microsoft/FASTER · error · FasterException

Object log device should not have fixed segment size. Set…

Error message

Object log device should not have fixed segment size. Set preallocateFile to false when calling CreateLogDevice for object log

What it means

Thrown by the GenericAllocator constructor when the object log device was created with a fixed segment size (SegmentSize != -1). The object log stores variable-size serialized objects and relies on dynamic sizing, so a preallocated, fixed-segment device is incompatible with it. FASTER rejects this configuration at construction time.

Solutions

Create the object log with preallocateFile: false so SegmentSize stays -1 (dynamic).
Check objectLogDevice.SegmentSize in config code and fall back to a dynamically-sized device.
If the backend forces fixed segments, choose a different device implementation (e.g. local file device without preallocation) for the object log.

Example fix

// before
LogSettings.ObjectLogDevice = Devices.CreateLogDevice("data.obj.log", preallocateFile: true);

// after
LogSettings.ObjectLogDevice = Devices.CreateLogDevice("data.obj.log", preallocateFile: false);

Defensive patterns

Strategy: validation

Validate before calling

// Ensure the object log device was created dynamically sized
if (settings.ObjectLogDevice != null && settings.ObjectLogDevice.SegmentSize != -1)
    throw new InvalidOperationException("ObjectLogDevice must be created with preallocateFile: false (SegmentSize == -1)");

Type guard

static bool IsDynamicSegmentDevice(IDevice d) => d == null || d.SegmentSize == -1;

Try / catch

try
{
    return new FASTERKeyedStore(settings);
}
catch (FasterException ex) when (ex.Message.Contains("fixed segment size"))
{
    settings.LogSettings.ObjectLogDevice?.Dispose();
    settings.LogSettings.ObjectLogDevice = Devices.CreateLogDevice("data.obj.log", preallocateFile: false);
    return new FASTERKeyedStore(settings);
}

Prevention

Always pass preallocateFile: false when calling CreateLogDevice for the object log.
Never reuse the main log's device-creation flags for the object log.
Check SegmentSize in a config sanity check before constructing the store.

When it happens

Trigger: Calling Devices.CreateLogDevice(path) for the object log with preallocateFile: true (or a device type that yields a fixed SegmentSize) and passing it as LogSettings.ObjectLogDevice to an allocator whose keys/values contain objects.

Common situations: Copying device-creation code from the main log (which commonly preallocates) to the object log; storage backends like AzureStorageDevice where preallocation is a flag; template setups that hardcode preallocateFile: true.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

AI-assisted analysis of microsoft/FASTER@321d872eab (2026-09-15). Data as JSON: /api/errors/e956e84ac78e3e9c. Report an issue: GitHub.

Appendix: source

Thrown at cs/src/core/Allocator/GenericAllocator.cs:85

            {
#if DEBUG
                if (typeof(Value) != typeof(byte[]) && typeof(Value) != typeof(string))
                    Debug.WriteLine("Value is not blittable, but no serializer specified via SerializerSettings. Using (slow) DataContractSerializer as default.");
#endif
                SerializerSettings.valueSerializer = ObjectSerializer.Get<Value>();
            }

            values = new Record<Key, Value>[BufferSize][];
            segmentOffsets = new long[SegmentBufferSize];

            objectLogDevice = settings.ObjectLogDevice;

            if ((settings.LogDevice as NullDevice == null) && (KeyHasObjects() || ValueHasObjects()))
            {
                if (objectLogDevice == null)
                    throw new FasterException("Objects in key/value, but object log not provided during creation of FASTER instance");
                if (objectLogDevice.SegmentSize != -1)
                    throw new FasterException("Object log device should not have fixed segment size. Set preallocateFile to false when calling CreateLogDevice for object log");
            }
        }

        internal override int OverflowPageCount => overflowPagePool.Count;

        public override void Reset()
        {
            base.Reset();
            objectLogDevice.Reset();
            for (int index = 0; index < BufferSize; index++)
            {
                ReturnPage(index);
            }

            Array.Clear(segmentOffsets, 0, segmentOffsets.Length);
            Initialize();
        }

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