{"id":504,"date":"2020-12-06T08:52:36","date_gmt":"2020-12-06T08:52:36","guid":{"rendered":"https:\/\/picockpit.com\/raspberry-pi\/?p=504"},"modified":"2023-11-13T11:42:54","modified_gmt":"2023-11-13T11:42:54","slug":"monitor-sd-card-health-of-raspberry-pi","status":"publish","type":"post","link":"https:\/\/picockpit.com\/raspberry-pi\/monitor-sd-card-health-of-raspberry-pi\/","title":{"rendered":"All about SD card health on the Raspberry Pi"},"content":{"rendered":"\n
\"SD<\/figure>\n\n\n\n

The SD card is – next to the power supply – a critical additional component of the Raspberry Pi. Monitoring it’s health is really important to ensure a smooth operation of your Raspberry Pi operating system, and a good user experience. This article will show you several ways how to check and monitor the health of your microSD card.<\/p>\n\n\n\n

First, I’ll give an in-depth overview of how memory cards work, so you can understand the possibilities and limitations of checking the health state of your SD card.<\/p>\n\n\n\n

Then I’ll explain how to protect your microSD card by reducing common problems Raspberry Pi users experience. We’ll also go into the best microSD card brands for Raspberry Pi we recommend.<\/p>\n\n\n\n

If you want, you can also skip down further, to just get the Linux commands to check the current microSD card state.<\/p>\n\n\n\n

Basics: Inside the microSD card<\/h2>\n\n\n
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Image: Illustration of the inside of an SD card. The microSD card has a similar structure. Image source: CC-BY-SA Korpsvart<\/a>, Wikimedia Commons<\/figcaption><\/figure>\n<\/div>\n\n\n

The microSD card contains a flash memory chip (on the left of the picture), and a micro-controller (on the right of the picture, usually ARM-based).<\/p>\n\n\n\n

Flash<\/h3>\n\n\n\n

Flash memory stores information by “trapping” electrons<\/strong>, which are “injected” using high voltage through a non-conductor into a so-called floating gate<\/strong>(**). The electrons are thus part of a transistor that may or may not allow a connected current to flow, depending on the charge of the floating gate. Theoretically, they cannot flow away, because the floating gate is electrically isolated<\/strong>. This means that the information stays put even after the current supply is switched off.<\/p>\n\n\n\n

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Images: CC-BY-SA \u0414.\u0418\u043b\u044c\u0438\u043d Wikimedia Commons<\/a> \/  Jlochoap CC-0<\/a><\/em><\/figcaption><\/figure>\n<\/div><\/div>\n<\/div>\n\n\n
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The information is always read between source (S) and drain (D). Electrons introduced into the floating gate increase e.g. the threshold voltage of the transistor, starting from which current would flow. The transistor then blocks at a normal read voltage (does not conduct).<\/p>\n\n\n\n

For programming the floating gate, significantly higher electrical voltages (e.g. 10 V) are necessary than for the normal read operation (e.g. 3.3 V). To do this, additionally the control gate (V1\/V2\/V3) plays a key role.<\/p>\n\n\n\n

To erase everything, the control gate drives electrons out of the floating gate by applying a high negative voltage.<\/p>\n\n\n\n

NAND flash components used in microSD cards group the individual memory transistors into pages, and several of the pages into blocks. A page has between 512 and 8192 bytes, a block can contain up to 256 pages (thus a total of 2048 kB with 8kB page size).<\/p>\n\n\n\n

Writing (for a logical “1”) can be done bitwise or at least byte\/wordwise. Erasing (for a logical “0”) can only be done blockwise. If there’s any unaltered information, it has to be programmed in again.<\/p>\n\n\n\n

Flash memories have a limited lifetime due to programming and erasing, which we calculate in erase cycles.<\/strong><\/span><\/p>\n\n\n\n

The reason for the limited lifetime is damage to the insulating oxide layer, which protects the floating gate from charge leakage, caused by the high voltages. As soon as this layer becomes conductive, the memory cell can’t hold more information.<\/p>\n\n\n\n

