Fibre Channel disk subsystem
69
Symmetrix DMX-3 Hardware
Deleting (and then
adding) devices
online
Enginuity supports removing and then adding devices online, which
facilitates the following configuration enhancements:
◆ Change Device Emulation Online — Remove a CKD volume and
add an FBA volume.
â—† Convert between mirrored and RAID protected volumes.
â—† The optimal order is to delete devices and then add. If done in the
reverse order, unnecessary global memory will be allocated for
the deleted devices.
â—† When attempting to add or delete devices, or change protection
type of devices, a new minimum cache value will be calculated.
In rare cases this new value could prohibit the changes until
additional memory is added to the system.
Open system disk
emulation
On open systems hosts, the Symmetrix DMX logical disk volumes
appear as standard SCSI disk drives with data stored in Fixed Block
Architecture (FBA) format. All host logical volume manager software
can be used with Symmetrix disk volumes. The following paragraphs
describe the FBA disk format and logical volume structure.
FBA data and
command format
Fixed Block Architecture (FBA) disk drives store data in fixed-sized
blocks (typically 512 bytes). A disk drive using FBA format is viewed
as a large array of blocks. The physical position of each block
(cylinder and track) is usually not significant to the host. When
requesting disk access for read or write, the host addresses a file by
the logical block address (LBA) of the starting block and a count of
the total blocks used by the file. Symmetrix channel directors and
disk directors control access to global memory and disk drives,
responding to host requests as a standard disk drive.
Note: Disk drives in Symmetrix DMX systems attached to iSeries hosts are
configured in 520-byte blocks.
Logical volume
structure (open
systems)
The channel directors interact with global memory. Therefore, there is
no physical meaning to cylinders, tracks, and heads on the
Symmetrix logical volume from the front-end point of view.
However, Symmetrix systems use a logical geometry definition for
their logical volume structure. This geometry is reflected in the mode
sense data available to the host.
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