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faq: rewrote IntegrityError answer
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Marian Beermann
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2 changed files with 68 additions and 26 deletions
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@ -1,3 +1,6 @@
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.. _important_notes:
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Important notes
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===============
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91
docs/faq.rst
91
docs/faq.rst
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@ -194,41 +194,80 @@ repo. It will then be able to check using CRCs and HMACs.
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I get an IntegrityError or similar - what now?
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----------------------------------------------
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The first step should be to check whether it's a problem with the disk drive,
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IntegrityErrors can be a sign of drive failure or other hardware issues.
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A single error does not necessarily indicate bad hardware or a Borg
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bug. All hardware exhibits a bit error rate (BER). Hard drives are typically
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specified as exhibiting less than one error every 12 to 120 TB
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(one bit error in 10e14 to 10e15 bits). The specification is often called
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*unrecoverable read error rate* (URE rate).
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Using the smartmontools one can retrieve self-diagnostics of the drive in question
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(where the repository is located, use *findmnt*, *mount* or *lsblk* to find the
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*/dev/...* path of the drive)::
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Apart from these very rare errors there are two main causes of errors:
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# smartctl -a /dev/sdSomething
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(i) Defective hardware: described below.
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(ii) Bugs in software (Borg, operating system, libraries):
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Ensure software is up to date.
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Check whether the issue is caused by any fixed bugs described in :ref:`important_notes`.
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Attributes that are a typical cause of data corruption are *Offline_Uncorrectable*,
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*Current_Pending_Sector*, *Reported_Uncorrect*. A high *UDMA_CRC_Error_Count* usually
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indicates a bad cable. If the *entire drive* is failing, then all data should be copied
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off it as soon as possible.
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Some drives log IO errors, which are also logged by the system (refer to the journal/dmesg).
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IO errors that impact only the filesystem can go unnoticed, since they are not reported
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to applications (e.g. Borg), but can still corrupt data.
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.. rubric:: Finding defective hardware
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If any of these are suspicious, a self-test is recommended::
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.. note::
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# smartctl -t long /dev/sdSomething
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Hardware diagnostics are operating system dependent and do not
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apply universally. The commands shown apply for popular Unix-like
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systems. Refer to your operating system's manual.
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Running ``fsck`` if not done already might yield further insights.
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Checking hard drives
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Find the drive containing the repository and use *findmnt*, *mount* or *lsblk*
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to learn the device path (typically */dev/...*) of the drive.
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Then, smartmontools can retrieve self-diagnostics of the drive in question::
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# smartctl -a /dev/sdSomething
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The *Offline_Uncorrectable*, *Current_Pending_Sector* and *Reported_Uncorrect*
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attributes indicate data corruption. A high *UDMA_CRC_Error_Count* usually
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indicates a bad cable.
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I/O errors logged by the system (refer to the system journal or
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dmesg) can point to issues as well. I/O errors only affecting the
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file system easily go unnoticed, since they are not reported to
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applications (e.g. Borg), while these errors can still corrupt data.
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Drives can corrupt some sectors in one event, while remaining
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reliable otherwise. Conversely, drives can fail completely with no
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advance warning. If in doubt, copy all data from the drive in
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question to another drive -- just in case it fails completely.
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If any of these are suspicious, a self-test is recommended::
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# smartctl -t long /dev/sdSomething
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Running ``fsck`` if not done already might yield further insights.
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Checking memory
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Intermittent issues, such as ``borg check`` finding errors
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inconsistently between runs, are frequently caused by bad memory.
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Run memtest86+ (or an equivalent memory tester) to verify that
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the memory subsystem is operating correctly.
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Checking processors
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Processors rarely cause errors. If they do, they are usually overclocked
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or otherwise operated outside their specifications. We do not recommend to
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operate hardware outside its specifications for productive use.
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Tools to verify correct processor operation include Prime95 (mprime), linpack,
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and the `Intel Processor Diagnostic Tool
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<https://downloadcenter.intel.com/download/19792/Intel-Processor-Diagnostic-Tool>`_
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(applies only to Intel processors).
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.. rubric:: Repairing a damaged repository
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With any defective hardware found and replaced, the damage done to the repository
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needs to be ascertained and fixed.
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:ref:`borg_check` provides diagnostics and ``--repair`` options for repositories with
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issues. We recommend to first run without ``--repair`` to assess the situation and
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if the found issues / proposed repairs sound right re-run it with ``--repair`` enabled.
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When errors are intermittent the cause might be bad memory, running memtest86+ or a similar
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test is recommended.
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A single error does not indicate bad hardware or a Borg bug -- all hardware has a certain
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bit error rate (BER), for hard drives this is typically specified as less than one error
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every 12 to 120 TB (one bit error in 10e14 to 10e15 bits) and often called
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*unrecoverable read error rate* (URE rate).
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issues. We recommend to first run without ``--repair`` to assess the situation.
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If the found issues and proposed repairs seem right, re-run "check" with ``--repair`` enabled.
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Security
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########
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