EIDE Disk Arrays and Its Implement

Transcription

1 EIDE Disk Arrays and Its Implement Qiong Chen Turku Center for Computer Science, ÅBO Akademi University Turku, Finland Abstract: Along with the information high-speed development, RAID, which has large capacity, high availability, and high performance, has played an important role in storage system. Comparison with SCSI disk drive, the body of EIDE disk drive is similar to SCSI s, but for long time, EIDE disk drive performance is lower than SCSI disk drive s. Recently,an EIDE drive is rapidly developed. New EIDE drive almost offers similar performance to SCSI s. At the same time, PC motherboard provides a lot of technology supports. Thus, it is possible that RAID is consisted of EIDE disk drives. Although many technologies and methods of SCSI RAID can continue to be used in EIDE RAID, there are other problems required to solve in EIDE RAID. The research work is processing. In this paper, for the first time, we propose a kind of architecture based on system integration, and a method of protocol conversion between SCSI and ATA. They should support a host to send an I/O request from a SCSI adapter to an EIDE RAID. They also support heterogeneous channel disk arrays. The test result is the performance and price ratio of EIDE RAID is better than SCSI disk arrays. Keywords: SCSI RAID, EIDE RAID, Protocol conversion, Performance/Price ratio. 1. Introduction Recently, microprocessor performance has been largely improved, and computer applications, which include parallel processing, multimedia, computer network etc, also have been largely developed. Therefore, I/O storage system is faced with greatly challenge, it is demanded with large-capacity, high-performance, and high-availability. One innovation that improved both availability and performance of storage system is redundant arrays of independent disks, acronym RAID [1,2,3,4] ; it has been promoted as the most promising technology to fit these needs. And SCSI (Small Computer System Interface) or Fiber Channel (FC) always occupies the disk drive areas of RAID. SCSI disk drive or Fiber Channel has a lot of well-known advantages; such as high data transfer rate, large capacity, and quickly access time. But it is more expensive, and it needs expensive adapter, therefore the price of RAID is very high. Comparing with SCSI disk drive, the price of EIDE (Enhanced Integrated Drive Electronics) is relevant low, its price is only 60% of SCSI disk drive, and EIDE 1

2 disks could directly be connected with ATA (Advanced Technology Attachment) interface of PC motherboard without any additional adapter. But for long time, its performance normally is lower than SCSI or Fiber Channel s. However, the biggest change was happened in late 1997, this was the creation of the Ultra ATA/DMA standard. For the first time, the new EIDE drive offers similar performance to Ultra Wide SCSI. ATA-5 defines transfer rates up to 66MB/s, with ATA-6 slated to include a 100MB/s rate, and 32-bit ATA could increase this rate to 200MB/s [5]. Since UDMA s introduction, there have been dramatic drops in cost per megabyte of the drives, this has expanded the market of the drivers, even into areas where only SCSI drivers have been seen like Mac, Sun, and SGI platforms, and RAID. Besides, some motherboards now have provided many technology supports. Until 2000, there are more than four ATA/66 EIDE interfaces on one motherboard. This means it is completely possible to construct an EIDE disk arrays. Although many technologies and methods of SCSI RAID can continue to be used in EIDE RAID, there are a lot of problems being solved in EIDE RAID, such as the architecture, the protocol, and etc. In this paper, we propose a flexible integrated EIDE RAID architecture, where a host would connect the EIDE RAID by a SCSI bus. In other words, the EIDE RAID is accessed as a SCSI disk drive. Thus, there is protocol conversion between host SCSI standard and ATA standard, in Section 4; we propose a method of protocol conversion based on direct translating and simulative translating. Lastly we compare the performance of EIDE RAID with a similar size SCSI RAID. And conclusions are given in Section Integrated Architecture An integrated EIDE RAID architecture that provides more than one of RAID levels is shown in Fig 1. It has a RAID controller, more than two EIDE (or called ATA) interfaces with one or two EIDE disk drives. The RAID controller, which is a personal computer system, receives/sends SCSI commands and data from/to a host by a SCSI bus, and carries out parity calculations, places data blocks, reconstructs and rebuilds data if a disk drive is failure. The disk drive interfaces pass commands from the RAID controller to the disk drives via an AT attachment bus. There is a PCI_SCSI I/O processor in RAID controller, which is a target, and an adapter of EIDE RAID. It would receive SCSI commands and data from a host through a SCSI bus, and also send data or message to the host. EIDE interface operation is dependent on RAID controller, only when EIDE disk drive finishes a physical operation, the RAID controller can schedule other task. 2

