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Servers Hard Drive Buying Guide

Types of Hard Drives

SSD– very fast drives but very expensive in servers, mainly used for OS drives. Flash based technology

HDD – Slower than SSD, writes to disk drives, used for OS and Data drives - Most commonly used type of drive

Flash Storage (SD, Micro SD, USB drives)

Connection Types (applies to both SSD and HDD)

IDE/PATA - Legacy connection type, uses 40 pin connector via a serial style cable - No longer used in the industry

SCSI - Legacy (to an extent replaced by Serial Attached SCSI (SAS))

SAS – fast and reliable drives; these are used for OS and DATA drives, 10k and 15k speeds are available. These should be used for primary storage where the data is accessed a lot.

SATA – cheaper cost per GB and larger capacities in 3.5" versions. Used mainly for secondary storage where data is accessed occasionally.

PCIe – Not commonly used, faster than SAS/SATA is starting to be popular connection type for SSD drives and offers 12GB throughput.

Hot Swap vs Non Hot Swap

Any disk drive can be made 'hot swappable' it is the PBC/disk controller on the SAN/Server and the caddy the drive is put in that makes drives hot swappable, drives can only be hot swap when RAID is configured so even if a server states hot swap this may not be the case depending on the disk configuration.

In the entry level servers the drives are non-hot swap, this means if there is a hard drive failure to swap the drive you need to power the system down to swap the drive then reboot. Hot swap drives allow you to remove a hard drive when the server is still running, so if you get a drive failure you can replace this with the server still running so there will be no downtime. Also if you need to add more drives to the server you can do this without powering off the server.

SFF (2.5") Drives vs LFF (3.5") drives

SFF drives are in the mid-high end servers and tend to be SAS drives, there are more capacity options and are available in 10k or 15k spin speeds, capacity ranges from 146GB to 1.8TB SAS drives and 1TB-2TB in SATA (2.5" SATA drives are more expensive than 3.5"). Normally if the server takes 2.5? drives you can get more drives in the server.

LFF drives tend to be SATA as you can get larger capacity drives, these are useful when a customer has larger storage requirements and doesn?t need the speed of the SAS drives. You can get SAS drives in LFF but there aren?t as many options – 300GB, 450GB and 600GB 15k (these tend to be more expensive than the SFF versions).

6G vs 12G drives

The Gb/s stands for Gigabits per second. This is the throughput speed the SAS 3.0, 2.0 and 1.0 specs have. 12G = 4800MB/s, 6G = 2400MB/s and 3G = 1200MB/s. These speeds are theoretical and real world results are often less depending on which controller card, sas expander, drives and raid level are used. 12G SAS controller cards came out recently and they provide huge performance gains over 6G products.

See the table below for an example of the speed improvements: -

- 6G Seek Time 12G Seek Time
Single Track - ms 0.3 0.1
Average - ms 4.6 3.7
Full-Stroke - ms 8.9 7.3

Seek time measures the time it takes the head assembly on the actuator arm to travel to the track of the disk where the data will be read or written.

When the drive needs to read or write a certain sector it determines in which track the sector is located. It then uses the actuator to move the head to that particular track. If the initial location of the head was the desired track then the seek time would be zero. If the initial track was the outermost edge of the media and the desired track was at the innermost edge then the seek time would be the maximum for that drive. Seek times are not linear compared with the seek distance travelled because of factors of acceleration and deceleration of the actuator arm.

Single Track is the time taken by the head positioning servomechanism to move the read/write track to the adjacent track.

Average Seek Time is an average of seek times for all possible seek lengths

Full-Stroke is the time required to move from the outermost track to the innermost track. This is the longest (slowest) possible seek time.

Rotational Latency

Rotational latency (sometimes called rotational delay or just latency) is the delay waiting for the rotation of the disk to bring the required disk sector under the read-write head

HDD Spindle [rpm]Average Rotational Latency (ms)
4,2007.14
5,4005.56
7,2004.17
10,0003.00
15,0002.00

SSD VS HDD

- Hard Disk Drive Solid State Drive
Speed HDD has higher latency, longer read/write times, and supports fewer IOPs (input output operations per second) compared to SSD. SSD has lower latency, faster read/writes, and supports more IOPs (input output operations per second) compared to HDD.
Heat, Electricity, Noise Hard disk drives use more electricity to rotate the platters, generating heat and noise. Since no such rotation is needed in solid state drives, they use less power and do not generate heat or noise.
Fragmentation The performance of HDD drives worsens due to fragmentation; therefore, they need to be periodically defragmented. SSD drive performance is not impacted by fragmentation. So defragmentation is not necessary.
Components HDD contains moving parts - a motor-driven spindle that holds one or more flat circular disks (called platters) coated with a thin layer of magnetic material. Read-and-write heads are positioned on top of the disks; all this is encased in a metal case SSD has no moving parts; it is essentially a memory chip. It is interconnected, integrated circuits (ICs) with an interface connector. There are three basic components - controller, cache and capacitor.
Weight HDDs are heavier than SSD drives. SSD drives are lighter than HDD drives because they do not have the rotating disks, spindle and motor.
Dealing with vibration The moving parts of HDDs make them susceptible to crashes and damage due to vibration. SSD drives can withstand vibration up to 2000Hz, which is much more than HDD.
  • The cost of SSD is a lot higher than SAS/SATA
  • The sustainable I/O performance of flash can be 40 to 60 times greater than that that of SAS and SATA
  • SSDs have limited read/write limits SAS/SATA drives don't.
  • Capacity is a lot higher on SATA/SAS than SSD
  • Cost per GB is higher on SSD
  • Typical SSDs will have a seek time between 0.08 and 0.16 ms
  • IOP performance increases with SSD – see below

SSD 300GB x 4

- Capacity IOPS
RAID 10 536 GB 30,769
RAID 5 804 GB 21,053

SAS 300GB 10K x 4

- Capacity IOPS
RAID 10 536 GB 385
RAID 5 804 GB 263

SAS 300GB 15k x 4

- Capacity IOPS
RAID 10 536 GB 600
RAID 5 804 GB 411

IOP (Input / Output Operations Per Second)

IOPs is a common performance measurement used to benchmark computer storage devices like hard disk drives (HDD), solid state drives (SSD), and storage area networks (SAN). As with any benchmark, IOPS numbers published by storage device manufacturers do not guarantee real-world application performance.

IOPS can be measured with applications and is primarily used with servers to find the best storage configuration.



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