# Sprites fsync issues

**URL:** <https://community.fly.io/t/sprites-fsync-issues/27068>\
**Category:** General\
**Tags:** storage, sprites, login-only\
**Created:** [February 6, 2026, 1:05am UTC](https://community.fly.io/t/sprites-fsync-issues/27068 "2026-02-06T01:05:00Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![daniel-horizon3](https://sea1.discourse-cdn.com/fly1/user_avatar/community.fly.io/daniel-horizon3/32/43375_2.png) [@daniel-horizon3](https://community.fly.io/u/daniel-horizon3)\
**Post date:** [February 6, 2026, 1:05am UTC](https://community.fly.io/t/sprites-fsync-issues/27068/1 "2026-02-06T01:05:00Z")

</div>

# **Sprite Disk I/O Report: Severe fsync Latency**

## **Summary**

Experiencing very slow disk operations across multiple sprites. Any operation requiring durable writes (git, package installs, config writes, login) is consistently slow – 3+ seconds for simple operations. The issue has been getting worse over time.

## **Environment**

- Affected sprites: `do-svsg`, `chorus-test-worker` (and others)

- Filesystem: overlay on ext4 (`/dev/vdb`, virtio-blk)

- Kernel: Firecracker microVM, virtio-mmio transport

- I/O scheduler: none

## **Diagnostic Results**

### **Buffered I/O is fine**

| Test | Result |
| --- | --- |
| Sequential write (100MB, dd) | 756 MB/s |
| Sequential read (100MB, dd) | 4.1 GB/s |
| 100 small file creates (no sync) | 7ms |
| 100 small file reads | 152ms |

### **fsync is extremely slow**

10 consecutive fsync operations took **9.56 seconds** total.

Individual fsync latencies (write one byte + sync):

| Call | Latency |
| --- | --- |
| 1 | 3,678 ms |
| 2 | 558 ms |
| 3 | 184 ms |
| 4 | 1,730 ms |
| 5 | 221 ms |

**Expected:** \<1ms on NVMe storage. **Observed:** 184ms - 3,678ms (200x-3600x slower than expected).

### **Additional observations**

- `virtio_balloon: Out of puff! Can't get 1 pages`appearing repeatedly in dmesg

- Load average spiked to 2.86 shortly after boot (1-min avg), suggesting I/O wait from background processes

- No I/O scheduler configured on any block device

## **Impact**

fsync is called by virtually every tool that writes data durably:

- **git** – status, commit, index updates

- **package managers** – npm, pip, apt

- **databases** – SQLite, Postgres

- **editors/IDEs** – saving files

- **shell** – writing history, config

This makes the sprites feel sluggish for all interactive use, not just specific workloads. A simple `git status` on a small repo takes 3+ seconds.

## **Reproduction**

```auto
for i in 1 2 3 4 5; do
  t=$(date +%s%N)
  echo "x" > /tmp/sync_test && sync
  t2=$(date +%s%N)
  echo "fsync $i: $(( (t2 - t) / 1000000 )) ms"
done
Copy

```

Expected: \<10ms per call. Actual: hundreds to thousands of ms.

---

<div class="post-metadata">

**Author:** ![system](https://global.discourse-cdn.com/fly1/original/3X/6/e/6e8955d00cb218e186ebbe3a6ab06fefdce15290.svg) [@system](https://community.fly.io/u/system)\
**Post date:** [February 13, 2026, 1:43am UTC](https://community.fly.io/t/sprites-fsync-issues/27068/3 "2026-02-13T01:43:55Z")

</div>

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