Azure and Azure files MMAP broken

We found out (or rather re-discovered) that Azure Files CIFS implementation does not correctly support mmap function that is used in some softwares.

It has been previously reported on https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=900821 but we thought it might be worth clarifying this for people spending a lot of time googling why their apache2 fails to deliver files larger than 1MB.

The default setting in apache2 does not mmap files smaller than 1MB hence why the issue is not noticed on small files or files not hosted on the CIFS volume. Large files from the affected volume will however cause a ”Read error at byte X/ X (Bad address). Retrying.” error if you try to wget or download the files via apache2 on a web app using Azure Files mounted CIFS storage (path mapping).

Some searches might lead you to investigate apache2 file limitations preventing files larger than 1MB from beint transmitted.
LimitRequestBody is not a fix for this issue.

Scenario:
Azure web app or VM with Apache2 or other mmap using software fails to read files properly from Azure Files mounted ”path mapping” volumes which uses CIFS.

Workaround:
EnableMMAP off

This configuration line can be done per affected Directory, global apache2 config or in the vhost.

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