Add section on mount options #254
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vvolkl
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Nov 27, 2025
Passthrough is a recent FUSE feature (kernel 6.17.0+, libfuse 3.17+)
enabling bypass of userspace on all the read requests. When the FUSE
daemon provides a "backing file descriptor" to FUSE on "open" request,
the kernel reads directly from it without calling into the FUSE daemon.
This is a win for performance. The test supplied demonstrates
measurable speedup:
```
+ dd status=progress bs=512 if=/cvmfs/test.repo.dumbdumb/big.bin of=/dev/null
10485760 bytes (10 MB, 10 MiB) copied, 3.35429 s, 3.1 MB/s
+ dd status=progress bs=512 if=/cvmfs/test.repo.passthru/big.bin of=/dev/null
10485760 bytes (10 MB, 10 MiB) copied, 0.073326 s, 143 MB/s
```
```
+ pushd /cvmfs/test.repo.dumbdumb
+ parallel -N0 'sha1sum --check --status CHECKSUMS' ::: {1..100}
real 0m3.440s
user 0m4.782s
sys 0m3.345s
+ pushd /cvmfs/test.repo.passthru
+ parallel -N0 'sha1sum --check --status CHECKSUMS' ::: {1..100}
real 0m1.221s
user 0m3.904s
sys 0m1.709s
```
Enabling new functionality requires
* building with sufficiently recent libfuse to have FUSE_CAP_PASSTHROUGH
* running with similarly recent enough libfuse
* running on Linux kernel 6.17.0 or newer
* specifying mount option fuse_passthrough (or fuse_passthru)
Related documentation update:
cvmfs/doc-cvmfs#254
---------
Co-authored-by: Valentin Volkl <[email protected]>
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This changeset assumes this feature is merged: cvmfs/cvmfs#4006