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FlashDisk RAID 15

“Three times better than RAID 5”

Reliability Imperative
Most network and system managers prefer RAID disk arrays today because they provide a measure of protection against drive failures. However, those who must attempt to keep the shop running without interruption typically require servers and storage with higher standards of fault tolerance (e.g., “No Single Point of Failure”). Unfortunately, standard RAID systems simply do not provide this measure of protection. Further, these enterprises typically run business critical database applications that are seldom, if ever, closed - even for backup - often 24 hours a day, 7 days a week.

Graceful Failure
Mechanical failure is inevitable. Disks, power supplies, fans and computers all fail. It is the network manager's job to anticipate the costs of these predictable failures and compare them to the costs of prevention. FlashDisk RAID 15 solutions from Winchester Systems offer several added elements of protection for those network server environments where the cost of data loss and downtime are very high. For these situations, the cost of extra equipment redundancy is low compared to the anticipated costs of downtime or data loss. We all know equipment will fail - we want it to fail gracefully and not take the enterprise or our data with it. To reduce downtime due to component failures, FlashDisk RAID 15 offers the three cost effective solutions below.

RAID 15
Better than “No Single Point of Failure!”
In response to requests from our key customers, Winchester Systems has crafted solutions that offer three levels of protection for these critical situations - at reasonable cost. These solutions take full advantage of FlashDisk speed and continue to offer the full benefits of write caching - even if an entire RAID array fails. They offer true “No Single Point of Failure” solutions at the storage array and optionally at the server.

RAID 15, Level 1 - Storage Redundancy

“Three drives can fail”
FlashDisk RAID 15 is a storage architecture designed to provide a full measure of fault tolerance via component redundancy. Unique to this RAID 15 architecture, this plan avoids the vulnerabilities to single failure points commonly found in typical storage arrays. This “No Single Point of Failure” design in the RAID storage architecture is accomplished by utilizing a pair of RAID 5 systems that are each connected to a server that supports host based mirroring. Operating systems that perform mirroring include Netware, Windows NT, Solaris, HP-UX, AIX, OpenVMS (Volume Shadowing), Digital UNIX, SGI Irix and others. Figure 1 shows how simple this is to set up. Unlike a standard RAID 5 array, this configuration can withstand up to three simultaneous drive failures and still continue to run properly - all transparently to system users. Each RAID 5 array can sustain a drive failure and continue operating with parity information. If a hotspare is present or a replacement drive is inserted, the system continues with either one or even both data rebuilds in progress simultaneously. During this critical time, another drive can fail, taking down an entire RAID 5 array. This would normally disable the server and all the users and risk data loss. However, FlashDisk RAID 15 keeps running. One RAID 5 array with a failed drive is still sufficient to run the server and keep data continuously accessible to users. This provides ample time to fix the disabled RAID 5 array and avoid potential data loss. With optional “hot-spare” drives installed, the RAID 15 array can withstand the subsequent faiure of up to two additional drives.

The RAID 15 array can also withstand the failure of multiple fans and multiple power supplies. Each RAID 5 array is typically connected to a separate UPS and has redundant AC connections. An AC power line to each array can fail and a UPS can fail during a power outage or even an entire RAID 5 array can completely fail and the RAID 15 array will continue to operate.

Flash Disk RAID15 Level 1
Server Redundancy
(Figure 1)

FlashDisk RAID can withstand a host bus failure and
controller failure plus multiple disk, multiple power
supply, multiple AC power source, multiple UPS and
multiple fan failures - all simultaneously.

RAID 15, Level 2 - Server Redundancy
“Server can fail”
The multi-host configuration shown in Figure 2 takes advantage of the FlashDisk multi-hosting capabilities. Each server is connected to both RAID 5 arrays. In most cases, the second server is a standby server - ready to take over if the first server fails. This configuration also withstands the failure of a bus since there are two RAID arrays connected to each server via separate buses. Bus hang-ups do occur and while they are easily reset by rebooting, it is unacceptable in many environments to do so when users expect the system to be in full operation. Thus the environment continues operating with the loss of a server, bus or storage element.

RAID 15, Level 3 - Clustering
RAID 15, Level 3 optional cluster links provide the capability for hosts to coordinate and share data

“Hosts share data”
Sharing data between multiple servers enables network managers to distribute workloads onto multiple servers without the need to arbitrarily decide how to split up the data. It is a wonderful capability that also provides the ability to have “No Single Point of Failure” in the entire environment when combined with RAID 15 storage. It also eliminates the obvious idleness of the backup server. This capability is supported by Digital UNIX and OpenVMS, HP-UX, AIX and Solaris. This architecture creates a server environment with true “No Single Point of Failure” fault tolerance, maximizes the utility of all installed components and promotes ease of access to shared data.

Flash Disk RAID15 Level 2/3
Server Redundancy/ Share Data
(Figure 2)

RAID15 level 2 adds the ability to withstand a
server failure and multiple host bus failures to the
already formidable list of simultaneous failures
covered by RAID 15, Level 1

On-line Backup
In 24-hour operations, where there is scarcely time to backup, the FlashDisk RAID 15 storage architecture offers the unique option of backing up on-line while the database is fully operational. This strategy involves breaking the RAID 15 mirror set by dismounting one RAID 5 array. Critical databases are momentarily closed to perform this first step. Second, one RAID 5 array is remounted as an independent device and backup is run against it. The third and final step is to remount the second RAID 5 set and remirror it to the first to recreate an up-to-date RAID 15 array. Of course, the fault tolerance level is reduced to that of a standard RAID 5 array during this period of time. The inherent reliability of a RAID 5 array coupled with the flexibility to run the application continuously even during backup makes this an attractive option in certain busy environments. If standard RAID 15 protection is not sufficient during backup, a third array added to the mirror set may be used; when the third mirror is removed and mounted as an independent volume for backup, the online service is still protected with the full benefits of RAID 15.

Summary
FlashDisk RAID 15 solutions are inexpensive insurance when the cost or inconvenience of server or storage downtime or loss of data is very expensive. Thus, when losing data is incomprehensible - when losing system availability is inconceivable - when standard RAID 5 solutions are simply not sufficient - you need FlashDisk RAID 15 Solutions.

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