硬碟存儲文件

❶ 硬碟是怎麼來存儲數據的

硬碟不是直接存儲我們現在人看到的數據，計算機中，通過2進制，將數據轉化為可以用2進製表示的數字數據，再對應機器的高電平低電平等可以用兩種機器物理狀態的狀態。

硬碟儲存數據的原理和盒式磁帶類似，只不過盒式磁帶上存儲是模擬格式的音樂，而硬碟上存儲的是數字格式的數據。寫入時，磁頭線圈上加電，在周圍產生磁場，磁化其下的磁性材料；電流的方向不同，所以磁場的方向也不同，可以表示 0 和 1 的區別。

讀取時，磁頭線圈切割磁場線產生感應電流，磁性材料的磁場方向不同，所以產生的感應電流方向也不同。

(1)硬碟存儲文件擴展閱讀

硬碟使用注意事項：

1、在工作時不能突然關機。

硬碟當硬碟開始工作時，一般都處於高速旋轉之中，如果我們中途突然關閉電源，可能會導致磁頭與碟片猛烈磨擦而損壞硬碟，因此要避免突然關機。關機時一定要注意麵板上的硬碟指示燈是否還在閃爍，只有在其指示燈停止閃爍、硬碟讀寫結束後方可關閉計算機的電源開關。

2、防止灰塵進入。

灰塵對硬碟的損害是非常大的，這是因為在灰塵嚴重的環境下，硬碟很容易吸引空氣中的灰塵顆粒，使其長期積累在硬碟的內部電路元器件上，會影響電子元器件的熱量散發，使得電路元器件的溫度上升，產生漏電或燒壞元件。

3、要防止溫度過高或過低。

溫度對硬碟的壽命也是有影響的。硬碟工作時會產生一定熱量，使用中存在散熱問題。溫度以20～25℃為宜，過高或過低都會使晶體振盪器的時鍾主頻發生改變。溫度還會造成硬碟電路元器件失靈，磁介質也會因熱脹效應而造成記錄錯誤。

❷ 硬碟是如何存儲和讀取零碎的文件的

如果數據量不是很大（G級別以下），文件不是特別零碎，可以直接存在硬碟上。
但是如果數據量已經/可能超過T級別，或者文件小且零碎，建議還是放在HDFS等分布式文件系統上。
我存儲爬蟲的html以及圖片數據，是通過HDFS的MapFile格式存儲的。MapFile是個已排序的鍵值對文件格式，我的鍵採用的是url的hash+採集時間，值就是文件內容。並且封裝了原生的MapFile.Reader實現了讀取和一定程度的緩存（目前只用了LRU）。
在HDFS提倡一次寫入，多次讀取的前提下，文件的更新只能是通過失效舊，使用新的策略。即把舊的元數據標記為失效，插入新的元數據，並把更新的文件寫入HDFS。讀取是通過新的元數據定位到文件。同時，要定期的清除已失效的文件，即把未失效的元數據讀出來，將對應的文件寫到新的MapFile，刪除舊的MapFile，即可實現物理刪除。
當然還可以使用HBase。HBase是面向列的，二進制存儲的，可橫向拓展的NoSQL。可以把不大於64M的數據作為單元格數據直接寫進去。但是有一定的學習成本，而且對集群的硬體要求比較

❸ 2020-12-02 硬碟如何存儲文件

系統中所有內容是以文件（文件夾是特殊的文件）存在的，而文件分為屬性（元信息）和內容兩部分，磁碟一部分被操作系統虛擬為塊用來存儲數據，同時也分出一部分虛擬為Inode用來存儲文件屬性，這樣磁碟就分為塊區和inode區。

扇區：磁碟存儲數據的最小物理單元，每個扇區很小512位元組左右。
讀取數據：OS要想讀取磁碟數據，首先讓磁頭徑向尋道（最慢），然後旋轉磁碟（較快），使磁頭到達目標扇區，開始讀取數據。

磁碟塊：OS日常工作中，一個扇區的512位元組數據很小，不足以支撐絕大部分工作場景，所以需要頻繁讀取單個扇區，而磁碟讀取數據速度相對CPU處理太慢了，所以讀磁碟時一次就多拿出幾個扇區（臨近的，無需耗費額外時間）的數據，於是在OS層面邏輯虛擬出磁碟塊（簇）的概念，一個磁碟塊一般對應8個連續扇區（也可4、16個等，由OS決定），這樣OS層面就使用磁碟塊作為最小數據存儲單元。
這樣的好處當然是更高效，缺點則是會？

inode：用於存儲文件的元信息（除名稱外的所有屬性，名稱存在文件夾的內容中）
Inode number is also known as index number. An inode is a unique number assigned to files and directories while it is created. The inode number will be unique to entire filesystem.

