EMC SYMMETRIX: DIFFERENT TYPES OF DEVICES AVAILABLE IN SYMMETRIX ARRAYS (PART 1)

Emc symmetrix arrays support different types of devices, each type has its own signification. In this post I am going to tell what are different types of devices we can create and what is the significance/purpose of the device.

lets start with a differentiation of devices

In DMX3 we can see below listed devices

1) Standard Devices                  2) GateKeeper Devices
3) Meta devices                         4) BCV devices
5) SRDF Devices                      6) Dynamic RDF devices
7) Virtual Devices                     8) Save Devices
9) Device Masking (VCM) devices
10) DRV Devices



For thin provisioning there is a THIN device which is also very important in DMX4 and VMAX

(If i miss any devices i will update them when i came to know about them)

In this post I am going to explain the first 4 device Types ( i listed above ) and the rest will be covered in later posts.

1) Standard Devices :

 These are basic devices that can be assaigned or addressible to host for normal    symmetrix operations. these devices can be created either with protection or without protection (EMC doesn't support non protected devices). The meaning of protection here is RAID protection. In DMX3 EMC supports RAID1(or 2-way-mirror), RAID5 and RAID6 protection.

examples:

below command creates 4 open system accessible luns or standard devices of size 10000 cylinders with RAID-1 protection in disk group 1

         create dev size=10000, count=4, config=2-way-mir, emulation=FBA, disk_group=1


to create 4 Mainframe host accessible devices of size 10000 cylinders with raid5(3+1) protection in disk group 1
 
create dev size=10000, count=4, config=RAID-5, data_member_count=3, emulation=CKD, disk_group=1

to create 4 open system host accessible devices of size 10000 cylinders with raid6(6+2) protection in disk group 1

create dev size=10000, count=4, config=RAID-6, data_member_count=6, emulation=FBA, disk_group=1

" symconfigure " command like below executes and creates the devices specified in the above commands in symmetrix array

symconfigure -sid 1234 -cmd " Create dev size=10000, count=4, config=2-way-mir, emulation=FBA, disk_group=1;" commit -nop

these devices can be combined to create meta device or can be mapped and masked to give access to host for I/O operations

2)Gatekeeper Devices:

 Gatekeeper is a small device that is used by a management host to communicate with the symmetrix array. To be detailed, the host issues SCSI commands to symmetrix array that performs the software operations. To make this possible there should be a device that acts as a gatekeeper and gives the access of symmetrix array to the management host. There is no normal data processing is involved in gatekeeper devices. These are of 6 cylinders in size and EMC recommends as large as the smalest host accessible LUN.

3)Metadevices: 

The maximum size of a standard device is limited in symmetrix arrays. For example in DMX3/4 with enginuity 5773 and earlier it is 65520 cylinders i.e 61425MB and with enginuity version 5874 it is 262668 cylinders i.e 240gb. so if a host connected to DMX3/4 requires a 100gb lun as single device the only solution is metadevice(/metalun) Metadevices are created by concatinating two or more standard devices to form a single addressible device. A meta device will have a meta head, meta members, and meta tail. Meta head is responsible in processing the SCSI commands related to that device.

We can create metadevices of two configuration types namely concatinated metadevices and striped metadevices. In concatinated metadecices, data will occupies one device till it is completely filled and then moves to next device in the meta members. where as in striped metadevices the data is written to each and every meta member. a stripe size of 1920 512byte blocks is used. first 1920 blocks are written to first device(hopefully meta head) in the metadevice then the second 1920 blocks are written to next device (first available meta member) in metadevice like wise data is written to each and every device that a metadevice formed from.

Examples:

create a text file with the below commands to create a meta device whose metahead is 0001 and members are 0002,0003,0004

a) striped meta

form meta from dev 0001 config=striped stripe_size=1920
add dev 0002:0004 to meta 0001 

symconfigure -sid 1234 -f meta.txt commit

b)concatinated meta

form meta from dev 0001 config=concatinated
add dev 0002:0004 to meta 0001 

symconfigure -sid 1234 -f meta.txt commit

4) BCV devices: 

Business Continuance Volume. BCV device is a standard symmetrix device that contains a copy of
data from symmetrix standard device that is online for regular I/O operation  from one or more hosts. It is a specialized device used in TimeFinder operations as a local replications. In a simple words BCV devices are used for mirroring critical data and can be accessed from other hosts. Each BCV device has its own host address and is configured as a stand alone device The uses of BCV copies include Backup, restore, decession support, and applications testing.

BCV devices can be metadevices
BCV devices cannot be thin devices


A BCV device first need to be paired with a standard device as a mirror. At this time the BCV device is not accessible via its original device address. when the pair is established and the data is synchronised, the BCV device can be separated from the pair. now the BCV device is available for backup or other host processes through its original device address. This BCV device is again can be paired or mirrored with the same or new standard device