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The number of slots for drives in the form factor 3.5", provided in the design of the server.

Initially, 3.5 "is the traditional, most popular form factor of drives for server systems. It is noticeably larger than 2.5", but it allows you to create capacious, inexpensive (in terms of gigabytes) and reliable media, in which it is also easier to implement various additional functions. That is why, specifically in NAS servers, this form factor is also the most popular; slots under 2.5" are much less common in such equipment, and in most cases they complement 3.5".

As for the number of slots, it can vary from 2 (or even 1) in the most basic desktop systems to 8 or more in professional rack-mount solutions. And not only their maximum capacity depends on the specific number of drives, but also some other features of work — first of all, the physical possibility of using one or another RAID level.

NAS server supports RAID technology. The term is an abbreviation for "redundant array of independent disks", that is, "redundant array of independent disks". Accordingly, only models with more than one drive slot can have this feature (see “Drive Slots”).

There are several options for combining disks into a redundant array, they differ in a number of characteristics: some focus on increasing speed, others focus on fault tolerance. However, all RAIDs have two key differences from non-arrayed systems. The first is that the RAID array is perceived by the system as one single drive. The second is “redundancy”: the total volume of disks included in the array must be greater than the volume of data that is planned to be stored on them. This is due to the fact that the array uses service information, which must be stored on the same disks (however, the exception is RAID 0, see below).

The most common RAID versions today are:

— RAID 0. An array of two or more disks, information on which is written by interleaving: first, the data is divided into blocks of the same length, and then each of these blocks is written to its “own” disk in turn. For example, if a RAID 0 array consists of 3 disks, and the file is divided into 7 parts, then parts 1, 4 and 7 will be on the first disk, 2 and 5 on the second, and 3 and 6 on the third. that it is not actually a RAID, as it devoid of "redundancy" — the volume of the array corresp...onds to the sum of the disk volumes. The main advantage of RAID 0 is a significant increase in performance; it is higher, the more disks are included in the array. On the other hand, the reliability of such systems is lower than that of individual drives: in the event of a failure of any of the drives, the entire array becomes inaccessible, and the more drives are used, the higher the likelihood of this. The minimum number of drives for RAID 0 is two.

— RAID 1. In arrays of this type, information is recorded according to the principle of mirroring: two disks, the information on which is completely identical. This provides a very solid system fault tolerance: the data contained in the array will be available in full, without additional tricks and serious drops in performance, even if one of the disks fails completely. In addition, some gain in read speed is achieved in this way, and "hot swapping" (see above) usually does not cause problems. The disadvantage is the high cost of building: you have to pay for two hard drives, getting the volume of one. However, in some cases this can be quite an acceptable price for increased reliability.

— RAID 5. In such arrays, unlike RAID 0 and 1 (see above), not only basic information is stored on disks, but also service information — in the form of data for error correction (so-called checksums). In this case, both types of information are distributed evenly across all disks. For example, in RAID 5, consisting of 4 disks, the first "portion" of data to be written will be divided equally between disks 1,2 and 3, and the checksum will be written to disk 4; the second portion is between disks 1,2 and 4, with a checksum written to disk 3, etc. This provides good fault tolerance: the array provides data access in the event of a complete failure of any of the drives. In addition, RAID 5 is characterized by a very low level of redundancy: the working volume of the array is equal to the volume of the smallest disk multiplied by (n-1), where n is the total number of disks. The main disadvantages of RAID 5 are relatively low performance, which drops even more in the event of a failure; this is due to the abundance of additional operations associated with the use of checksums. In addition, if one of the drives fails, the reliability of the remaining array is reduced to the RAID 0 level (see above), and the remaining drives experience very significant loads, which further increases the risk of additional failure; if two disks fail, data can be recovered only by special methods. The minimum required number of drives for RAID 5 is three.

— RAID 10. A combination of arrays of the RAID 0 and RAID 1 types (see above): the disks are combined in pairs into mirror RAID 1 arrays, and the whole system operates on the RAID 0 principle, with sequential information writing to each pair of disks. This scheme allows you to maintain the high performance characteristic of the classic RAID 0, while eliminating its main drawback — unreliability. Regardless of the number of drives, a RAID 10 array is completely insensitive to a single drive failure and can easily survive the loss of half the drives if they are all in different mirrored pairs. At the same time, the simultaneous failure of one pair leads to an irreversible loss of information. Another drawback is the high redundancy characteristic of RAID 1: the useful volume of the array is half the sum of the volumes of all disks. At least 4 drives are required to build RAID 10, and anyway, their number must be even.