Disk inodes contain the following information:

Owner identifier
Type of file (regular, directory, character or block device)
Access permissions
Times and dates
· file creation time

· last file access time

· last inode modification time

Number of links to the file
Array of pointers to data blocks on disk
File size (in bytes, sometimes also in blocks)

文件：
上文提及文件屬性存在磁碟inode區的inode（每個都有編號）內，而內容存儲在塊區的塊中。

文件夾：
作為特殊文件，其組織文件及目錄，屬性也是存在inode內，而存儲的內容是一個包含多個{ 文件名:對應inode Id} 的列表，內容亦存在塊區的塊中。

這樣在OS中查看一個文件（比如/etc/fstab）的內容，大概是：
首先OS獲取到根目錄的inodeId >在inode區中讀取到其屬性（某項是內容所在塊）>在塊區讀取到根目錄內容>在內容中找到名為/etc對應發inodeId>/etc在inode區的屬性>讀取到塊中/etc的內容（包含/etc/fstab對應inodeId）>/etc/fstab Inode Id > 在inode區讀取到/etc/fstab屬性 >/etc/fstab塊。

可能有誤，望指點。

Within each file system, the mapping from names to blocks is handled through a structure called an i-node. There's a pool of these things near the "bottom" (lowest-numbered blocks) of each file system (the very lowest ones are used for housekeeping and labeling purposes we won't describe here). Each i-node describes one file. File data blocks (including directories) live above the i-nodes (in higher-numbered blocks).

Every i-node contains a list of the disk block numbers in the file it describes. (Actually this is a half-truth, only correct for small files, but the rest of the details aren't important here.) Note that the i-node does not contain the name of the file.

Names of files live in directory structures. A directory structure just maps names to i-node numbers. This is why, in Unix, a file can have multiple true names (or hard links); they're just multiple directory entries that happen to point to the same i-node.

refer: https://unix.stackexchange.com/questions/432655/why-does-using-indirect-pointers-in-inodes-not-incur-the-same-amount-of-space

less:by direct list blocks in node?.
large :by two-level indirect block
larger : multi-level indirect block.

The original hierarchy of the inodes levels works roughly like this:

You can store one or a few block numbers directly in the inode. This means you use a few bytes more for the inode, but for small files, you don't have to allocate a complete block, which is mostly empty.

The next level is one indirection: You allocate a block to store the block pointers. Only the address of this indirect block is stored in the inode. This doesn't use somehow "less space", and most filesystems, even early ones, worked like that (have a pointer near the inode/filename which points to a block, which stores the block numbers of the file).

But what do you do when the space in this block runs out? You have to allocate another block, but where do you store the reference to this block? You could just add those references to the inode, but to store largers files, the inode would get large. And you want small inodes, so as many as possible inodes can fit into a single block (less disk access to read more inodes).

So you use a two-level indirect block: You just add one pointer to the inode, then you have a whole block to store pointers to indirect blocks, and the indirect blocks store the block address of the file itself.

And so on, you can add higher-level indirect blocks, or stop at some stage, until you reach the maximal size of a file possible with the structure you want.

So the point is not "use up less space in total", but "use a scheme that uses blocks efficiently for the expected distribution a files wrt. to size, i.e. many small files, some larger files, and very few huge files".

Page tables on the other hand work very differently.

Edit

To answer the questions in the comment:

Data blocks are of fixed sizes (originally 512 bytes, IIRC), which is a multiple of the block size of the underlying harddisks. So data block size can't "decrease".

As I tried to describe above, the whole point of having the inodes not use up too much space is to make inode access faster (or, alternatively, make caching inodes use up less memory - back then when the unix file system with inodes was invented, computers had a lot less memory than today). It's not about somehow saving space in total. As you say yourself, everything has to be stored somewhere, and if it doesn't use up space at location X, it will use up space at location Y.

Just adding a variable number of block pointers to the inode is not practical, because the inode must take up a fixed amount of space - you want to use the inode number to calculate the block address and the offset inside the block where the inode information is stored. You can't do that if every inode has a different size. So there must be some form of indirection.

Page tables work differently because hardware implements them differently - that's just how it is. The hierarchy has a fixed depth, always the same (though sometimes configurable. And while reading a block from disk is slow, that doesn't matter for page tables. So the design issues are completely different.

http://www.cems.uwe.ac.uk/~irjohnso/coursenotes/lrc/internals/filestore/fs3.htm

Assuming, for the purposes of illustration, that each disk data block is 1024 bytes in size, then these ten data block pointers will allow files to be created that are up to 10 Kb in size. As you can see, for the large majority of files it should be possible to access the data with nothing more than a direct lookup required to find the data block that contains any particular data byte.

With this scheme, once a file has grown to 10 Kb, there are only three block pointers in the inode left to use, whatever the eventual size of the file. Obviously, some new arrangement must be found so that the three remaining block pointers will suffice for any realistic file size, while at the same time not degrading the data access time too much.

This goal is achieved by using the idea of indirect block pointers. Specifically, when an 11th data block needs to be allocated to the file, the 11th inode block pointer is used, but instead of pointing to the block which will contain the data, the 11th pointer is a single indirect pointer which points to a data block filled with a list of direct block pointers. In our example, if we assume that a data block number is a 32-bit value, then a list of 256 of them will fit into the single indirect block. This list will point directly to the data blocks for the next 256 Kb of our file. This means that with 11 block pointers in the inode, files of up to 266 Kb (10 + 256) can be created. True, it takes a little longer to access the data beyond the first 10 Kb in the file, but it takes only one extra disk block read to find the position on the disk of the required data.