3 RAID controller AT attachment bus IDE disks host interconnect SCSI BUS Figure Figure1 1 EIDE IDE RAID RAID Architecture architecture 3. Operate Model of EIDE RAID In ATA standard, disk drives on the different interface can work concurrently, but the disk drives on the same interface are not operated synchronously, while there are over eight disk drives (Narrow SCSI), 16 disk drives (Wide SCSI), 32 (Very Space (1,1) (0,1) (1,0) (0,0) st st t1 t2 t3 t4 Time Figure 2 Time-Space Relation of Disk Work Wide SCSI) or 126 (FireWire) disk drives connecting a SCSI interface [6]. There is time coherence between disk drives on the same ATA interface. It means all disk drives on EIDE interfaces can t parallel work. Fig. 2 is a chart of time-space relation of two EIDE interfaces; each EIDE interface connects two disk drives. Disk (0,0) and Disk (0,1) are connected with No.1 EIDE interface, Disk (1,0) and Disk (1,1) are connected with No.2 EIDE interface. Disk (i, 0) and Disk (i, 1) cannot work synchronously, where i=0,1. Supposing t i denotes the time to start a disk drive. Disk (0,0) is started at t 1, and Disk (1,0) (maybe could be Disk (1,1)) is started at t 2. When Disk (0,0) has been completed, Disk (0,1) can be started at t 3. In the same way, when Disk (1,0) has been completed, Disk (1,1) can be started at t 4. So there are only m disk drives concurrently working at anytime, where m denotes the number of EIDE interface. 4. Protocol Conversion 3

4 The different protocols are used in EIDE RAID, one is SCSI protocol, and the other is ATA protocol. Therefore, protocol conversion between SCSI and ATA is major problem in EIDE RAID. Assuming X is the protocol translated, and Y is the target protocol, it is the general method for protocol conversion to execute a group of Y commands for an X command. However, in EIDE RAID, some SCSI commands are not interpreted by EIDE, such as MODE SENSE, INQUIRY etc. So a method called simulative conversion is proposed, which simulates the SCSI command function that is not interpreted by EIDE commands. The data structure of protocol conversion with C language is described in Table 1. Table 1 Protocol Conversion Data Structure typedef struct Trans{ char ImpMd; char ScsiCmd[10]; char *SimuData; struct IdeType *IdeCmd; struct Trans *next ; }Trans; Variable ImpMd denotes the SCSI command implement mode, ScsiCmd[10] describes a SCSI command from a host, the simulative data is pointed out by pointer variable SimuData. And the IdeCmd points to the first EIDE command that will translate a SCSI command. If direct conversion is used, ImpMd =0, and IdeCmd points at a corresponding EIDE command or a set of EIDE commands that completes the SCSI command function, while SimuData equals to NULL. Otherwise, ImpMd =1, simulative method is used, then SimuData points to the simulated data directly sent to the host, and IdeCmd equals to NULL. 4.2 Form Simulative Data The simulative data is usually the information regarding parameters of target and its attached peripheral device, such as detailed vital product data, the target wide data transfers and synchronous data transfers. The simplest method obtained the simulative data is from a SCSI disk drive, then modifying the data according to EIDE RAID configuration. For example INQUIRY command [7,8], it requests that information regarding parameters of the target and a component logical unit be sent to the application client, options allow the application client to request additional information about the target and logical unit or information about SCSI commands supported by the device server. The 4