— JBOD. Abbreviation for "Just a bunch of disks" — "just a bunch of disks." This name, although rough, but quite accurately describes the features of arrays of this type: JBOD does not provide "redundancy", does not use additional technologies such as checksums (see RAID 5), and the volume of the array is equal to the total volume of all disks included in it. The discs are connected in a kind of series. This means that when writing each next file, the remaining free space on the previous disk in the queue is first filled, and if there is not enough space, the rest of the data is written to the next one. For example, if you write two 70 GB files to an empty JBOD array of 100 GB disks, the first file will fit entirely on the first disk, and the second will take up the remaining 30 GB on the first and 40 GB on the second. Similarly, if the volume of the file exceeds the volume of the entire disk — in our example, a 120 GB file will occupy the entire first disk and 20 GB on the second. The advantages of JBOD are good performance with a small load on the processor and the ability to combine disks with different sizes and speeds. In addition, they are somewhat more fault-tolerant than similar RAID 0 in many respects (see above): the failure of one disk does not necessarily lead to the irreversible loss of data of the entire array. At the same time, the reliability of JBODs is still somewhat lower than that of single disks, and therefore they can only be considered as a tool for improving performance.

Note that the variety of RAID standards used in modern NAS servers is not limited to the above. Additional options may include but are not limited to:

— RAID 3 and RAID 4 — similar to RAID 5 described above, however, in these formats, checksums are written to one dedicated disk, and are not distributed evenly across all disks. This improves performance (for RAID 3 — only in some cases), but reduces the reliability of the control disk. For a number of reasons, they are rather uncommon.

— RAID 6 is another analogue of RAID 5, differs in that it uses not one, but two sets of checksums, also evenly distributed over all disks. This significantly increases reliability, but reduces performance and increases the level of redundancy — the volumes of not one, but two disks “fall out” of the total volume.

— RAID 0+1. It can mean 2 options. The most common is an array of two RAID 0 (striped) combined into a RAID 1 (mirror). Some manufacturers use RAID 0+1 as a designation for an advanced technology that allows you to “mirror” information on an odd number of disks: for example, in a three-disk array, the first piece of data will be mirrored on disks 1 and 2, the second — on 2 and 3, the third — on 3 and 1 etc.

— RAID 50 and RAID 60. RAID 5 and RAID 6 arrays, respectively, composed of groups of disks combined in RAID 0. Provide high reliability and performance, but are expensive and difficult to maintain.

There are also other options for "combined" RAID — for example, in RAID 51, two RAID 5 arrays are made into a "mirror" pair.

— Web server. Ability to use the device as a Web server. It is on servers of this type that the Internet in its current form is built: the user's computer sends a request to a web server through a browser and receives a response in the form of a page, picture, video / audio stream, etc. Accordingly, the presence of this function in the NAS allows you to view its contents in the form of web pages using a regular browser — roughly speaking, "walk through the server, like on the Internet." In this case, the device can be used not only as a local resource, but also as a web host — for example, host the company's official website on it.

— FTP server. FTP is an abbreviation for File Transfer Protocol, i.e. file transfer protocol. This feature allows you to use the NAS server as a shared data storage: users can "upload" their own files to storage and download them from there. FTP tools provide ample features for configuring access to server content — for example, you can set restrictions on writing information to individual users or to individual folders, close part of the content with passwords, etc. Due to this, this protocol is much more convenient for working with individual files than the HTTP used in web servers (see above). Therefore, if you plan to create shared storage on the network, it is advisable to have a NAS with FTP server function.

— Print server. The print server feature makes it easy to share the same printer among network users. The printer is connected to the NAS, usually via a USB interface (see above), and the NAS serves as an intermediate link: it receives print jobs from users and sends them to the printer. Additional print server features can include sequencing optimization, local job storage (document will be printed even if you turn off the computer from which the job was sent), deletion of "overdue" jobs, and even accounting for the number of pages and remaining consumables. Using a NAS with a print server function is often more convenient than connecting a printer through one of the regular computers on the network.