Aside: Multi-level cell memory cells<\/h3>\n\n\n\n

Initially, there were only two charge states (1 bit of information<\/strong>) per memory cell. Now, thanks to several floating gates per transistor, multi-level cell memory cells store different charge states and thus several bits per memory transistor. During readout, the system evaluates how the applied current is conducted differently by the transistor.<\/p>\n\n\n\n

On the one hand, this makes it possible to increase the density of the memory cells significantly, but on the other hand, readout is slower and the memory cells react much more sensitively with bit errors to charge losses. With single-level cells, 100,000 to 1,000,000 write-erase cycles are possible, with TLCs (triple-level cells with three bits per memory cell) approx. 1000 write-erase cycles.<\/p>\n\n\n\n

This is the reason that industrial SD cards usually have lower memory densities, and use SLC (single level cells), for better data integrity<\/strong>.<\/span><\/p>\n\n\n\n

The Controller<\/h3>\n\n\n\n

The task of the controller is to manage the flash, and in particular to perform wear leveling and read error correction<\/strong>. The performance and longevity of the microSD card depends decisively on the algorithms used in the controller.<\/p>\n\n\n\n

Flash memory cannot be rewritten as often as required due to damage to the insulating oxide layer of the floating gates as described above. To avoid damage to individual areas that are used particularly frequently, the controller varies the physical allocation to the blocks that can be addressed logically by the file system (= wear leveling).<\/strong><\/p>\n\n\n\n

This variation of the physical allocation is also the reason that write-testing the SD card (by writing & reading with bad block tools, etc.) will actually not identify the real bad blocks & allow you to avoid them on the operating system \/ file system level! Only the flash memory controller inside the microSD card knows which block is written to \/ read from at any given moment, and as discussed this can change over time.<\/strong><\/span><\/p>\n\n\n\n

Defective blocks (bad blocks) are already present in brand-new flash memory. These defective blocks are marked in a special area of the flash memory.<\/p>\n\n\n\n

Error correction information for individual blocks is also managed so that read errors can be corrected by checksums. The controller adds blocks with clustered read errors to the list of bad blocks, and shifts the actual physical allocation of the logical block.<\/p>\n\n\n\n

The microSD card typically has – depending on the manufacturer – about 10% spare capacity to exchange the bad blocks with good “reserve blocks”.<\/p>\n\n\n\n

Dirty little secrets: Flash memory problems<\/h3>\n\n\n\n

Deletion is only blockwise<\/h4>\n\n\n\n

Data can only be erased block by block. Erasing stresses the memory cells, and shortens their lifetime – new bad blocks are created.<\/p>\n\n\n\n

Defective blocks from the factory<\/h4>\n\n\n\n

Flash memories are already shipped with defective blocks. In the course of operation, further defective blocks (bad blocks) are added. The controller therefore tries to write \/ erase blocks as evenly distributed as possible by wear-leveling.<\/p>\n\n\n\n

MLC and TLC particularly sensitive<\/h4>\n\n\n\n

Multilevel cell memory cells (MLCs) reduce the number of erase cycles and thus long-term reliability.<\/p>\n\n\n\n

Read Disturb<\/h4>\n\n\n\n

A phenomenon not yet mentioned by me, but particularly perfidious, is Read Disturb<\/strong>. Even when only reading from the card, it can – just by reading – cause neighboring memory cells in the same block to change their programming. The probability of this happening increases sharply after a few 100,000 reads.<\/p>\n\n\n\n

To avoid Read Disturb the controller therefore logs the number of accesses to a block in order to copy it in one piece to a new location when a threshold is exceeded, and to delete the original block. After that, the block can be re-used again.<\/p>\n\n\n\n

All these are things a controller has to compensate for in order to pretend to us that it is a “perfect memory card” on the outside, while it looks anything but perfect on the inside!<\/p>\n\n\n\n

Last but not least, bits written by X-rays could be erased unintentionally. Here, only an X-ray-safe design of the card can ensure that the data remains intact.<\/p>\n\n\n\n

Manufacturer & selection of a good card<\/h2>\n\n\n\n

Both micro-controller and flash device and the finished microSD card can come from different manufacturers – the Panasonic SD card in the example photo has Samsung flash. The controller was made in Japan.<\/p>\n\n\n\n

There are four manufacturers that produce NAND flash devices:<\/p>\n\n\n\n