November, 2012 IBM announced new low end system Storwize V3700. After the new software version came, the system began to support more useful options and configurations. In this article we will make a brief overview of the system and talk about some of its features.

The competition in the storage market requires from manufacturers to use certain tricks to win in competitive battles and reduce the cost of creating and maintaining hardware and software. We’ll talk a little later about apparatus features of V3700, and now let’s look at the main characteristics of system.

Basis things (Type – Model: 2072-24С):

• Each controller has 2-core 64-bit Intel CPU, based on Sandy Bridge. Thus, one pair of controllers has 4 cores.
• The amount of memory on the pair of controllers is 8 GB, i.e. 4 GB per controller. Optionally, the amount of cache memory on the pair of controllers can be increased up to 16 GB.
• An access to hosts may be organized in the following external interfaces to block access: FC – 8Gbps, iSCSI – 1Gbps and 10Gbps, FCoE – 10Gbps.
• The standard PCIE V2 is used for Backend IO connection. The throughput of this standard allows the use of SAS Backend at the speed up to 6Gbps.
• The maximum number of disk drives per pair of controllers is 240.
• A form factor of shelve with a pair of controllers is 2U in the 19-inch rack.
• Up to 24 disk drives size 2.5″ can be installed in the shelf with a pair of controllers (2072-24С).
• Up to 12 disk drives size 3.5″ can be installed in the shelf with a pair of controllers (2072-12С).
• Up to 24 disk drives size 2.5″ can be installed in the disk shelf (2072-24E).
• Up to 12 disk drives size 3.5″ can be installed in the disk shelf (2072-12E).
• The Flash disk drives 2.5” sizes 200, 400, 800 GB with 12Gbps SAS interface are supported.
• The SAS disk drives 2.5” sizes 146, 300, 600 GB 15K rpm with 6 Gbps SAS interface are supported (600 GB – 12 Gbps SAS).
• The SAS disk drives 2.5” sizes 600, 900, 1200, 1800 GB 10K rpm with 6Gbps SAS interface are supported (1800 GB – 12 Gbps SAS).
• The SAS disk drives 3.5” sizes 300, 600 GB 15K rpm with 6Gbps SAS interface are supported (600 GB – 12 Gbps SAS).
• The SAS disk drives 3.5” sizes 900, 1200, 1800 GB 10K rpm with 6Gbps SAS interface are supported (1800 GB – 12 Gbps SAS).
• The NL-SAS disk drives 2.5” 7.2K rpm size 1 TB with 6Gbps SAS interface are supported.
• The NL-SAS disk drives 3.5” 7.2K rpm size 2, 3, 4, 6 TB with 6Gbps SAS interface are supported (6 TB – 12 Gbps SAS).
• 12 Gbps SAS ports of new drives work at 6 Gbps as SAS ports on the controllers.
• The different disk drive types with the same form factor can be supported in one shelf.
• One pair of controllers can support up to 9 disk shelves.
• Thin Provisioning technology.
• Hybrid pools with automatic data migration between storage tires (Easy Tier).
• Limited external block storage virtualization (image mode) is used for data migration only.
• Local replication (FlashCopy).
• Remote replication synchronous and asynchronous (Remote Mirror).
• 12 Gbps SAS drives are supported with software version 7.4 or above.

Software

Software based on SAN Volume Controller (SVC) code is used in Storwize V3700. The scheme of organization RAID groups and Pools was borrowed from High end systems DS8xxx series. Base platform – Linux.

Support of additional features appears with each update the software of controllers. In this case, the main components such as processors, memory, PCIe fabric, SAS chips, in fact, don’t change, but the support of new drives and host adapters appears.

Let’s look briefly at the chronology of support additional options to the latest versions of software.

These features appeared in 7.1:

• Support for connecting hosts with SAS cables. 4 SAS ports were on each controller in the first release in 2012, but only one port used for connecting disk shelves. An ability to connect hosts, via 3 unused SAS ports on each controller, appeared with this version. This is an interesting feature, which helps to reduce the summary cost of solution, because we don’t need to use SAN, based on FC or FCoE for hosts’ connections. But at the same time, we obtain a significant limitation on the number of hosts that we can connect to the storage system.
• Support new 4 port SAS 6 Gbps cards for controllers to connect hosts. It gave 7 SAS ports 6 Gbps on each controller ( 3 embedded and 4 on the card). Up to 14 SAS ports on the entire system.
• The automatic migration of extents in pools between storage tiers appeared (EasyTier). In this version, only two tiers are available, there is no balance at the same level. Averaged statistics about utilization of each extent is formed every 5 minutes and collected in 24 hours. Further, a map of the most loaded extents (heat map) and the plan of extents migration are formed. The rate of migration is about 15 MBps. Thus, up to 1200 GB of data can be migrated during 24 hours (15MBps * 86400s * 1/1024).
• The new option FlashCopy upgrade expands the number of links between the original volumes and snaps and between snaps up to 2040. 64 links are supported on the system by default.
• Synchronous and asynchronous data replication to a remote system (Remote mirror) over FC and FCoE.
• The option of improving system performance (Turbo performance). The throughput increases when you use this option. But it is not clear how many, 10 – 20%?! The hardware does not change, where is the extra performance coming from?! Perhaps, this is due to the fact that IBM made the performance margin for some components such as Backend (SAS chips, etc.) and software increases throughput without changing hardware. It is also interesting to note that IBM recommends to use this option when the system has more than 5 Flash disks or 80 SAS disks.

These features appeared in 7.2:

• Data replication to a remote system (Remote mirror) over IP.
• Data migration between hosts which are connected by SAS.