— Multimedia (DLNA, iTunes, uPnP).... NAS server supports various functions related to the exchange of multimedia content. For example, DLNA (Digital Living Network Alliance) is a standard, one of the functions of which is the general access of various network devices to video, audio and photos stored on a local network; while streaming is also supported. The iTunes player has multimedia networking features similar to DLNA, but was created specifically for Apple electronics and is used primarily in it. uPnP (Universal Plug and Play) is a technology that facilitates the automatic configuration of local networks, including to share content. A server with multimedia functions should be looked for in the first place if the ability to work with streaming video / audio is important for your local network.

— Transcoding. A function that allows you to convert audio and video materials from one format to another directly during playback. In other words, the file on the NAS server is stored in one format, and it can be sent to an external device in another, the server itself will provide the conversion. It should be taken into account that the set of supported formats and general transcoding capabilities may be different (in particular, the maximum video resolution is inevitably limited); these nuances should be clarified in each case separately. However, this function anyway significantly expands the possibilities for playing multimedia content and reduces the likelihood of compatibility problems.

— BitTorrent client. The presence in the device of its own torrent client or other data exchange protocol (HTTP, FTP, etc.). This feature allows you to work with file-sharing networks, which are built on the principle of "everyone's own server": the downloaded information is not on a separate computer on the network, but on the computers of the same users. At the same time, the same file can be opened for download in several places and the torrent client simultaneously downloads different parts of it from different sources - this significantly increases the speed. Using a torrent client on a device is convenient in two ways. Firstly, it allows you to offload the main computers of users - an important advantage, given that the torrent client can consume a lot of resources, especially with an abundance of simultaneous downloads / distributions. Secondly, network equipment tends to stay on at all times, allowing downloads and uploads to continue even when users' PCs and laptops are turned off. However, it should be taken into account that despite the presence of such functionality in devices, the open placement of content in torrent networks can violate copyrights. Therefore, use torrent clients in compliance with legal regulations.

— Mail server. Ability to operate the NAS in server mode for e-mail processing. On such a server, you can create mailboxes in the format [user]@[company_name].com, it works as a storage for incoming letters and as a forwarding service for outgoing ones. Additional features may include automatic redirects, spam protection, custom filters, and so on. Having this feature is indispensable if you need a corporate email system: your own internal storage is more reliable in terms of security than external email services, and the ability to create unique email addresses can also come in handy.

— Database server. As the name implies, this function is useful for creating databases — systematized arrays of information designed to be accessed and processed from a computer. It is usually implemented through support for the SQL language. Its peculiarity lies in the fact that the network user does not need to know the specific location of the information in the database — it is enough to issue a request, what kind of data needs to be obtained, and the server itself searches for it. This is very convenient when working with large volumes of information, while high power is not required from user computers — the main load falls on the server.

— Surveillance server. A set of software tools that allow you to use the NAS as a storage for video recordings from surveillance cameras. Features of video storage may be different. For example, in some servers, a certain part of the working volume of drives can be allocated for this, and when it overflows, the oldest records are automatically deleted, freeing up space; in others, deletion is carried out not by volume, but by date — for example, materials are stored for a month, then deleted. Both the volume and the shelf life, usually, can be set by the user himself. And some models with a VGA output (see above) can also be used as live surveillance systems — the image from the cameras is displayed on the monitor in real time, which can be useful, for example, for organizing security. The specific features of the operation of the NAS in the video server mode may vary from model to model, this point is best specified according to the manufacturer's official data.

— Backup. The backup function is designed to create a backup copy of data (so-called backup) in case of loss or damage to information on the primary media. Backup can be done to the built-in or external drive, and even to another device over the network. To facilitate this task, many developers create various specialized software tools; in this case, it is assumed that the NAS server supports one of these tools. Also, software capabilities can be supplemented by hardware ones — for example, a separate quick copy button.

