UDisk

The fastest ACID-transactional persisted Key-Value store designed for NVMe block-devices with GPU-acceleration and SPDK to bypass the Linux kernel.

Configuration Parameters

Most important parameters:

• directory: Where to keep the configs, schemas, data, and logs? The boot drive?
• memory_limit: How much RAM to use? 80% of it?
• value_max_size: How large the values can get?

Tuning performance?

• directories_data: Want to separate large data and put it on some RAID array?
• gpu_devices: Do you a have a GPU we can borrow for acceleration?

Versioning

Disk IO

Path where there database would store metadata:

"directory": "/etc/udisk/"

Paths where the database would store data, with tuples containing the path and a limit:

"data_directories": [
    {
        "path": "/mnt/md0/ukv/",
        "max_size": 1e12,
    },
    {
        "path": "/mnt/md1/ukv/",
        "max_size": 1e12,
    }
]

Choose the underlying IO mechanism:

"io_mechanism": "posix"
"io_mechanism": "pulling"
"io_mechanism": "polling"

For asynchronous IO mechanisms you can set the queue depth:

Random Access Memory

Limit the total amount of RAM available to the database:

"memory_limit": 1000000000

Limit the amount of memory to be used for caching. Greatly affects the performance of single reads. Doesn't affect the performance of batch reads.

Limit the maximum number of entries to be collected in-memory (and in the Write Ahead Log), before flushing them into files.

"write_buffer_max_elements": 100000

Limit the maximum combined size of entries to be collected in-memory (and in the Write Ahead Log), before flushing them into files.

"write_buffer_max_bytes": 100000000

Hardware Acceleration

Threads to be used for compactions, non-blocking garbage collection, and polling mechanisms. For optimal performance with high-end NVMe SSDs we suggest reserving 2 or 3 threads per SSD.

Define, how many threads may be accessing the database concurrently:

Optional array of integer identifiers for the GPUs to be used for background compactions:

"gpu_devices": "all"
"gpu_devices": [-1]
"gpu_devices": [0, 1, 2, 3]

The amount of Video RAM available to UDisk on the GPU:

"gpu_memory_limit": 1000000000

Structure

Define, how large the biggest value can be:

In some rare cases, you may have all of your values of the same size:

Define, how much larger every layer of the Log-Structured-Merge Tree can get:

"level_enlarge_factor": 4

How large the top-level of the Log-Structured-Merge Tree should be?

"first_level_max_bytes": 1000000000

Transactions

The maximum number of entries to be updated with a single transaction:

"transaction_max_elements": 100000

The maximum capacity of a single transaction, limiting the size of all updates in it:

"transaction_max_bytes": 100000000

Full Example

{
    "version": 1,
    "threads": 1,
    "memory_limit": 100000000000,
    "gpu_devices": [-1],
    "gpu_memory_limit": 1000000000,
    "directory": "./tmp/",
    "data_directories": [
        {
            "path": "/mnt/disk1/",
            "max_size": 100000000000
        },
        {
            "path": "/mnt/disk2/",
            "max_size": 100000000000
        }
    ],
    "io_mechanism": "pulling",
    "io_queue_depth": 4096,
    "transaction_max_elements": 1000,
    "transaction_max_bytes": 13421773,
    "concurrency_limit": 128,
    "cache_limit": 134217728,
    "write_buffer_max_elements": 100000,
    "write_buffer_max_bytes": 134217728,
    "first_level_max_bytes": 4831838208,
    "level_enlarge_factor": 4,
    "value_size": 0,
    "value_max_size": 4194304
}