is.pennwellnet.com
Guidelines for SAN deployment
Focus on fault resiliency, storage resource management, data
recovery, and virtualization techniques.
By Mark Teter
The ability to manage and control today's storage demands is tenuous. In
the world of e-business, storage capacity requirements change rapidly,
with only a short window of time to change storage allocations. Worse
yet, many companies are concerned about emerging industry standards,
such as SCSI over TCP/IP, storage over IP, and Fibre Channel over IP (see
Software and hardware solutions are emerging from vendors such as
Compaq, Datacore Software, DataDirect Networks, StorageApps, and
Veritas Software that erase the traditional boundaries between
multi-vendor storage systems. These vendors have developed SAN
appliances, or storage domain managers, that virtualize storage resources
into "pools."
Storage pools can be managed as a single resource, providing a true
"plug-in" storage model. The only requirement for hosts is a pair of Fibre
Channel host bus adapters (HBAs) to gain access to an unlimited amount
of disk capacity. Whether or not you are ready to deploy virtualized pools
in storage area network (SAN) configurations, or are waiting to see how
the standards settle out, there are a number of strategies that can help
manage and deploy disk storage.
No-fault insurance
Perhaps the most important storage consideration is fault resiliency. The
underlying storage network and devices must have no single points of
failure, providing multiple access paths to redundant switches and HBAs
with automatic failover and load-balancing functions. Companies must
address the cabling infrastructure, storage management issues, backup
and recovery processes, and security requirements.
An ideal fault-resistant storage fabric has multiple ports for front-end
server connectivity and multiple ports for back-end storage devices. Other
ports can be allocated to Ethernet LANs or IP clients. Figures 1 and 2
show fabrics built with no single point of failure using Fibre Channel
switches. In these configurations, a single switch could be disabled and
the fabric would not drop connections.
Fibre Channel switch vendors include Ancor, Brocade, Gadzoox, Inrange,
McData, and Vixel. To reduce the number of required switches use Fibre
Channel directors available from vendors such as Computer Network
Technology (CNT) and McData. Directors are high-end switches with
built-in fault resiliency (or redundancy) features that can provide 99.999%
uptime availability.
Figure 2 illustrates a highly available, scalable fabric. It is a cross-connect
design that can scale in terms of port density and fabric throughput. If
more front-end ports are needed, you can add edge switches. If more
performance is required, you can add a cross-connect switch.
Click here to enlarge image
There should be no single point of failure with N_PORTs that are attached
to the fabric. Redundancy is the key element. All devices must have
redundant field replacement units (FRUs) so that component upgrades do
not affect storage availability.
There are two general types of SAN disk storage: RAID and JBOD. RAID, or
controller-based storage, provides a good way to keep "logic" and LUN
management local. With controller-based arrays, SCSI targets can be
presented with multiple LUNs, all behind a single F_PORT. LUN limitations
normally do not present difficulty in server-attached storage
environments, but with SANs they can be troublesome. RAID arrays offer
other beneficial features such as online code updates, automatic RAID and
failover functions, data replication, cache, and event management at the
hardware level.
JBOD (or FC-AL storage) is well suited for application storage
consolidation, where disk resources are divided among several
application-specific hosts. JBOD presents multiple SCSI targets, each
having one LUN, making it an effective long-term solution if it has native
dual-ported Fibre Channel drives.
Health and general welfare
The storage infrastructure must have management tools to facilitate easy
allocation and management of storage resources. Storage resource
management (SRM) software provides policy-based event and performance
management that elevates the scope of management to higher-level
attributes. SRM features include centralized views of physical and logical
storage resources, device status monitoring, capacity allocation,
performance management, and asset inventory. (For more information on
SRM, see the Case Study, "User guidelines for network storage policies,"
on page 76.)
When you can remove the management burden of storage, you remove
the biggest reason to outsource it. If storage management were not a
problem, no one would sign up for a three-year lease for a commodity (disk
drives) that is rapidly decreasing in value. With the cost of disk drives
falling (recent vendor announcements list 100GB drives for less than
$100), the problem no longer is not having enough storage but, rather,
having too much of it to manage.
At a minimum, storage management must provide notification that
something is broken, and an indication of what caused the problem. The
most common difficulty is differentiation between failed connections and
failed devices in the fabric. SRM tools help solve data placement and
transport problems. Data placement problems include LUN management,
RAID levels, partition space thresholds, backup sizing, and caching
policies. Data transport deals with problem isolation and detection,
performance measurement, and load monitoring and leveling.
SRM software provides a single point of management for LUN placement,
device and interconnect path configuration, and zoning. Disk storage
vendors generally provide these management functions for use with their
products. However, hardware-independent products are available from
vendors such as BMC, Computer Associates, HighGround, and Veritas. With
the cost of managing storage spiraling skyward, companies must decide to
either manage their storage resources or outsource them.
Point-in-time saves nine
The cost of data recovery has become prohibitive due to today's online
storage availability requirements. The most important issue is not a matter
of how fast backups are, but, rather, how fast recovery is-as long as
neither affects application server performance.
