# Upgrading to Varnish 7.6

In general, upgrading from Varnish 7.5 to 7.6 should not require any changes besides the actual upgrade.

The changes mentioned below are considered noteworthy nevertheless:

## Noteworthy changes when upgrading varnishd

### Warning about failed memory locking

The `Warning: mlock() of VSM failed` message is now emitted when locking of shared memory segments (via `mlock(2)`) fails. As Varnish performance may be severely impacted if shared memory segments are not resident in RAM, users seeing this message are urged to review the `RLIMIT_MEMLOCK` resource control as set via `ulimit -l` or `LimitMEMLOCK` with `systemd(1)`. This is not new at all, just now the warning has been added to make administrators more aware.

### Warning if tmpfs is not used

On Linux (when the new `linux` jail is used), the `Working directory not mounted on tmpfs partition` warning is now emitted if the working directory is found to reside on a file system other than `tmpfs`. While other file systems are supported (and might be the right choice where administrators understand how to avoid blocking disk IO while `varnishd` is writing to shared memory), `tmpfs` is the failsafe option to avoid performance issues.

## Upgrading VCL

An issue has been addressed in the `builtin.vcl` where backend responses would fail if they contained a `Content-Range` header when no range was requested. According to RFC9110, this header should just be ignored, yet some Varnish users might prefer stricter checks. Thus, we decided to change the `builtin.vcl` only and users hitting this issue are advised to call `vcl_beresp_range` from custom VCL.

## Changes for developers and VMOD authors

The VDP filter API has changed. See VDP filter API changes for details.

The signature of `ObjWaitExtend()` has changed. See Object API changes for details.

`varnishd` now creates a `worker_tmpdir` which can be used by VMODs for temporary files. See Event functions for details.

eof