.e. installing
additional storage nodes) on some larger hosts over time. Today I have 2
distinct SANs which are mirror copies of each other for availability, and
have zoning implemented so that only hosts of a given OS type can see each
other and their storage ports. Tech environment includes: IBM ESSs, AIX,
Solaris, Win2k, Netware, and coming soon 6 x IBM 3590H and 6 x IBM LTO-2
drives.
In order to add tape devices to the SAN I have some questions:
1) Is it wise to mix disk and tape on the same SAN? My initial reaction is
why not if you zone correctly, but perhaps there are other issues?
tl> In general, we have found that this works fine, especially if you are
using Fibre Channel switches (instead of hubs), so each connection has its
own dedicated bandwidth through the switch (instead of all connections
sharing the bandwidth through a hub).
2) Can the HBAs in our hosts be used for both disk and tape access? Again,
my initial reaction is why not but perhaps there are other issues? Is the
answer different for each OS and if so which are OK and which not?
tl> Again, in general, we have found that this works just fine. When SANs
were first being deployed, there was concern about mixing tape and disk
traffic, and there was some folklore about problems seen when you did.
Personally, I've never seen this myself, and have been doing SAN backups
involving tape and disk on the same SAN for several years.
I would prefer to share as much infrastructure between disk and tape as
possible and I suspect with proper zoning the same fabric can be used for
tape and disk. If the case is however that I can't share the existing HBAs
and need to add a 3rd HBA for tape use I have to pick which SAN (A or B) to
plug it into, or I have to add a 4th HBA to get visibility to all the
drives. At that point the complexity (i.e some drives/hosts on one SAN and
some on the other) suggests leaving SAN A and SAN B in place for disk, and
creating a SAN C for tapes. Creating SAN C however has significant costs
(more switches, more HBAs) and would need good justification.
tl> Yes, adding an additional adapter and/or SAN for tape makes things
complex very fast. And VERY expensive, too. I would anticipate having no
problems with attaching tape drives to one of your existing SANs, IF IF IF
you follow the recommended SAN interoperability matrix of your vendors. EMC
spends A LOT of time and money in these kinds of tests, and you see the
result in the 700+ page EMC Support Matrix
(http://www.emc.com/horizontal/interoperability/interop_support_matrices.jsp
). Look for an equivalent document from your vendors, and follow it
closely. NOTE, this is NOT a plug for EMC, but rather the Voice of
Experience saying that, if you don't follow SAN interoperability testing and
configuration very closely, it will not work.
tl> That said, here's another item to chew on. Disk storage arrays are
commonly built with redundant connections, allow them to connect to multiple
SANs. In addition, software has been written to take advantage of these
multiple connections for redundancy/fail over, load balancing, etc. EMC
PowerPath is an example of this genre. One of its features is that it
recognizes multiple connections to the same disk, but shows the operating
system only a single device; PowerPath manages the multiple connections,
fail over, load balancing, etc., and the operating system never sees this
occurring.
tl> BUT, the same software and features commonly don't exist for tape
drives. So, many FC tape drives, FC tape libraries and FC bridges do not
support multiple connections. Even if they did, I'm not aware of any
software that can take advantage of these multiple connections; instead,
current software (like NetWorker) would see all of the drives appearing ONCE
FOR EACH connection. So, if you had 5 drives with two connections to the
SANs, NetWorker would see 10 drives. This would confuse NetWorker.
tl> So, plan on being able to access your tape resources through a single
connection, and just live with the lack of redundancy. Given that
businesses can survive minutes without tape access (IMO), this give time to
manually move cables, if this is ever needed.
Fellow NetWorkers, what would you do in my shoes?
tl> I would add tape resources to one of your existing SANs, make use of
the NetWorker SAN-based storage node licenses (formerly, "SAN Storage Node",
now called "Dedicated Storage Node"), use dynamic drive sharing (to share
some/all of your tape drives - DDS is licensed per tape drive). I'd also
have as few tape drive types as possible in my environment (why get both
3590 and LTO2?).
tl> Hope this helps!
tl
Kind Regards,
Chris
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