d Vinca's new SnapShotServer NetWare loadable module (NLM) to the mix, you get snapshot backups that don't affect server availability.
Testing on a small NetWare network, we found that both Vinca products work well, although installation and instructions are rough enough to require considerable NetWare expertise. We tested the $2599 version of StandbyServer 32, which comes with two ISA adapter cards. EISA and Micro Channel architecture versions cost $2999. A LAN Server version for OS/2 is also available. SnapShotServer costs an additional $1299.
Disk Mirroring Goes the Distance
Unlike other server-mirroring solutions (see the sidebar
"Mirroring: Different Strokes"
), StandbyServer 32 doesn't require the secondary server to be an identical twin of the primary server. It requires only enough memory and CPU power to take over when the primary server fails and to limp along u
ntil it's fixed. You can use a less expensive server and disks to mirror the information on your primary server, and you can opt to mirror only those volumes storing critical information.
You also save money by not having to buy an additional NetWare license. However, if you install a separate NetWare license on the secondary server, it can mount volumes of its own and accept user connections while mirroring data on the primary server. The secondary server doesn't have to be dedicated to mirroring the primary server.
StandbyServer 32 achieves server mirroring simply but effectively. Two adapter cards (one each for the primary and secondary servers) provide a high-speed link over a 25-foot connecting cable (a 50-foot cable is optional). When you install and connect the cards and load Vinca's software, the disks in the secondary server appear as local disks to the primary server. You then use NetWare's disk-mirroring capability to duplicate information from the primary server's local disks to disk
s in the secondary server.
The secondary server uses a run-time version of NetWare (included with StandbyServer 32) with an NLM that manages the mirrored disks. A second NLM monitors the primary server. If the primary server fails, the secondary server reboots and mounts its copies of the mirrored primary-server volumes. In doing this, it assumes the name and internal IPX address of the failed primary server.
Users experience a brief interruption during the reboot, and they have to know how to reconnect once the secondary server is up. Thus, StandbyServer 32 doesn't match the seamless and nonstop fail-over service that NetWare SFT III provides (see the feature-comparison table in the sidebar). It also doesn't require two identical Novell-certified servers that are configured with equal amounts of RAM and disk storage, as does NetWare SFT III.
Not Idly Standing By
Vinca's use of NetWare's disk-mirroring capability has its pros and cons. Introduced in the mid-1980s, the
technology is well established. Disk writes can take longer because they must be completed to two disks instead of one, but NetWare's caching insulates users from delays except in extremely write-intensive scenarios.
However, using an older secondary server can have performance drawbacks. To minimize performance consequences, the secondary server should have disks, a disk channel, and an I/O bus that compare favorably to the primary server. During normal operation, the CPU and memory performance of the secondary server matter little because the primary server handles all processing and disk caching. Obviously, when the secondary server takes over during a primary server's failure, its CPU speed and RAM quantity become significant. But if the secondary server needs to run only long enough to enable you to repair the primary server, you don't need to go first-class.
While NetWare SFT III is truly fault-tolerant, and thus provides the best defense against hardware failure, it may provide less prot
ection than StandbyServer 32 against software or OS failures. With NetWare SFT III, both servers run configurations that are duplicated. If an errant NLM causes the OS to fail and crash the server, both sides of the mirrored-server pair are likely to crash. With StandbyServer 32, the secondary server does not have to be configured to run the same drivers and NLMs when it reboots to become the primary server.
Freeze-Framing Your Data
SnapShotServer, an add-on to StandbyServer 32, elegantly solves one of the more vexing challenges of operating a server on a nonstop basis: backing up files that may be open. It does this by maintaining a snapshot of server data at a given moment.
The program is an NLM that runs on the secondary server. Once it takes a data snapshot of a mirrored-server volume, you can mount the snapshot as a regular NetWare volume and archive it using your regular backup software.
SnapShotServer doesn't interfere with Vinca's mirroring process. It tracks
disk blocks that change after the snapshot and stores an unchanged copy of each changed block in a special buffer volume. The size of this buffer is typically 5 percent to 10 percent that of the volume you're backing up.
A snapshot volume consists of
the unchanged blocks from the original volume plus any blocks from the snapshot buffer. Because all the activity takes place on the secondary server, backing up doesn't impact performance, nor does it threaten the stability of the primary server with a software crash.
While Vinca's snapshot method is more reliable than backing up a live copy of open files directly from the primary server, it isn't risk-free. You could take the snapshot at the moment a database transaction is incomplete, for example. To reduce this risk, SnapShotServer will take snapshots only after there have been no disk writes for a specified number of seconds, so you can catch the volume at an idle moment when transactions are likely to be complete.
We found
that SnapShotServer works well, but it whetted our appetite for more capability. There is no way, for example, to automatically keep multiple point-in-time snapshots of the same volume. You can configure SnapShotServer to make a snapshot at intervals of as short as every 15 minutes, but you must archive them with your backup software to preserve each iteration.
Not for Novices
Vinca's products worked as advertised in our tests, with one exception. StandbyServer 32 couldn't detect the failure of a server that was running NetWare 4.x unless bindery emulation was enabled. Vinca's technical-support staff responded promptly and knowledgeably when we reported this problem, and we received a preview copy of the next release, which fixes the problem.
Installation and configuration of StandbyServer 32 are complex tasks, and Vinca's attempts to automate the procedure with an installation utility caused more problems than the utility was worth. We resorted to manually installing Vinca
's drivers and NLMs in the appropriate subdirectories and carefully configuring the AUTOEXEC.NCF and STARTUP.NCF files of the two servers ourselves. According to a Vinca spokesperson, the company is rewriting the documentation and retooling the installation procedure.
Meanwhile, you'll need expertise on NetWare start-up files to make Vinca's products work. Set aside plenty of time for trial and error, and keep Vinca's support number handy. If you need low-cost and flexible server failure recovery, the result will be well worth your effort.
PRODUCT INFORMATION
StandbyServer 32..............$2599 with ISA cards;
..............................$2999 with EISA or Micro
Channel architecture cards
SnapShotServer $1299
Vinca Corp.
Orem, UT
Phone: (800) 934-9530 or (801) 223-3100
Fax: (801) 223-3107
E-Mail:
info@vinca.com
Internet:
http://www.vinca.com
Circle 1093 on Inquiry Card.
NetWare Mirror with a Twist
