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Chapter Seven : Managing the Storage Appliance 201
StorTrends Tiered Storage and CSM (Scaling within tiers)
There are various reasons for requiring multiple logical drives in a tier. Expansion on the
logical drive level can take place by adding new physical disks in two ways:
•
Expanding existing RAID arrays with new disks.
•
Creating a new RAID array with new disks.
Creating a new RAID array with new disks is used more often to avoid a lengthy re-
striping process after expansion (during which time the I/O performance on the old data
is largely reduced). This will result in multiple Logical Drives (LDs) in the same tier and
data has to be striped across them.
There can also be restrictions given by the RAID card in the number of disk that can be
added to one LD (maximum expansion support per LD). Some RAID cards may have
restrictions on creating arrays across enclosures. If a storage system contains multiple
enclosures (or JBODs), it might be necessary to have independent LDs in each of them.
With a substantial spread in the speed and the cost of physical drives available in the
market today, it is conceivable that different LDs may have different RAID levels but
may still fall into the same tier. For example, a RAID 10 array with 7500 RPM SATA
drives and a RAID 50 array with 10000 RPM SATA drives may be considered to belong
to the same tier. Hence data will need to be stored using CSM on both of these entities.
CSM makes sure that the maximum number of spindles may be made available to any
I/O load, by distributing the data equally amongst all spindles, even if they fall in
different LDs.