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A similar limitation exists for Fibre Channel. Although there is a theoretical limit of 255 devices per FC
port on a host or switch, the actual limit appears to be 128 for many switches and HBAs. You should
either balance drives across FC ports or configure less than 128 drives per library.
Some backup applications will deliver less than optimum performance if managing many concurrent
backup tape drives/streams. Balancing the load across multiple backup application media servers can
help here.
Cartridge sizing
The size of a virtual cartridge has no impact on its performance; it is recommended that cartridges are created to
match the amount of data being backed up. For example, if a full backup is 500 GB, the next larger
configurable cartridge size is 800 GB, so this should be selected.
If backups span multiple cartridges, there is a risk that this will impact performance because housekeeping
operations will start on the first backup cartridge as soon as the backup application spans to the next cartridge. If
backups do span a cartridge, the effect of the housekeeping process should be quite small because it will hold off
due to the remaining backup traffic to the appliance taking priority. The performance overhead might become
higher, if several backup jobs are running concurrently and they all behave in the same way so that several
cartridges are performing low level housekeeping activity.
It is best practice to configure housekeeping blackout windows, as described on page 71.
Note that if backups are to be offloaded to physical media elsewhere in the network, it is recommended that the
cartridge sizing matches that of the physical media to be used. For G1 products, direct attach tape offload
cannot span physical cartridges so if a cartridge is too large, the offload operation will fail. Some backup
application-controlled copies can accommodate spanning of a single cartridge onto multiple physical cartridges,
however.
Number of libraries per appliance
The D2D appliance supports the creation of multiple virtual library devices. If large amounts of data are being
backed up from multiple hosts or for multiple disk LUNs on a single host, it is good practice to separate these
across several libraries (and consequently into multiple backup jobs). Each library has a separate deduplication
“store” associated with it. Reducing the amount of data in, and complexity of, each store will improve its
performance.
Creating a number of smaller deduplication “stores” rather than one large store which receives data from
multiple backup hosts could have an impact on the overall effectiveness of deduplication. However, generally,
the cross-server deduplication effect is quite low unless a lot of common data is being stored. If a lot of common
data is present on two servers, it is recommended that these are backed up to the same virtual library.
For best backup performance, configure multiple virtual libraries and use them all concurrently.
For best deduplication performance, use a single virtual library and fully utilize all the drives in that one
library.
Backup application configuration
In general backup application configurations for physical tape devices can be readily ported over to target a
deduplicating virtual library with no changes; this is one of the key benefits of virtual libraries – seamless
integration. However considering deduplication in the design of a backup application configuration can improve
performance, deduplication ratio or ease of data recovery so some time spent optimizing backup application
configuration is valuable.
Blocksize and transfer size
As with physical tape, larger tape block sizes and host transfer sizes are of benefit. This is because they reduce
the amount of overhead of headers added by the backup application and also by the transport interface. The
recommended minimum is 256 KB block size, and up to 1 MB is suggested.
For HP Data Protector and EMC Networker Software a block size of 512 KB has been found to provide the best
deduplication ratio and performance balance and is the recommended block size for this application.