docs(posts): measure what the missing Viridian flags cost - #37
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The recipe tells readers to build the VM from the "Other install media" template so XCP-ng does not impose an OS-specific default. That template sets platform:viridian=true and none of the other five Viridian keys, while the Windows Server 2025 and 2022 templates set all six. Verified on an XCP-ng 8.3 host, where the VM built by following this post carries one key of six and a VM created from the Windows 11 template carries all six. So the post has been quietly telling people to build under-configured Windows guests. The same gap catches any VM imported from a VHD or migrated in from VMware rather than created from a Windows template. The new section gives the comparison, the check, and the fix, and stops short of claiming a speed-up: the before and after was never benchmarked. It cites only what the documentation attributes to the missing synthetic timer, which is Windows Server 2025 hanging at 0% CPU. Placed as an unnumbered section rather than a renumbered step, so the existing "Step 5" and "The payoff" anchors do not move.
The section added earlier said the before and after was never benchmarked and declined to claim a speed-up. It has now been measured, so the hedge is replaced by numbers. Method: one VM, one quiet host, diskspd with caching disabled, five iterations per run, measured at one flag then six then one then six again so a reboot could not account for the difference. First iteration of each run discarded as cache-warm. 4K random read, medians of eight measurements per group: queue depth 1 latency 0.312 ms -> 0.195 ms (-38%) queue depth 1 IOPS 3,143 -> 5,127 (+63%) queue depth 8, 4 threads 45,147 -> 71,674 (+59%) The two groups do not overlap: the worst six-flag run beat the best one-flag run on both tests. Run-to-run spread also fell from roughly a quarter to a few percent, which matters more than the medians, since unpredictable I/O is what users actually report. Caveats kept in the text: local storage rather than iSCSI or NFS, one VM on one host, and one run drifted upward mid-way in a manner consistent with host caching, so the table quotes medians and not peaks.
The check told the reader to look for missing platform keys, which walks straight past a key that is present and set to false. A VM carrying viridian_stimer=false passes that test and still meets the documented hang. The condition now reads missing-or-false, and includes viridian itself, which the list had omitted even though the set command below it already wrote that key. The 0% CPU hang was also framed as one command away from being ruled out. The troubleshooting page prescribes two steps rather than one: check the host flags, then revert useplatformclock and useplatformtick if Windows boot parameters were ever set to disable the synthetic timer. Checking the flags alone leaves the guest side untested, so the section now carries both. Both docs.xcp-ng.org pages were re-read to confirm the six parameter names, the xe syntax and the two-step remedy. No measured number moved.
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📝 WalkthroughWalkthroughThe post adds instructions to enable six Viridian settings, benchmark results for those settings, and troubleshooting steps for Windows Server 2025 hangs caused by missing synthetic-timer support. ChangesWindows VM guidance
Estimated code review effort: 2 (Simple) | ~10 minutes Merge Risk: 🔵 Low · up to The post is mergeable with owner awareness: the troubleshooting command should specify elevated execution and a restart, and the benchmark should clarify its cache controls so readers do not overgeneralize the results. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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…tled The numbers were written up as a finding. They are one VM on one host, five flags changed at once, and no mechanism. Three things follow that the earlier wording did not admit. Which flag matters is unknown, because all five were flipped together. The mechanism is unknown: there is no good account of why timer and APIC enlightenments would move 4K random read throughput this far, and the plausible one about VM exits is invented rather than established. And the effect may be something that correlates with the config change rather than the flags themselves. The percentage column is dropped from the table. It was the most quotable and least defensible part, and a reader lifting "+59%" out of a single-VM local-disk test into a capacity plan is the likely failure mode. Absolute medians stay, so the measurement is still legible. The template comparison is separated out and marked as the part that is certain, since it is only reading parameters back and any reader can reproduce it in seconds. The guest-side timer revert and the missing-or-false condition from the previous commit are untouched.
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Inline comments:
In `@content/posts/2026-06-29-fastest-windows-on-xcp-ng.adoc`:
- Around line 229-235: Update the bcdedit instructions in the Windows Server
2025 troubleshooting section to require an elevated Administrator PowerShell
prompt and explicitly tell readers to restart Windows after modifying the BCD
store so the changes take effect.
- Around line 214-227: Expand the diskspd measurement description to state the
exact command and cache controls used. If -Sh was used, clarify that it bypasses
Windows software caching and requests write-through, but does not establish that
host-side or storage caches were bypassed; also reconcile the methodology as two
five-iteration runs per configuration yielding eight retained measurements after
discarding first iterations.
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Two things the block left implicit and a reader would only find out by running it. bcdedit refuses to write the BCD store from a non-elevated prompt, and the store is consulted at boot, so deleting the values changes nothing in the running guest. Someone chasing the 0% CPU hang would run both lines, see no change, and conclude the flags were not the problem after all.
"diskspd with caching disabled" claimed more than was controlled. -Sh is guest-side: it makes Windows open the file unbuffered and write-through, and says nothing about dom0, blkback or tapdisk. Nothing was done about those layers, so the sentence contradicted the caveat three paragraphs down that attributes one run's upward drift to host caching. Both now point the same way. The two invocations replace the paraphrase, so the numbers can be reproduced rather than taken on trust. -w0 means pure read, which is why the write-through half of -Sh never came into play. The discarded first iteration is worth its line too: on the queue-depth-1 test it returned 28.8 MiB/s against a 10.4-13.4 body. The warm-cache effect is large and sits in the raw data, which is evidence for the uncontrolled lower layers rather than a reason to wave the discard away. An earlier three-iteration run exists and is deliberately not in the post. It ran with four other VMs up and showed a 3.8x spread between identical runs.
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Summary
Adds a post-publication section to the Windows-on-XCP-ng post about the Viridian platform flags that the "Other install media" template leaves unset.
A colleague's support question sent me back to look at my own VM. Building from that template is still the right call for getting the imported disk to boot, but it sets
viridian: trueand stops there, where the Windows Server 2025 template sets all six parameters. My VM had been running on one flag out of six the whole time.The first half of the section is only reading template parameters back, and I stand behind it. The second half is a benchmark, and it is deliberately left unexplained. diskspd, 4K random read, caching disabled, alternating between one flag and six so that a reboot could not quietly take the credit. The two groups did not overlap and the run-to-run spread tightened. But I flipped five flags at once, so I cannot say which one matters, and I have no mechanism to offer for why timer and APIC enlightenments would move 4K random read this far. I can invent one, which is the thing I am trying not to do. The table gives medians and no percentages, and the text asks people not to quote it as a general figure.
Pre-PR review
Two of the four automated reviewers were available. The other two had spent their quota for the period, so this was a two-of-four gate, not a clean pass from four.
Fixed before opening:
falsewalks straight through that test, and I had leftviridianitself off the list even though thexe vm-param-setblock below it already wrote that key. It now reads missing-or-false, with all six named.useplatformclockanduseplatformtickif boot parameters were ever set to disable the synthetic timer. Both are in the post now.I checked both against docs.xcp-ng.org rather than taking the reviewer's word for it. The six parameter names and the
xesyntax in the post match that page exactly.A reviewer also flagged that the table said "average latency" while the caveat underneath said medians, and that "run" was doing two jobs in neighbouring paragraphs. The last commit settles both by stating the aggregation once above the table and dropping the percentage column.
Still open:
Test plan
build.ymlrenders the post. No local build here: hugo is not installed and the localasciidoctorbinary cannot execute{current}shows up literally in the bcdedit block