For files bigger than 266 Kb the double indirect (12th) inode block pointer is used. This is the same idea as the previous inode pointer except that the double indirect pointer points to a list of pointers in a data block, each of which is itself a single indirect block pointer which points to a list of 256 direct block pointers. This means that the 12th inode block pointer gives access to the next 65536 Kb (256x256) of data in our file.

By now, you should be able to spot the pattern and see that when the file grows bigger than 64 Mb (actually 65802 Kb), the inode's 13th data block pointer will be used, but this time as a triple indirect pointer, which will give access to a staggering 16 Gb (256x256x256 Kb) of extra file space. A single file bigger than 16Gb sounds huge. However, even though the calculation we have just done suggests that this file size is possible with the inode layout as given, in fact there are other factors which limit the maximum size of a file to a smaller value than this. For example, the size of a file, in bytes, is stored separately in its inode in a field of type unsigned long. This is a 32-bit number which limits the size of a file to 4 Gb, so that 13 data block pointers in an inode really are enough.

10.4. How a file gets looked up
Now we can look at the file system from the top down. When you open a file (such as, say, /home/esr/WWW/ldp/fundamentals.xml) here is what happens:

Your kernel starts at the root of your Unix file system (in the root partition). It looks for a directory there called 『home』. Usually 『home』 is a mount point to a large user partition elsewhere, so it will go there. In the top-level directory structure of that user partition, it will look for a entry called 『esr』 and extract an i-node number. It will go to that i-node, notice that its associated file data blocks are a directory structure, and look up 『WWW』. Extracting that i-node, it will go to the corresponding subdirectory and look up 『ldp』. That will take it to yet another directory i-node. Opening that one, it will find an i-node number for 『fundamentals.xml』. That i-node is not a directory, but instead holds the list of disk blocks associated with the file.

The surface area of your disk, where it stores data, is divided up something like a dartboard — into circular tracks which are then pie-sliced into sectors. Because tracks near the outer edge have more area than those close to the spindle at the center of the disk, the outer tracks have more sector slices in them than the inner ones. Each sector (or disk block ) has the same size, which under modern Unixes is generally 1 binary K (1024 8-bit bytes). Each disk block has a unique address or disk block number .

Unix divides the disk into disk partitions . Each partition is a continuous span of blocks that's used separately from any other partition, either as a file system or as swap space. The original reasons for partitions had to do with crash recovery in a world of much slower and more error-prone disks; the boundaries between them rece the fraction of your disk likely to become inaccessible or corrupted by a random bad spot on the disk. Nowadays, it's more important that partitions can be declared read-only (preventing an intruder from modifying critical system files) or shared over a network through various means we won't discuss here. The lowest-numbered partition on a disk is often treated specially, as a boot partition where you can put a kernel to be booted.

Each partition is either swap space (used to implement virtual memory ) or a file system used to hold files. Swap-space partitions are just treated as a linear sequence of blocks. File systems, on the other hand, need a way to map file names to sequences of disk blocks. Because files grow, shrink, and change over time, a file's data blocks will not be a linear sequence but may be scattered all over its partition (from wherever the operating system can find a free block when it needs one). This scattering effect is called fragmentation .

Within each file system, the mapping from names to blocks is handled through a structure called an i-node . There's a pool of these things near the "bottom" (lowest-numbered blocks) of each file system (the very lowest ones are used for housekeeping and labeling purposes we won't describe here). Each i-node describes one file. File data blocks (including directories) live above the i-nodes (in higher-numbered blocks).

Every i-node contains a list of the disk block numbers in the file it describes. (Actually this is a half-truth, only correct for small files, but the rest of the details aren't important here.) Note that the i-node does not contain the name of the file.

Names of files live in directory structures . A directory structure just maps names to i-node numbers. This is why, in Unix, a file can have multiple true names (or hard links ); they're just multiple directory entries that happen to point to the same i-node.

In the simplest case, your entire Unix file system lives in just one disk partition. While you'll see this arrangement on some small personal Unix systems, it's unusual. More typical is for it to be spread across several disk partitions, possibly on different physical disks. So, for example, your system may have one small partition where the kernel lives, a slightly larger one where OS utilities live, and a much bigger one where user home directories live.

The only partition you'll have access to immediately after system boot is your root partition , which is (almost always) the one you booted from. It holds the root directory of the file system, the top node from which everything else hangs.

The other partitions in the system have to be attached to this root in order for your entire, multiple-partition file system to be accessible. About midway through the boot process, your Unix will make these non-root partitions accessible. It will mount each one onto a directory on the root partition.

For example, if you have a Unix directory called <tt class="filename">/usr</tt>, it is probably a mount point to a partition that contains many programs installed with your Unix but not required ring initial boot.