| 1 | <?xml version='1.0' encoding='UTF-8'?>
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| 2 | <!DOCTYPE topic PUBLIC "-//OASIS//DTD DITA Topic//EN" "topic.dtd">
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| 3 | <topic xml:lang="en-us" id="ts_config-periodic-flush">
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| 4 | <title>Guest Shows IDE/SATA Errors for File-Based Images on Slow Host File
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| 5 | System</title>
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| 6 |
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| 7 | <body>
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| 8 | <p>
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| 9 | Occasionally, some host file systems provide very poor writing
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| 10 | performance and as a consequence cause the guest to time out
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| 11 | IDE/SATA commands. This is normal behavior and should normally
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| 12 | cause no real problems, as the guest should repeat commands that
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| 13 | have timed out. However, guests such as some Linux versions have
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| 14 | severe problems if a write to an image file takes longer than
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| 15 | about 15 seconds. Some file systems however require more than a
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| 16 | minute to complete a single write, if the host cache contains a
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| 17 | large amount of data that needs to be written.
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| 18 | </p>
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| 19 | <p>
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| 20 | The symptom for this problem is that the guest can no longer
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| 21 | access its files during large write or copying operations,
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| 22 | usually leading to an immediate hang of the guest.
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| 23 | </p>
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| 24 | <p>
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| 25 | In order to work around this problem, the true fix is to use a
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| 26 | faster file system that does not exhibit such unacceptable write
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| 27 | performance, it is possible to flush the image file after a
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| 28 | certain amount of data has been written. This interval is
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| 29 | normally infinite, but can be configured individually for each
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| 30 | disk of a VM.
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| 31 | </p>
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| 32 | <p>
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| 33 | For IDE disks use the following command:
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| 34 | </p>
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| 35 | <pre xml:space="preserve">VBoxManage setextradata <varname>VM-name</varname>
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| 36 | "VBoxInternal/Devices/piix3ide/0/LUN#[<varname>x</varname>]/Config/FlushInterval" [<varname>b</varname>]</pre>
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| 37 | <p>
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| 38 | For SATA disks use the following command:
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| 39 | </p>
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| 40 | <pre xml:space="preserve">VBoxManage setextradata <varname>VM-name</varname>
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| 41 | "VBoxInternal/Devices/ahci/0/LUN#[<varname>x</varname>]/Config/FlushInterval" [<varname>b</varname>]</pre>
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| 42 | <p><codeph>[<varname>x</varname>]</codeph> specifies the
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| 43 | disk. For IDE, <codeph>0</codeph> represents device 0 on the
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| 44 | primary channel, <codeph>1</codeph> represents device 1 on the
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| 45 | primary channel, <codeph>2</codeph> represents device 0 on the
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| 46 | secondary channel, and <codeph>3</codeph> represents device 1
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| 47 | on the secondary channel. For SATA, use values between
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| 48 | <codeph>0</codeph> and <codeph>29</codeph>. This
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| 49 | configuration option applies to disks only. Do not use this
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| 50 | option for CD or DVD drives.
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| 51 | </p>
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| 52 | <p>
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| 53 | The unit of the interval
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| 54 | (<codeph>[<varname>b</varname>]</codeph>) is the
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| 55 | number of bytes written since the last flush. The value for it
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| 56 | must be selected so that the occasional long write delays do not
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| 57 | occur. Since the proper flush interval depends on the
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| 58 | performance of the host and the host filesystem, finding the
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| 59 | optimal value that makes the problem disappear requires some
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| 60 | experimentation. Values between 1000000 and 10000000 (1 to 10
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| 61 | megabytes) are a good starting point. Decreasing the interval
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| 62 | both decreases the probability of the problem and the write
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| 63 | performance of the guest. Setting the value unnecessarily low
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| 64 | will cost performance without providing any benefits. An
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| 65 | interval of 1 will cause a flush for each write operation and
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| 66 | should solve the problem in any case, but has a severe write
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| 67 | performance penalty.
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| 68 | </p>
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| 69 | <p>
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| 70 | Providing a value of <codeph>0</codeph> for
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| 71 | <codeph>[<varname>b</varname>]</codeph> is treated as
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| 72 | an infinite flush interval, effectively disabling this
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| 73 | workaround. Removing the extra data key by specifying no value
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| 74 | for <codeph>[<varname>b</varname>]</codeph> has the
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| 75 | same effect.
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| 76 | </p>
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| 77 | </body>
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| 78 |
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| 79 | </topic>
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