5 standard INQUIRY data shall contain at least 36 bytes, where if 16 bits wide bus is used in EIDE RAID adapter, then the Wbus16 bit of inquiry data must be set. Same as the MODE SENSE command. 4.2 Conversion Implement The SCSI commands could be plotted out three kinds of conversion implement. The simplest doesn t need to execute any EIDE command. It only needs to check some status of Registers, e.g. TEST UNIT READY [7,8]. TEST UNIT READY command provides a means to check if the logical unit is ready. This is not a request for a self-test. If the logical unit is able to accept an appropriate medium-access command without returning CHECK CONDITION status, this command shall return a GOOD status (0x00). If the logical unit is unable to become operational or is in a state such that an application client action (e.g., START UNIT command) is required to make the unit ready, the device server shall return CHECK CONDITION status (0x01) with a sense key of NOT READY. So only respectively read the Status or Alternate Status Register of EIDE drives, if the BYS bit is equal to 0 then return 0x00 status, otherwise return 0x01 status. The second is one-by-one conversion implement, e.g. READ CAPACITY [9,10,11]. READ CAPACITY command could be directly implemented through IDENTIFY DEVICE EIDE command. The third is one-to-multi commands implement, it means that one SCSI command needs more than one EIDE commands to implement, e.g. READ/WRITE command. IdeCmd data structure definition is shown in Table 2 [12]. If a SCSI command needs more than one EIDE command to complete its function, the EIDE commands will link together by the next pointer in the data structure. In ATA standard, the addressing of the device can be either by cylinder-head-sector, for short CHS, or by the logical block address, for short LBA. Since the LBA is used in SCSI command format, the LBA is used in EIDE RAID, and there are four registers containing the LBA for any media access in ATA interface standard, so four variables are used for describing LBA address. Table 2 EIDE Command Data Structure typedef struct IdeType{ unsigned char CmdCode; // Command Code unsigned char DataLength; / / the number of sectors of data requested to be read // or written operation on a disk drive unsigned char Address1; ///Bit 7-0 of the LBA unsigned char Address2; // Bit 15-8 of the LBA 5

6 unsigned char Address3; // Bit of the LBA unsigned char Address4; // Bit of the LBA struct data_list *DataHead; // pointing data buffer in main memory struct IdeType *Next; }IdeType; A procedure of translating a SCSI READ/WRITE command into EIDE commands is as follow: 1) For simplified writing, the variable Datalength denotes IdeType->Datalength in Table 2, same as others. And the SCSI commands are respectively shown in Table 3 and Table 4 [9,10,11]. In the tables, the Logical Unit is defined in the IDENTIFY message, and It is recommended that the logical unit number in the command descriptor block will be set to zero. The Logical Block Address field specifies the logical block where the read/write operation shall begin. The Transfer Length field specifies the number of contiguous logical blocks of data to be transferred. A Transfer Length of zero indirectly indicates that 256 logical blocks shall be transferred. Any other value directly indicates the number of logical blocks that shall be transferred. 2) The Logical Block Address is evaluated to variable startaddress, and the Transfer Length is evaluated to variable datalength. 3) If datalength<0 then stop. Else turning to next step 4) If datalength>255 then DataLength=255, else DataLength=dataLength 5) datalength=datalength-255 6) Adrress1=startAddress & 0xff, Adrress2=(startAddress >> 8) & 0xff, Adrress3=(startAddress >> 16) & 0xff, Adrress4=(startAddress >> 24) & 0x0f; 7) Connecting this EIDE command into the Next pointer. 8) If datalength<0 then stop. Else startaddress= startaddress+255, and turning to step 3. Fig. 3 shows a SCSI read command to be translated into a group EIDE commands. The mean of SCSI command is that read data from the start address 0, 768 sectors length. Then it needs four EIDE read commands, their start address is different while the length of first three commands is all 255 sectors, and the last length is only 3 sectors. Table 3 READ/WRITE(6) Command Table 4 READ/WRITE(10) Command 6

