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Add UBLK docs

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Vitaliy Filippov
2025-08-24 16:55:58 +03:00
parent 31b7021330
commit db63e58b3d
13 changed files with 146 additions and 49 deletions
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docs/usage/qemu.en.md
+8 -15
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@@ -130,23 +130,16 @@ Linux kernel, starting with version 5.15, supports a new interface for attaching
to the host - VDUSE (vDPA Device in Userspace). QEMU, starting with 7.2, has support for
exporting QEMU block devices over this protocol using qemu-storage-daemon.
VDUSE is currently the best interface to attach Vitastor disks as kernel devices because:
- It avoids data copies and thus achieves much better performance than [NBD](nbd.en.md)
- It doesn't have NBD timeout problem - the device doesn't die if an operation executes for too long
VDUSE advantages:
- VDUSE copies memory 1 time instead of 2, and is thus faster than [NBD](nbd.en.md) for linear read/write.
- It doesn't have NBD timeout problem - the device doesn't die if an operation executes for too long.
- It doesn't have hung device problem - if the userspace process dies it can be restarted (!)
and block device will continue operation
- It doesn't seem to have the device number limit
and block device will continue operation (UBLK can do it too).
- It doesn't seem to have the device number limit (UBLK also doesn't).
Example performance comparison:
| | direct fio | NBD | VDUSE |
|----------------------|-------------|-------------|-------------|
| linear write | 3.85 GB/s | 1.12 GB/s | 3.85 GB/s |
| 4k random write Q128 | 240000 iops | 120000 iops | 178000 iops |
| 4k random write Q1 | 9500 iops | 7620 iops | 7640 iops |
| linear read | 4.3 GB/s | 1.8 GB/s | 2.85 GB/s |
| 4k random read Q128 | 287000 iops | 140000 iops | 189000 iops |
| 4k random read Q1 | 9600 iops | 7640 iops | 7780 iops |
At the same time, VDUSE may be slower or faster than [UBLK](ublk.en.md) for linear read/write,
and iops-wise it's sometimes even slower than NBD. See performance comparison examples at the page [UBLK](ublk.en.md).
To try VDUSE you need at least Linux 5.15, built with VDUSE support
(CONFIG_VDPA=m, CONFIG_VDPA_USER=m, CONFIG_VIRTIO_VDPA=m).