UbiqStor: A Remote Storage Service for Mobile Devices

In Ubiquitous computing environment the mobile devices such as PDAs necessarily connect to remote storage servers. We present an iSCSI caching system that localizes iSCSI target to overcome the shortcomings of iSCSI performance dropping sharply as the latency increases. 1 Motivation Mobile devices such as PDAs are evolving to be incorporated into Ubiquitous computing environment. Due to lightly equipped storage, they lack enough capacity to process application of large data, thus it has been necessitated supplying vast storage capacity from remote machine. For mass storage service, SCSI has been representative protocol in its widespread application. We have built a remote storage system for mobile appliances using iSCSI protocol, which mobile devices can use the storage of a remote server through wireless link but just as their own local storage. It enables mobile appliances to overcome the limitation of storage capacity, as well as the ability to adapt various applications of wired environment in need of mass scale data. 1.1 iSCSI The iSCSI (Internet Small Computer System Interface) is an emerging standard storage protocol that can transfer a SCSI command over IP network. Since the iSCSI protocol can make clients access the SCSI I/O devices of server host over an IP Network, client can use the storage of another host transparently without the need to pass through a server host's file system[1]. Fig. 1 illustrates iSCSI protocol linkage. In iSCSI layer common SCSI commands and data are encapsulated in the form of iSCSI PDU (Protocol Data Unit). The iSCSI PDU is sent to the TCP layer for the IP network transport. The encapsulation and the decapsulation of SCSI I/O commands over TCP/IP enable the storage user to access a remote storage device of the remote server directly[2]. 1.2 iCache iCache is developed to improve iSCSI performance using local cache of a client system. Initiator’s systems have specific cache space for iSCSI data, and iSCSI block M. Ok, D. Kim, and M.-s. Park 686 Fig. 1. Remote Storage Service with iSCSI protocol Fig. 2. iCache Architecture data is cached to minimize network block I/O. Thus iSCSI does not send I/O requests through the network every time the disk I/O happens. Instead it reads cached blocks or sends blocks cached in LogDisk at once to the server for improving iSCSI performance. iCache's buffer space consists of two hierarchical caches comprising Non-Volatile RAM and LogDisk. Data is stored sequentially in NVRAM. When enough data is gathered, iCache process moves data from NVRAM to LogDisk. Blocks which are frequently accessed, are kept in NVRAM where access speed is fast. iCache stores less accessed data in the LogDisk. Caching techniques used in iCache are based on DCD technology, [3] proposed to improve Disk I/O performance. However storage subsystem like iCache is not adequate to mobile devices since it needs additional NVRAM and LogDisk to embody the local cache. Data Data Data SAN Disk Bunch Storage Server UbiqStor Server UbiqStor Server