These features appeared in 7.3:

• The memory architecture on the controllers was changed.
• The ability to connect more than 4 disk shelves appeared. The maximum number of disk shelves is 9. The quantity of disks is 240.
• Support of 3 tiers in hybrid Pools (EasyTier V3).

Version 7.4 adds support for new SAS drives 12 Gbps (NL-SAS 6TB 7K 3.5 “, SAS 1,8TB 10K 3.5″, SAS 1,8TB 10K 2.5 “).

Controllers

The form factor of V3700 controllers doesn’t differ much from that of V7000 Gen1. However, they are located horizontally at the same level rather than one above another. In this case, as in V7000 Gen1 except for embedded ports, each controller has one PCIE slot that can be used for various cards.

Embedded ports on the controller

• Up to 2 ports Ethernet 1Gbps, (1 port is also used for service needs, initial install and etc). These 2 ports can be used to block access iSCSI 1Gbps protocol.
• Up to 2 USB ports (used for configuration purposes).
• Up to 4 SAS ports on the controller. The first 3 ports are used to connect hosts; the 4th port is used for connecting disk shelves. One connector consists of 4 bidirectional serial lanes, each one operates at the speed up to 6 Gbps (max theoretical bandwidth).

Connections of disk shelves

As it was mentioned before, the 4th SAS port of the controllers is used for Backend connectivity. 9 disk shelves (9 ESM modules) can be connected by chain to one SAS port. A typical connection scheme is shown in the figure below.

PCIE cards for controllers

At the moment, you can install different cards into PCIE slot, these are:

• 4 ports FC 8Gbps card. It comes with Shot Wave SFP 8Gbps transceivers. There is a possibility to use Long Wave SFP 8Gbps transceivers.
• 2 ports iSCSI, FCoE 10Gbps card. It comes with SFP+ transceivers.
• 4 ports iSCSI, FCoE 1Gbps card.
• 4 ports SAS 6Gbps card.

The table contains possible ports configurations:

embedded ports		additional ports
SAS 6Gbps	iSCSI 1Gbps	SAS 6Gbps	iSCSI 1Gbps	FC 8Gbps	iSCSI/FCoE 10Gbps
6	4
6	4	8
6	4		8
6	4			8
6	4				4

Features

Among the new things I want to note that it is now possible to make the replication via Ethernet, using the integrated ports 1Gbps and 10Gbps ports on the cards.

Host connection thru FCoE 10 Gbps is supported only when the network infrastructure is used, i.e. direct connection to the host is not supported.

Besides, there is a recommendation to separate FCoE and iSCSI protocols between different ports.

Batteries and power outage

The batteries are fairly compactly located in the controllers. Their main task is to allow the controllers to save the current configuration and the write cache to the internal Flash drive, in case of failure of the guaranteed supply.

Key components of solution

The photo of elements inside the controller is presented in the figure below:

The key element of the controller is the use of 2-core CPU Intel Celeron G530T, based on Sandy Bridge architecture. Cores frequency is 2 Ghz. An embedded video core. Support PCE V2. Configurations with 1 port x16 or 2 port x8 are supported. This is a typical low cost CPU designed for desktop stations.
It supports UDIMM memory without correcting errors (ECC).

In general, what do you want of low end system?! Of course, IBM has tried to reduce the price of the system. At the moment, Intel no longer produces these processors and you can see the inscription «End of Life» on the official website, so we expect that the 2nd generation of the system will apear soon with new processors with memory ECC.

Each controller has a SAS chip PMC-Sierra Inc. PM8018 Tachyon SPCv 16x6G. This chip has a large number of SAS ports. It can support PCIE V3, but uses PCIE V2. The same SAS chip is used to connect disk shelves (Backend) and hosts (Frontend).

Not less than 2 SAS ports are used to connect the drives in the controllers’ shelf (2 controllers). We assume that the disks in the controllers shelf are connected to SAS chips more than 2 SAS ports, because SAS chips have a large number of ports. This gives increased bandwidth for this disks shelf. Hence follows the recommendation to install high-performance Flash drives in the controller shelf.

The same FC HBA is used to connect hosts by FC, as in the first generation V7000. This is 4 ports HBA PM8032 Tachyon QE8 designed by PMC-Sierra, Inc., it supports PCIE v2.

Low cost 8 ports PCIE switches PEX8609 PLX technology are used for connection PCIE devices and data exchange between controllers.

You can see information about PCI device ids for V3700 here.

Cache memory architecture and hybrid pools EasyTier V3

The advent of the 7.3 version has changed the cache memory architecture and has provided an ability to use hybrid pools with 3 tiers. You can find more detailed information about these matters in the article «IBM Storwize V7000 Gen2».This information is valid for V3700 with the exception of small features associated with the equipment. For example, Upper Cache is equal to 128 MB, instead of 256 MB, due to the small size of memory. Besides, less memory is allocated for Linux kernel.

The controllers performance estimations, made on the basis of storage backend calculator can be found in the article «IOPS Estimation for IBM V3700».

We finish our review with a small comparative table:

IBM Storwize	V3700
Max drives per controller pair (IO Group)	240
Memory per controller, GB	8
Memory per controller pair (IO Group), GB	16
CPU type	Celeron
CPU per controller	1
CPU core q-ty	2
CPU core clock, GHz	2
Max FC 8Gbps ports per controller (FE)	4
Max FC 8Gbps ports per controller pair (IO Group) (FE)	8
Max iSCSI 10Gbps ports per controller (FE)	2
Max iSCSI 10Gbps ports per controller pair (IO Group) (FE)	4
RAID types	0,5,6,10
Max Disks per RAID Group	16
Max Disks per Pool	128