– ZFS file system. An advanced file system that uses a transactional copy-on-write model. Active data is never overwritten - ZFS places the new block in a different location on disk and updates the metadata, which allows you to write a link to the new block of information and save older versions of the data. The key features of ZFS are snapshots (immutable copies of the file system made on the fly), advanced compression algorithms, and a built-in deduplication function. ZFS is also one of the most advanced file systems in terms of security.

— DDNS. Short for Dynamic DNS — "dynamic DNS". This feature allows you to assign a permanent domain name to a device with a dynamic IP address. A domain name is the name of a device on the local network or the address of a site on the Internet (for example, m.ua or e-katalog.ru). An IP address is service information in the form of a digital code; it is thanks to her that network equipment can find the desired device and issue the required data from it. Actually, IP is the primary network "coordinates"; however, remembering addresses as a sequence of numbers is quite difficult, so domain names appeared — they are much more convenient for a person. Both on the Internet and in local networks, the connection between a domain name and an IP address is responsible for the so-called DNS servers: for each domain in the database of such a server, its own IP is registered. However, for technical reasons, there are often situations where the NAS server has to use a dynamic (changing) IP; accordingly, in order for information to be constantly available on the same domain name, it is necessary to update the data on the DNS server with each IP change. It is this update that the DDNS function provides.

— Integration with. A software tool that makes it easy to integrate a NAS into an existing domain (computer network area). Each domain has a so-called a controller is a server that stores information about users, primarily logins, passwords, and access rights. When connecting a NAS with the integration function, all these settings can be automatically imported, so that all users have the same access rights to the contents of the NAS as to all contents of the domain. This saves the administrator the hassle of creating and configuring separate accounts (which can be quite a hassle on large networks).

— Airplay. NAS server support for AirPlay technology. This is an Apple proprietary development, originally created for wireless broadcasting of audio and video content from Apple technology to TVs, audio systems and other playback devices; however, nowadays, the role of the transmitter can also be performed by electronics from other manufacturers. This is what we are talking about in this case: AirPlay support allows you to broadcast video and/or audio content stored on the server from the NAS server to external devices. To do this, the NAS and the AirPlay-compatible signal receiver must be on the same network, and the signal receiver must be connected via Wi-Fi. Such a broadcast is usually controlled either through a browser on a computer, or through a proprietary application on a mobile device that plays the role of a remote control. Also note that, in addition to the original AirPlay, compatibility with AirPlay 2 can also be provided — this is an improved version of this technology, which introduced, in particular, the ability to work in the multiroom format (simultaneous broadcast of different audio tracks to different devices within the network).

— Chromecast. NAS server support for Chromecast technology. This technology, developed by Google, is in many ways similar to the AirPlay described above: it is intended primarily for broadcasting audio and video to playback devices wirelessly. Accordingly, the use of Chromecast is almost the same: a NAS server with this function can broadcast the content stored on it to a TV, projector, audio system, or other compatible playback device connected via Wi-Fi to the same local network. Management is also most often carried out through a web interface or using a mobile application.

The operating system (OS) installed on the NAS server as standard. The OS is the software basis for the functioning of any computer; it is impossible to use the machine without it. Accordingly, when purchasing a server with a pre-installed OS, you get a practically ready-to-use device — additional steps, in fact, come down to fine-tuning the system and installing (if necessary) additional software.

Various specialized applications are available for different operating systems to facilitate the use of NAS server functions; some of them (see "Programme Features") may also be preinstalled. Accordingly, knowing the name of the OS, it is possible, to a certain extent, to determine the tools available for working with the device.

Note that some operating systems are paid, and their cost is included in the price of the NAS.

The amount of RAM on the NAS server. Along with the processor, it is one of the indicators that determine the speed of the system — the more memory, the higher the computing power. However, in fact, it does not always make sense to chase large amounts of "RAM", which can reach 4 GB, 8 GB and even higher; see "Processor" for details.

The amount of power consumed by the NAS server during normal operation. Most often, we are talking about maximum power consumption — with all the occupied slots for drives, under high load.

Modern NAS, even high-performance ones, have rather modest power consumption — even among professional models with 10 or more drives, this figure rarely exceeds 1 kW. So there are no problems with connecting to a 230 V network. However, energy consumption information can be useful for some special applications, primarily for estimating the load on UPSs, emergency generators, stabilizers, and other special equipment.