Technical Reference

This section contains background on JBOD and the RAID levels used on the NAS.

204 Soft ECC

Correction Low This is the number of errors corrected by software ECC (Error Correction Code).

205 Thermal Asperity

Rate (TAR) Low This is the number of thermal asperity errors. Thermal asperity is a read signal spike caused by sensor temperature rise due to touching the disk surface or contaminant particles.

206 Flying Height This is the height of the hard drive’s read/write heads above the disk surface.

207 Spin High

Current This is the quantity of high current used to spin up the drive.

208 Spin Buzz This is the number of buzz routines to spin up the drive. When the arm holding the read/write heads is stuck, the motor driving it tries to oscillate the arm to free it. This causes an audible vibration.

209 Offline Seek

Performance This is the hard drive’s seek performance during offline operations. Offline operations are tests the drive does itself as opposed to those that an external diagnostic tool has it do. Seek performance is how quickly the drive moves from track to track.

220 Disk Shift Low This is how far the disk has moved relative to the spindle (this kind of shift is usually due to shock).

221 G-Sense Error Rate

Low This is the number of errors that have resulted from external vibration and shock.

222 Loaded Hours This is how long the hard drive has operated under data load (this requires movement of the magnetic head armature).

223 Load/Unload

Retry Count This is how many time the magnetic head has changed position.

224 Load Friction Low This is resistance caused by friction in mechanical parts during operation.

225 Load/Unload Cycle Count

Low This is the total number of load cycles.

226 Load 'In'-time This is the total time that the magnetic heads actuator has had a load (not been in the parking area).

227 Torque Amplification Count

Low This is the number of attempts to compensate for variations in platter speed.

228 Power-Off

Retract Cycle Low This is how many times the magnetic armature was automatically retracted because the power was cut.

230 GMR Head

Amplitude This is the amplitude of thrashing (or the distance of repetitive forward and reverse head motion).

231 Temperature Low This is the hard drive’s temperature.

240 Head Flying Hours

This is the total time that the head has been positioning.

250 Read Error Retry

Rate Low This is the number of errors in reading from the disk.

Table 20 S.M.A.R.T. Attributes (continued)

ID ATTRIBUTE

NAME BETTER DESCRIPTION

NAS540 User’s Guide

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• Total capacity: Sum of the member disks

• Advantages: Maximum storage capacity, especially for disks of mixed sizes. Flexibility (you can add disks to the JBOD

• Disadvantages: Not as fast or reliable as RAID.

JBOD allows you to combine multiple physical disk drives into a single virtual one, so they appear as a single large disk. JBOD can be used to turn several different-sized drives into one big drive. For example, JBOD could convert 100 GB, 200 GB, 250 GB, and 500 GB drives into one large logical drive of 1050 GB. Since data isn’t striped across disks, if one disk fails, you should just lose the data on that disk (but you may lose data in the whole array depending on the nature of the disk failure). You can add disks to the JBOD array later (using the Add disk to JBOD feature) and even remove them so JBOD offers a lot of flexibility. However JBOD read performance is not as good as RAID as only one disk can be read at a time and they must be read sequentially. The following figure shows three disks in a single JBOD array. Data is not written across disks but written sequentially to each disk until it’s full.

RAID 0

RAID 0 spreads data across two or more disks (data striping) with no mirroring nor parity for data redundancy, so if one disk fails the entire array will be lost. The major benefit of RAID 0 is

performance. The following figure shows two disks in a single RAID 0 array. Data can be written and read across disks simultaneously for faster performance.

RAID 0 capacity is the size of the sum of the capacities of the disks in the RAID 0. For example, if you have four disks of sizes 1 TB, 2 TB, 3 TB and 2 TB respectively in one RAID 0 array, then the maximum capacity is 8 TB.

Typical applications for RAID 0 are non-critical data (or data that changes infrequently and is backed up regularly) requiring high write speed such as audio, video, graphics, games and so on.

Table 21 JBOD

RAID 1

RAID 1 creates an exact copy (or mirror) of a set of data on another disk. This is useful when data backup is more important than data capacity. The following figure shows two disks in a single RAID 1 array with mirrored data. Data is duplicated across two disks, so if one disk fails, there is still a copy of the data.

As RAID 1 uses mirroring and duplexing, a RAID 1 array needs an even number of disks (two or four for the NAS).

RAID 1 capacity is limited to the size of the smallest disk in the RAID array. For example, if you have two disks of sizes 150 GB and 200 GB respectively in one RAID 1 array, then the maximum capacity is 150 GB and the remaining space (50 GB) is unused.

Typical applications for RAID 1 are those requiring high fault tolerance without need of large amounts of storage capacity or top performance, for example, accounting and financial data, small database systems, and enterprise servers.

RAID 6

RAID 6 can tolerate two simultaneous drive failures by calculating dual distributed parity data on striped data across disks. Dual parity provides extra data protection, however, it is slower to write than most other RAID levels.

RAID 6 uses parity to store redundant data on space equal to the size of two disks for later data recovery. Therefore, on a RAID 6 array, only 50% of the space is available as usable capacity. If you have four disks of sizes 1TB, 1TB, 2TB, 2TB respectively in one RAID 6 array, then the maximum Table 23 RAID 1

NAS540 User’s Guide

70

capacity of the array is the capacity of the smallest drive (1TB, 1TB, 2TB, 2TB) * (Number of disks - 2) = 1TB * (4-2) = 2TB. The remaining space (2 TB) is unused.

RAID 10

RAID 10 (RAID 1+0) is a nested RAID where two RAID 1 arrays are stored on the physical disks with a RAID 0 array on top. It is a stripe of mirrors. RAID 1 provides redundancy while RAID 0 boosts performance. The following figure shows two disks in two RAID 1 arrays. Data is duplicated across two disks, so if one disk fails, there is still a copy of the data. These two arrays are

configured as a single RAID 0 array for faster performance

.

Typical applications for RAID 10 are those requiring both high performance and reliability such as enterprise servers and high-end moderate-sized database systems. RAID 10 is often used in place of RAID 1 or RAID 5 by those requiring higher performance. It may be used instead of RAID 1 for applications requiring more capacity.

RAID 5

RAID 5 provides the best balance of capacity and performance while providing data redundancy. It provides redundancy by striping data across three disks and keeps the parity information (AP) on the fourth disk (in each stripe). In case of disk failure, data can be recovered from the surviving disks using the parity information. When you replace the failed disk, the reconstructed data is written onto the new disk. Re-synchronize the array to have it return to its original state. The Table 24 RAID 6

following example shows data stripped across three disks (A1 to A3 in the first strip for example) with parity information (AP) on the fourth disk

.

The capacity of a RAID 5 array is the smallest disk in the RAID set multiplied by one less than the number of disks in the RAID set. For example, if you have four disks of sizes 150 GB, 150 GB, 200 GB and 250 GB respectively in one RAID 5 array, then the maximum capacity is 450 GB (3 * 150 GB, the smallest disk size) and the remaining space (300 GB) is unused.

Typical applications for RAID 10 are transaction processing, relational database applications, enterprise resource planning and other business systems. For write-intensive applications, RAID 1 or RAID 1+0 are probably better choices, as the performance of RAID 5 will begin to substantially decrease in a write-heavy environment.