7 Bit Byt e 0 Operation Code 1 Logical (MSB) 2 3 Unit MSB Logical Block Address LSB 4 Transfer Length 5 Control Bit Byt e 0 Operation Code 1 Logical Reserved Unit MSB Logical Block Address LSB 6 Reserved 7 MSB 8 Transfer LSB 9 Control length {28h, 00h, 00h, 00h, 00h, 00h, 00h, 03h, 00h, 00h} (a) A SCSI Read Command 21h FFh 00h 00h 00h 00h 21h FFh FFh 00h 00h 00h 21h FFh FEh 01h 00h 00h 21h 03h FDh 02h 00h 00h NULL (b) Corresponding EIDE commands Figure 3 Command Translation from SCSI to EIDE 5. EIDE RAID Experimental Performance and Comparison According to above idea, we construct two RAID experimental platforms, which provide levels with RAID 0, RAID 1, and RAID 5. The EIDE RAID consists of 4 ST36810A EIDE disk drives, an Asus motherboard, a Pentium MMX 200MHZ CPU, 64MB memory, and a NCR53C875 PCI-SCSI adapter. While the SCSI RAID consists of 4 ST34520N SCSI disk drives, an Asus motherboard, a Pentium MMX 7

8 200MHZ CPU, 64MB memory, and 3 NCR53C875 PCI-SCSI adapters. For availably comparing performance, the RAIDs are composed of two strings, which can work concurrently, and each string will be connected two drives, the disk drives in the same string do not work concurrently. NCR53C875 support synchronous data transfer rate up to 40MB/s. The AT attachment on Asus motherboard is Ultra ATA/66, which transfers data at burst rates up to 66Mbytes/sec. The host connects RAID via a SCSI bus. Table 5 shows the parameters of SCSI drive and EIDE drive used in the platforms [13 ]. Table 5 Disk drive parameters Drive type ST36810A ST34520N Interface Ultra ATA/66 Ultra SCSI Cylinder Number of heads Capacity 6.8GB 4.55GB Sectors/Track (avg.) Bytes per sector Spindle speed 7200rpm 7200rpm Average seek time 8.6ms 9.5ms We use Qbench [14] I/O traces as the input to RAID experimental platforms. The workload is respectively sent from the same host to SCSI RAID and EIDE RAID. It varies from 1 sector, 2 sectors, 4 sectors, 8 sectors, up to 128 sectors, where read/write one sector is 54.3%, and the other is respectively around 6.52%. And for all operations, 65% is sequential, 35% is random; read operation is 60%, write operation is 40%. We obtain the average weighted data access time and data transfer rate for all loads. The test results are shown in Fig. 4. Fig. 4 (a) shows that the data transfer rate of SCSI RAID is a little higher than EIDE RAID s for RAID 0 and RAID 1, but it is lower for RAID 5. Furthermore, Fig. 4 (b) shows that the access time of EIDE RAID is better than SCSI RAID when RAID architecture is RAID1 or RAID0, and for RAID5, EIDE RAID access time is also less than the access time of SCSI RAID. In a word, the holistic performance of EIDE disk arrays is similar to the performance of SCSI disk arrays. In other words, the performance /price ratio of EIDE disk arrays excels SCSI disk arrays. 8

9 Data Transfer Rate (MB/s) SCSI RAID IDE RAID RAID 0 RAID 1 RAID 5 RAID Level (a) Data Transfer Rate Comparison 6 Access Time (ms) SCSI RAID IDE RAID RAID 0 RAID 1 RAID 5 (b) Access Time Comparison RAID Level Figure 4 Performance Comparison 6. Conclusion The experimental results indicate that EIDE disk arrays can provide similar performance to SCSI disk arrays under the condition assumed. In other word, if the scale of I/O subsystem is not needed too large, it is very effective to structure a RAID by EIDE disk drives, and the EIDE disk arrays can provide a good performance/price ratio. Therefore it is very significative to study EIDE disk arrays. Actually, there are many kinds of disk drive, in theory, they could respectively compose a disk arrays, and the host will access the disk arrays by a SCSI-PCI adapter. In this case, there must be a protocol conversion between SCSI and other standard. This method of protocol conversion introduced in Section 4 can implement the conversion not only between SCSI and EIDE, but also between SCSI and other standard. References [1] D.Patterson, G. Gibson and R. Katz A Case for Redundant arrays of Inexpensive Disks (RAID) Proc. Of the ACM SIGMOD 88, (June),

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