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The difference between SATA and SAS and the use of SSDs

This article provides an overview of the differences between the two interfaces SATA and SAS. It also offers information on SSDs. Moreover, it deals with the question why the use of SSDs leads to an increase in performance in BMD NTCS.

When it comes to running BMD NTCS, one of the most important factors is IOPS performance (input/output operations per second). How much traffic/how many operations can a medium (e. g. a hard disk or an SSD) process per second? However, we should bear in mind that there are other vital factors for good performance than data throughput (bandwidth).
 

1. Bandwidth does not equal performance

MB/s and GB/s are important for:

  • Backbone networks 
  • Data streaming 
  • Disk-to-disk backup

 

IO/s are important for:

  • Transaction-oriented applications
  • Databases (e. g. SQL and Oracle) → BMD NTCS!
  • Data analysis

By now there are various interfaces which can be used to connect hard disks and SSDs. In this article we focus on SATA and SAS for HDDs. For SSDs interfaces NVMe (especially for servers) and M.2 (for consumers) can also be used.

Before addressing the differences of SATA and SAS interfaces, let us have a look at a speed comparison between an HDD and an SSD. For the use of database-oriented applications like BMD NTCS, critical factors include IOPS and latency of the respective media. Already, the advantage of an SSD over an HDD is readily apparent.
 

Test scenario

Consumer HDD

Consumer SSD

4k Ran. Read IOPS

~ 184 IOPS

~ 65.000 IOPS

4K Ran. Write IOPS

~ 102 IOPS

~ 94.000 IOPS

4K Write Latency (Avg)

~ 4.8ms

~ 0.45ms

 

2. Myth

 

SATA

SAS

Bandwidth

up to 6 Gb/s

up to 12 Gb/s

Capacity

up to 8 TB

up to 1.8 TB

Revolutions per minute

up to 7,200

up to 15,000

Average access time

7.6 – 12.0 ms

2.9 – 4.0 ms

Duplex mode

no

yes

Dual porting

no

yes

 

As for operating databases, only random I/O performance is relevant. When using one single SAS hard disk drive, random I/O performance is up to four times higher than when using a SATA drive (access time)! Moreover, a SAS hard disk drive can perform read and write operations at the same time (duplex mode)! 

 

In addition, hard disk performance scales with the number of disks. In case of hard disk overload, speed often drops massively, even though processor, main memory and network are underutilized.

 

The number of disks is therefore crucial!
 

3. Optimization strategies for the disk subsystem

  • SAS instead of SATA 
  • RAID controller instead of onboard controller 
  • Efficient RAID levels and the “right” number of hard disks 
  • Using the right strip size (e. g. 64 kB instead of 16 kB for RAID5) 
  • Write-back cache (BBWC/FBWC) 
    • Very good for random I/O data 
    • Neutral for sequential data (good for short bursts)
  • More hard disks 
  • Hard disks with 15k rpm 
  • Using 2.5” disks - smaller disks have shorter access times 
  • Solid-state drives (SSDs) 
    • can be used as a substitute for BBWC/FBWC
    • thus more cache memory
    • considerably higher random read performance
  • separating sequential and random I/O data to different RAID controllers and RAID levels
    • e.g. distributing operating system, log files and databases to separate controllers and separate RAIDs

 

4. Overview of the most common RAID levels

4.1. RAID 0 - Striping

Capacity

Number of disks times capacity of the smallest single disk

Speed

Very high, excellent write and read performance

Probability of failure

Very high, failure of one disk causes loss of all data

Costs

Very low, no capacity loss

Minimum number of disks

2

Application

Video editing, temporary storage

 

4.2. RAID 1 – Mirroring

Capacity

Half the number of disks times capacity of the smallest single disk

Speed

High, read performance very good, write performance same as single disk

Probability of failure

Low, failure of one disk does not cause data loss

Costs

High, use of capacity reduced by 50%

Minimum number of disks

2

Application

Operating system, database, high I/O load

 

4.3. RAID 10 – Striped mirror

Capacity

Half the number of disks times capacity of the smallest single disk

Speed

Very high, read performance very good, write performance good

Probability of failure

Low, failure of one disk does not cause data loss (security is 1 + x)

Costs

High, use of capacity reduced by 50%

Minimum number of disks

4

Application

Databases, high I/O load

 

4.4. RAID 5 – Distributed data guarding

Capacity

Number of disks minus 1 disk times capacity of the smallest single disk

Speed

Average, read performance good, write performance moderate

Probability of failure

Low, failure of one disk does not cause data loss

Costs

Low, loss of only one disk

Minimum number of disks

3

Application

File server, test system, archiving, backup to disk

 

4.5. RAID 50 – Striped RAID 5

Capacity

Number of disks minus 2 disks times capacity of the smallest single disk

Speed

High but slower than RAID 10

Probability of failure

Low, failure of one disk does not cause data loss (security 1 + x)

Costs

Average, loss of two disks

Minimum number of disks

6

Application

File server, database, high I/O load

 

4.6. RAID 6 – Advanced data guarding

Capacity

Number of disks minus 2 disks times capacity of the smallest single disk

Speed

Low, read performance good (~ RAID 5), write performance poor

Probability of failure

Very low, failure of two disks does not cause data loss

Costs

Average, loss of two disks

Minimum number of disks

4

Application

File server, test system, archiving, backup to disk

 

 

5. SSD (solid-state disk)

Since SSDs are now available on both consumer and enterprise markets, let us have a look at the different features of this technology.

