"Virtual machine Storage" storage media comparison and selection

SSD, SAS, SATA

Real-time SSD is best; SAS io is faster; SATA is the cheapest and should be used as a storage server (low frequency of access)

The following major sectors are excerpted from the network:

MLC SSD and SLC SSD

SSD (Solid state disk or Solid state Drive), also known as an electronic hard disk or solid-state electronic disk, is a hard disk consisting of a control unit and a solid-state storage unit (DRAM or Flash chip).

Solid-State hard disk interface specifications and definitions, functions and use of the same as the ordinary hard disk, in the product shape and size is also consistent with the common hard drive. Because the solid state hard drive does not have the common hard disk the rotating medium, therefore the earthquake resistance is extremely good. The operating temperature range of the chip is very wide (-40~85摄氏度). Currently widely used in military, automotive, industrial control, video surveillance, network monitoring, network terminals, power, medical, aviation and other fields such as navigation equipment.

In a single unit (SLC) flash device, a transistor can hold 1-bit data. You write data to a cell through an electronic channel, add enough voltage to the ends, create a strong enough electric field, and the electrons penetrate the oxide into the floating door. To eliminate the voltage, the electrons will continue to stay at the floating door. By passing the voltage through the channel without applying to the door, reversing the bias, the electrons will move in the other direction.

Simply put, this is how the Flash disk works-You have two states, 0 and 1, which are ideal for storage devices, even if the unit is out of power and the state is maintained.

Programming Flash memory is a recurring process. The controller applies the voltage to the door (or channel), allowing some electrons to pass through and check the critical voltage of the unit. When the critical voltage has reached some predetermined value, it begins to program and store the data.

1 The difference between MLC and SLC

currently SSD drives use two forms of NAND flash: Single-stage Unit (SLC) and multistage unit (MLC). The difference between the two is the amount of data stored per unit, and SLC stores 1 bits per unit and MLC 2 bits per unit. The key is that SLC and MLC occupy the same size of chip area. Therefore, at the same price, MLC can have a capacity of twice times the effect.

Stec's SLC and MLC Flash memory actually use the same transistor, except for how to read/write the two flash memory. SLC has only two voltage characters because it has only two states (0 or 1). MLC has four states (00,01,10 ,11), so it takes a long time to access, because you don't want to accidentally write the wrong data; you already have the same maximum, minimum voltage, and you now have only more ticks between them:  

&NBSP
 

  slc  (left)  vs. MLC  (right)

Here is a table of basic data on the performance of SLC and MLC:



The erasure performance of the SLC and MLC is the same, and the read performance of the MLC Flash takes twice times longer, and the write performance takes four times times the length of time. If you've ever heard of someone complaining about MLC write speed, that's part of the reason. But keep in mind that the numbers we're talking about here are ridiculously low--even 900 microseconds write MLC Flash faster than you can write to a normal mechanical hard drive.

The biggest advantage of SLC is not that its performance is good but its long service life. To understand the durability of flash memory, we first need to look at the internal structure of the storage device.
Flash level and data loss

We have confirmed that a flash deposit can be stored in one or two bits depending on whether it is an SLC or a MLC device. Bring together a group of units, you get a page. page is the smallest structure you can program (write) NAND flash devices.

Most of the MLC NAND Flash is 4KB per page. A block is made up of many pages, and a block in the Stec MLC SSD contains 128 page (128 pages x 4kb/page = 512kb/block = 0.5MB. Block is the smallest structure that you can erase.

2 Sas,sata

SAS (serial Attached SCSI), serial attached SCSI, is the next generation of SCSI technology, and is the same as the current serial ATA (SATA) hard drive, using serial technology to achieve higher transmission speed and improve internal space by shortening the link line. SAS is a new interface developed after the parallel SCSI interface. This interface is designed to improve the performance, availability, and extensibility of the storage system and to provide compatibility with SATA hard drives.

SAS interface technology can be backward-compatible with SATA. Specifically, the compatibility of the two is mainly embodied in the physical layer and the protocol layer compatibility. In the physical layer, the SAS interface and SATA interface are fully compatible, SATA hard disk can be directly used in the SAS environment, from the interface standard, SATA is a child standard SAS, so the SAS controller can directly control the SATA hard drive, but SAS is not directly used in the SATA environment, Because the SATA controller does not control the SAS hard drive, at the protocol layer, SAS consists of 3 types of protocols for data transmission based on the different devices connected. Where the serial SCSI Protocol (SSP) is used to transmit SCSI commands, SCSI Management Protocol (SMP) is used for the maintenance and management of connection devices, and the SATA Channel Protocol (STP) is used for data transmission between SAS and SATA. So with these 3 protocols, SAS can be seamlessly integrated with SATA and some SCSI devices.

Comparison

SATA and SAS are the same as the use of universal serial technology, the latter can be backward-compatible, the former is mainly used for PCs, the latter is mainly used for servers, the latter price is higher;