3.5" drive slots
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.
Pre-Installed storage
The total capacity of all drives supplied with the NAS server (see "Preinstalled Drives"). In some situations, it is more convenient to use a ready-made solution than to independently equip
an “empty” server with disks; Knowing the volume of pre-installed drives, you can evaluate how this or that model is suitable for your purposes.
Note that the specifications usually indicate the usual amount of disk volumes installed on the NAS server. At the same time, some RAID levels significantly reduce the available volume; see "RAID support" for details.
RAID
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.USB 3.2 gen1
The number of
USB 3.2 gen1 ports provided in the design of the NAS server.
USB connectors are used in computer technology to connect various external peripherals. In the case of NAS servers, we are most often talking about external drives — flash drives, hard drives, etc. In this way, you can transfer information from an internal drive to an external one (for example, for backup purposes) or vice versa, and even expand the total working volume of the server . In addition, on models with a VGA output (see below), a keyboard can also be connected to USB, and on models with a print server function (see "Software Features"), respectively, a printer. For added convenience, the USB connector can be placed on the front panel (see below).
Specifically, USB 3.2 gen1 (formerly known as USB 3.0 and USB 3.1 gen1) is the direct successor to USB 2.0 and is the most common USB standard today. This version provides data transfer rates up to 4.8 Gbps, as well as a fairly high power supply. At the same time, such connectors are backward compatible with peripherals using USB 2.0.
