VM.pool_migrate and /or VM.start call failed due to EMFILE error:

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varstored-guard: [error||0 ||backtrace] Raised Unix.Unix_error(Unix.EMFILE, "socket", "")
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EMFILE: too many files open error.

Workaround:

The current number in use can be checked by looking at:

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$ ls /proc/$(pidof varstored-guard)/fd | wc -l
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and if that number gets close to 4000 to restart toolstack:

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$ xe-toolstack-restart
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Hotfix XS82ECU1011 has been released for this issue, please download and apply the following hotfix to fix this issue:

https://support.citrix.com/article/CTX463430

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Problem Cause

The problem is that varstored-guard is running out of file descriptors; it can use up to 4096 before it runs out.
The number of file descriptors should increase slowly because one leaks every time a VM with UEFI (i.e. a Windows VM) is started on a host, resumed, or migrated to. These are relatively rare events.