btrfs-progs: docs: more hw considerations
Add section about NVME and hints what to do to some other sections. The rest are typo or style fixes. Signed-off-by: David Sterba <dsterba@suse.com>
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David Sterba
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@ -1480,11 +1480,11 @@ MAIN MEMORY
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The data structures and raw data blocks are temporarily stored in computer
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memory before they get written to the device. It is critical that memory is
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reliable because even simple bit flips can have vast consquences and lead to
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reliable because even simple bit flips can have vast consequences and lead to
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damaged structures, not only in the filesystem but in the whole operating
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system.
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Based on experience in the community, memory bitflips are more common than one
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Based on experience in the community, memory bit flips are more common than one
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would think. When it happens, it's reported by the tree-checker or by a checksum
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mismatch after reading blocks. There are some very obvious instances of bit
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flips that happen, e.g. in an ordered sequence of keys in metadata blocks. We can
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@ -1494,7 +1494,7 @@ entire filesystem to see the scope.
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If available, ECC memory should lower the chances of bit flips, but this
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type of memory is not available in all cases. A memory test should be performed
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in case there's a visible bitflip pattern, though this may not detect a faulty
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in case there's a visible bit flip pattern, though this may not detect a faulty
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memory module because the actual load of the system could be the factor making
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the problems appear. In recent years attacks on how the memory modules operate
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have been demonstrated ('rowhammer') achieving specific bits to be flipped.
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@ -1505,6 +1505,14 @@ Further reading:
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- https://en.wikipedia.org/wiki/Row_hammer
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What to do:
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- run 'memtest', note that sometimes memory errors happen only when the system
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is under heavy load that the default memtest cannot trigger
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- memory errors may appear as filesystem going read-only due to "pre write"
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check, that verify meta data before they get written but fail some basic
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consistency checks
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DIRECT MEMORY ACCESS (DMA)
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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@ -1515,8 +1523,14 @@ other pages. Storage devices utilize DMA for performance reasons, the
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filesystem structures and data pages are passed back and forth, making
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errors possible in case page life time is not properly tracked.
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There are lots of quirks (device-specific workarounds) in linux kernel
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drivers (regarding not only DMA) that are added when found.
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There are lots of quirks (device-specific workarounds) in Linux kernel
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drivers (regarding not only DMA) that are added when found. The quirks
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may avoid specific errors or disable some features to avoid worse problems.
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What to do:
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- use up-to-date kernel (recent releases or maintained long term support versions)
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- as this may be caused by faulty drivers, keep the systems up-to-date
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ROTATIONAL DISKS (HDD)
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~~~~~~~~~~~~~~~~~~~~~~
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@ -1539,6 +1553,10 @@ lower layers try hard to transfer the data correctly or not at all. The errors
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from badly-connecting cables may manifest as large amount of failed read or
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write requests, or as short error bursts depending on physical conditions.
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What to do:
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- check 'smartctl' for potential issues
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SOLID STATE DRIVES (SSD)
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~~~~~~~~~~~~~~~~~~~~~~~~
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@ -1602,6 +1620,34 @@ Further reading:
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- https://www.snia.org/educational-library/ssd-performance-primer-2013
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- https://www.snia.org/educational-library/how-controllers-maximize-ssd-life-2013
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What to do:
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- run 'smartctl' or self-tests to look for potential issues
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- keep the firmware up-to-date
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NVM EXPRESS, NON-VOLATILE MEMORY (NVMe)
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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NVMe is a type of persistent memory usually connected over a system bus (PCIe)
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or similar interface and the speeds are an order of magnitude faster than SSD.
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It is also a non-rotating type of storage, and is not typically connected by a
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cable. It's not a SCSI type device either but rather a complete specification
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for logical device interface.
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In a way the errors could be compared to a combination of SSD class and regular
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memory. Errors may exhibit as random bit flips or IO failures. There are tools
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to access the internal log ('nvme log' and 'nvme-cli') for a more detailed
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analysis.
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There are separate error detection and correction steps performed e.g. on the
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bus level and in most cases never making in to the filesystem level. Once this
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happens it could mean there's some systematic error like overheating or bad
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physical connection of the device. You may want to run self-tests (using
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'smartctl').
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* https://en.wikipedia.org/wiki/NVM_Express
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* https://www.smartmontools.org/wiki/NVMe_Support
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DRIVE FIRMWARE
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~~~~~~~~~~~~~~
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@ -1610,20 +1656,30 @@ software has bugs, so does firmware. Storage devices can update the firmware
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and fix known bugs. In some cases the it's possible to avoid certain bugs by
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quirks (device-specific workarounds) in Linux kernel.
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A faulty firmware can cause wide range of corruptions from small and localized
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to large affecting lots of data. Self-repair capabilities may not be sufficient.
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What to do:
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- check for firmware updates in case there are known problems, note that
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updating firmware can be risky on itself
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- use up-to-date kernel (recent releases or maintained long term support versions)
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SD FLASH CARDS
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~~~~~~~~~~~~~~
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There are a lot of devices with low power consumption and thus using storage
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media based on low power consumption, typically flash memory stored on
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media based on low power consumption too, typically flash memory stored on
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a chip enclosed in a detachable card package. An improperly inserted card may be
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damaged by electrical spikes when the device is turned on or off. The chips
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storing data in turn may be damaged permanently. All types of flash memory
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have a limited number of number of rewrites, so the data are internally
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translated by FTL (flash translation layer). This is implemented in firmware
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(software) and prone to bugs that manifest as hardware errors.
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have a limited number of rewrites, so the data are internally translated by FTL
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(flash translation layer). This is implemented in firmware (technically a
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software) and prone to bugs that manifest as hardware errors.
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Adding redundancy like using DUP profiles for both data and metadata can help
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in some cases.
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in some cases but a full backup might be the best option once problems appear
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and replacing the card could be required as well.
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HARDWARE AS THE MAIN SOURCE OF FILESYSTEM CORRUPTIONS
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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