LAN-free backup and third-party copy technology are now available for
SAN-based backup and recovery. However, database and file system
backup and recovery times can be reduced without much effort using
point-in-time (PIT) copy technology. PIT images (or snapshots) are virtual
replicas of data, without the requirement of physical copying.
One approach for creating PIT images is through RAID arrays at the
controller level-an approach referred to as a "triple mirror." This approach
requires an additional volume to be associated with the master data
volume (which may be RAID 1 mirrored, hence the term triple mirror). After
the mirror breaks, the new mirror contains a snapshot of the master data.
It can then be mounted as a volume on alternate servers (as long as they
are attached to the same array; otherwise, data replication is required)
allowing it to be backed up without impacting performance on the
production server.
Point-in-time copies can be generated with software. At the volume
management level, full volume copies are created. This affects I/O
performance on application servers. At the file system level, a virtual copy
(not a physical copy of the data) is generated via a copy-on-write
technique used to preserve the illusion of a master volume. All writes to
the master are copied first to the PIT image, thereby preserving the
original contents. By using only pointers for the original data blocks, and
copying only the changed ones, a virtual point-in-time volume is created.
The advantage of this approach is that the PIT image generally only
requires 20% to 30% of the original volume size, and it can be re-mounted
by alternate servers for backup/recovery.
By capturing frequent snapshots, or storage checkpoints of online data,
files and databases can be kept in production while images are backed up
in the background on the SAN without affecting application server
performance. Point-in-time images can additionally be checked for validity
before data is actually backed up to tape. Disaster rehearsals can be
performed to ensure that data can be restored from tape or that the
business can be moved to alternate data processing sites.
Database servers have similar technology for capturing point-in-time
copies by putting tablespaces in archive mode. However, performing
snapshots at the storage system level removes the undesirable side effect
of having multiple tablespaces simultaneously in "online mode" for a hot
backup.
Block-level incremental backups (BLIBs) are another solution. Rather than
backing up entire files that have been modified since the last backup, only
the data blocks that have been changed are backed up. This approach
again puts a load on the application server for backup/recovery.
To restore data, the master volume must be re-synchronized with the PIT
image. The differences between the master and PIT image are tracked by
software that enables selective refreshes without performing entire
disk-to-disk copies. In most cases, only the changes from the last
established point-in-time will need to be copied to the master volume. On
average, no more than 5% to 10% of data on file servers changes daily,
permitting multiple snapshots to be taken throughout the day. Snapshots
can eventually be taken from online storage to tape via LAN-free or
third-party copy techniques. Most RAID array vendors have point-in-time
technology, and hardware-independent approaches are available from
vendors such as CrosStor, Legato, and Veritas.
Virtual reality
Storage virtualization is a major step in delivering the true promise of SAN
technology. Virtualization enables online storage to be parceled out as
virtual SCSI disk groups, consolidating multiple heterogeneous storage
devices (RAID, JBOD, tape) behind a single management console. For
example, virtual RAID-3 storage could be allocated to volumes to store
video information, while lower-cost RAID-5 virtual storage could be used
for transactional applications. This type of virtual storage obsoletes
physical LUN management, replacing it with attribute-based virtual disk
controls, which simplifies storage management.
Using block-level mapping techniques, storage virtualization presents
servers with logical views of storage in the form of virtual disks, while
storing data blocks on physical storage devices in a way that is
transparent to servers. In theory, this allows storage to be allocated
based on price, availability, and performance. JBOD can be divided into
hundreds of volumes, each having its own caching policy. Multiple JBOD or
RAID arrays can be combined into single, large volumes. Virtualization
allows storage (regardless of vendor) to be re-purposed for use on the
SAN, in essence providing uniform "volume management" across the
enterprise.
Storage virtualization products on the market today are either
pre-packaged solutions (SAN appliances) or software. Vendor examples
include Compaq, Datacore Software, DataDirect Networks, Gadzoox
Networks, StoreAge, StorageApps, TrueSAN, and Xiotech (Seagate).
The Gartner Group IT consulting firm, in Stamford, CT, defines two
methods for implementing storage virtualization: symmetrical pooling and
asymmetrical pooling. Symmetrical pooling puts the abstraction layer
directly in the SAN data path, between storage devices and servers. A
server, or storage domain controller, owns all the SAN storage and dishes
it out (via cache over multiple HBAs) to clients. The domain controller
houses the cache, storage pooling, and event management.
Asymmetrical pooling is when the storage abstraction is outside of the
data path. With this approach, "appliances" control the overall storage
virtualization process through the use of client-based software that
maintains the virtual disk mapping tables.
With either approach, the virtualization process provides real-time
translation of virtual disk addresses to physical LUN block addresses.
Without this storage abstraction function, storage management will
continue to be a problem.
Virtual storage solutions are a combination of software, hardware, and
professional services-often from a storage integrator-used to build and
deploy the new storage architecture. Once in place, however, these
solutions allow companies to lower the cost of storage management and to
react more quickly to their storage requirements.
Companies need ease of administration in assigning storage to hosts, with
the ability to grow and shrink volumes online across multiple operating
systems. These SAN strategies will help you manage and deploy storage
resources more effectively.
Mark Teter is director of storage solutions at Advanced Systems Group
(www.virtual.com), an enterprise computing and storage consulting firm
in Denve |
