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第5章 Optimizing LVM-VDO performance

The VDO kernel driver speeds up tasks by using multiple threads. It breaks I/O requests into smaller parts handled by different threads that communicate with each other. This allows one thread to manage shared data efficiently without frequent locking and unlocking. When one thread finishes a task, VDO already has another task ready for it. This keeps the threads busy and reduces the time spent switching tasks. VDO also uses separate threads for slower tasks, such as adding I/O operations to the queue or handling messages to the deduplication index.

5.1. VDO thread types
リンクのコピー

VDO uses various thread types to handle specific operations including I/O processing, compression, deduplication, and other storage management tasks.

Following are the thread types used by VDO:

Logical zone threads (vdo:logQ)
Maintain the mapping between the logical block numbers (LBNs) presented to the user of the VDO device and the physical block numbers (PBNs) in the underlying storage system. They also prevent concurrent writes to the same block. Logical threads are active during both read and write operations. Processing is generally evenly distributed, however, specific access patterns may occasionally concentrate work in one thread. For example, frequent access to LBNs in a specific block map page might make one logical thread handle all those operations.
Physical zone threads (vdo:physQ)
Handle data block allocation and reference counts during write operations.
I/O submission threads (vdo:bioQ)
Handle the transfer of block I/O (bio) operations from VDO to the storage system. They handle I/O requests from other VDO threads and pass them to the underlying device driver. These threads interact with device-related data structures, create requests for device driver kernel threads, and prevent delays when I/O requests get blocked due to a full device request queue.
CPU-processing threads (vdo:cpuQ)
Handle CPU-intensive tasks that do not block or need exclusive access to data structures managed by other thread types. These tasks include calculating hash values and compressing data blocks.
I/O acknowledgment threads (vdo:ackQ)
Signal the completion of I/O requests to higher-level components, such as the kernel page cache or application threads performing direct I/O. Their CPU usage and impact on memory contention are influenced by kernel-level code.
Hash zone threads (vdo:hashQ)
Coordinate I/O requests with matching hashes to handle potential deduplication tasks. Although they create and manage deduplication requests, they do not perform significant computations. A single hash zone thread is usually sufficient.
Deduplication thread (vdo:dedupeQ)
Handles I/O requests and communicates with the deduplication index. This work is performed on a separate thread to prevent blocking. It also has a timeout mechanism to skip deduplication if the index does not respond quickly. There is only one deduplication thread per VDO device.
Journal thread (vdo:journalQ)
Updates the recovery journal and schedules journal blocks for writing. This task cannot be divided among multiple threads. There is only one journal thread per VDO device.
Packer thread (vdo:packerQ)
Works during write operations when the compression is enabled. It collects compressed data blocks from the CPU threads to reduce wasted space. There is only one packer thread per VDO device.
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