 

Almost all data that are processed in BMD NTCS are located in a database and, as previously mentioned, one of the most important factors for databases is I/O performance. Therefore, our application benefits from the use of SSDs. What do you have to consider when using SSDs?

 

5.1. Key components of an SSD

There are differences in the architecture of SSDs and regular hard disks:

  • As opposed to regular hard disks, SSDs have no moving parts but use flash memory.
  • The SSD has a controller that is responsible for “data maintenance” and thus organizing and managing data. 
  • Another component of an SSD is the interface, which is used to communicate with RAM, CPU etc. Just as for regular hard disk drives, we distinguish between SATA (common for PC or laptop) and SAS (for servers). By now, it is also possible to use PCI-E interfaces (NVMe) or M.2 for (consumer) SSDs.

 

5.2. Advantages/disadvantages of SSDs compared with hard disks

Advantages:

  • Better performance
  • Latency time (access time) is very low
  • Energy efficient
  • Soundless

Disadvantages:

  • Higher costs for large capacities and enterprise SSDs
  • Shorter lifespan

 

6. Consumer SSDs vs. enterprise SSDs

As is the case with hard disks, there are also differences between SSD models affecting mostly performance, lifespan, price and the area of application.

6.1. Memory used

The memory that is used in an SSD has a particularly significant impact on price and endurance of the SSD. Contrary to hard disks, SSD memory can only be written to a certain amount of times before you cannot use the SSD anymore. SSDs that are used for normal (consumer) operation already have a lifespan of several years. This is mostly dependent on how much the SSD is used, though. Endurance is often specified using DWPD (Disk Writes Per Day) or TBW (Terabytes Written). As for SSD memory, the term “write cycles” is fairly common. 

 

Below, you will find a list of the most common memory options that are used in SSDs depending on their respective area of application (PC, server, etc.).

 

TLC

Area of application

Consumer, mostly PCs, laptops, etc.

Advantages/Disadvantages

Low price, short lifespan compared to eMLC and SLC, lower speed than MLC, SLC, etc.

Write cycles

~ 1000

 

MLC

Area of application

Consumer, mostly PCs, laptops, etc.

Advantages/Disadvantages

More write cycles than TLC, often seen as a “pro” version of consumer SSDs

Write cycles

~ 3000

 

eMLC

Application

Enterprise, used for servers, workstations 

Advantages/Disadvantages

Fast memory, higher lifespan than MLC or TLC

Write cycles

~ 20,000-30,000

 

SLC

Application

Enterprise, mostly used for servers

Advantages/Disadvantages

Very fast memory, very high price

Write cycles

~ 100,000

 

6.2. Performance

The most important factor for us is the difference in performance between SSD and regular hard disk. In the brief comparison between SSD and HDD performance below, two consumer products are compared. If we have a look at the values, we get an idea of the performance benefits that an SSD can offer. (Since it is not our objective to make recommendations for hard disks or SSDs, we do not name manufacturers. The test results have been provided by an external website.) 
 

Test scenario

Consumer HDD

Consumer SSD

4k Ran. Read IOPS

~ 184 IOPS

~ 65,000 IOPS

4K Ran. Write IOPS

~ 102 IOPS

~ 94,000 IOPS

4K Write Latency (Avg)

~ 4.8ms

~ 0.45ms

 

Another test (different setting) compares the performance of hard disks (SAS) and the use of SSDs. Comparison: 14 x 10krpm SAS HDD (RAID5) vs. 6x SSD (SAS) RAID5. 
 

Test scenario

14x 10krpm HDD (SAS) RAID5

6x SSD (SAS) RAID5

8k Ran. Read IOPS

~ 5,100 IOPS

~ 118,000 IOPS

8K Ran. Write IOPS

~ 1,500 IOPS

~ 31,000 IOPS

8K Write Latency (Avg)

~ 44.00ms

~ 2.06ms

 

7. Conclusion

In view of the reasons and results stated above, we can conclude that the use of or the change to SSDs can lead to an increase in performance when it comes to BMD NTCS, whether as a standalone or as a server installation. 
 

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