LEARN MORE ON COMPARISON: NAS, FIBRE CHANNEL SAN AND ISCSI SAN

LEARN MORE ON  COMPARISON: NAS, FIBRE CHANNEL SAN AND ISCSI SAN

Fibre Channel SAN, iSCSI SAN and NAS are three techniques with which storage networks can be realized. Figure 4.15 compares the I/O paths of the three Techniques and Table 4.2 summarizes the most important differences. In contrast to NAS, in Fibre Channel and iSCSI the data exchange between servers and

storage devices takes place in a block-based fashion. Storage networks are more difficult Table 4.2 Comparison of Fibre Channel, iSCSI and NAS Fibre channel iSCSI NAS Protocol FCP (SCSI) iSCSI (SCSI) NFS, CIFS, HTTP Network Fibre Channel TCP/IP TCP/IP Source/target Server/storage device Server/storage device

to configure. On the other hand, Fibre Channel at least supplies optimal performance for the data exchange between server and storage device. NAS servers, on the other hand, are turnkey file servers. They can only be used as file servers, but they do this very well. NAS servers have only limited suitability as data

storage for databases due to lack of performance. Storage networks can be realized with NAS servers by installing an additional LAN between NAS server and the application servers (Figure 4.16). In contrast to Fibre Channel and iSCSI this storage network transfers files or file fragments. One supposed advantage of NAS is that NAS servers at first glance have a higher prefetch hit rate than disk subsystems connected via Fibre Channel or iSCSI (or just SCSI). However, it should be borne in mind that NAS servers work at file system level and disk subsystems only at block level. A file server can move the blocks of an opened

file from the hard disk into the main memory and thus operate subsequent file accesses more quickly from the main memory. Disk subsystems, on the other hand, have a prefetch hit rate of around 40% because they only know blocks; they do not know how the data (for example, a file system ordatabase) is organized in the blocks. A self-configured file server or a NAS server that uses hard disks in the storage network can naturally implement its own prefetch strategy in addition to the prefetch strategy of the disk subsystem and, just like a NAS server, achieve a prefetch hit rate of 100%. As yet, only Fibre Channel and NAS have been successfully implemented in production environments. Fibre Channel satisfies the highest performance requirements – it is currently (2003) the only transmission technique for storage networks that is suitable for I/O intensive databases. Since 2002, iSCSI has slowly been moving into production environments. It is said that iSCSI is initially being used for applications with low or medium performance requirements. It remains to be seen in practice whether iSCSI also satisfies high performance requirements (cf. Section 3.5.2, 'TCP/IP and Ethernet as an I/O technology'). NAS is excellently suited to web servers and for the file sharing of work groups. With RDMA-enabled NFS and CIFS, NAS could also establish itself as a more convenient data store for databases.

 

Hard disks provide their storage in the form of blocks that are addressed via cylinders, sectors and tracks. File systems manage the blocks of the hard disks and make their storage capacity available to users in the form of directories and files. Network file  systems and shared disk file systems make it possible to access to the common data set from various computers. Modern file systems have additional functions, which can increase the availability of data in various situations. Journaling ensures that file systems  become available again quickly after a system crash; snapshots allows data sets to be practically copied within a few seconds; the volume manager makes it possible to react to changed storage requirements without interrupting operation. Network file systems export

local file systems over the LAN so that it is possible to work on a common data set from different computers. The performance of network file systems is limited by two factors: (1) all data accesses to network file systems have to pass through a single file server; and (2) current network file systems such as NFS and CIFS and the underlying network protocols are not suitable for a high throughput. We introduced two approaches to circumventing these performance bottlenecks: RDMA-enabled file systems such as the Direct Access File System (DAFS) and shared disk file systems such as the General  Parallel File System (GPFS). Network Attached Storage (NAS) represents a new product category. NAS servers are preconfigured file servers, the operating systems of which have

been optimized for the tasks of file servers. Storage networks can also be realized with NAS servers. However, current NAS servers are not suitable for providing storage space for I/O intensive databases. In the previous chapters we have introduced all the necessary techniques for storage

networks. Many of these techniques deal with the virtualization of storage resources like RAID, instant copy, remote mirroring, volume manager, file systems, and file system snapshots. All these virtualization techniques have in common that they present given storage resources to the upper layers as logical resources which are easier to use and administrate and which very often are also more performant and more failure tolerant. The next chapter will discuss how the concept of storage virtualization changes in a storage-centric IT architecture.