numam-spdk/doc/blob.md

Blobstore Programmer's Guide

In this document

• @ref blob_pg_audience
• @ref blob_pg_intro
• @ref blob_pg_theory
• @ref blob_pg_design
• @ref blob_pg_examples
• @ref blob_pg_config
• @ref blob_pg_component

Target Audience

The programmer's guide is intended for developers authoring applications that utilize the SPDK Blobstore. It is intended to supplement the source code in providing an overall understanding of how to integrate Blobstore into an application as well as provide some high level insight into how Blobstore works behind the scenes. It is not intended to serve as a design document or an API reference and in some cases source code snippets and high level sequences will be discussed; for the latest source code reference refer to the repo.

Introduction

Blobstore is a persistent, power-fail safe block allocator designed to be used as the local storage system backing a higher level storage service, typically in lieu of a traditional filesystem. These higher level services can be local databases or key/value stores (MySQL, RocksDB), they can be dedicated appliances (SAN, NAS), or distributed storage systems (ex. Ceph, Cassandra). It is not designed to be a general purpose filesystem, however, and it is intentionally not POSIX compliant. To avoid confusion, we avoid references to files or objects instead using the term 'blob'. The Blobstore is designed to allow asynchronous, uncached, parallel reads and writes to groups of blocks on a block device called 'blobs'. Blobs are typically large, measured in at least hundreds of kilobytes, and are always a multiple of the underlying block size.

The Blobstore is designed primarily to run on "next generation" media, which means the device supports fast random reads and writes, with no required background garbage collection. However, in practice the design will run well on NAND too.

Theory of Operation

Abstractions

The Blobstore defines a hierarchy of storage abstractions as follows.

• Logical Block: Logical blocks are exposed by the disk itself, which are numbered from 0 to N, where N is the number of blocks in the disk. A logical block is typically either 512B or 4KiB.
• Page: A page is defined to be a fixed number of logical blocks defined at Blobstore creation time. The logical blocks that compose a page are always contiguous. Pages are also numbered from the beginning of the disk such that the first page worth of blocks is page 0, the second page is page 1, etc. A page is typically 4KiB in size, so this is either 8 or 1 logical blocks in practice. The SSD must be able to perform atomic reads and writes of at least the page size.
• Cluster: A cluster is a fixed number of pages defined at Blobstore creation time. The pages that compose a cluster are always contiguous. Clusters are also numbered from the beginning of the disk, where cluster 0 is the first cluster worth of pages, cluster 1 is the second grouping of pages, etc. A cluster is typically 1MiB in size, or 256 pages.
• Blob: A blob is an ordered list of clusters. Blobs are manipulated (created, sized, deleted, etc.) by the application and persist across power failures and reboots. Applications use a Blobstore provided identifier to access a particular blob. Blobs are read and written in units of pages by specifying an offset from the start of the blob. Applications can also store metadata in the form of key/value pairs with each blob which we'll refer to as xattrs (extended attributes).
• Blobstore: An SSD which has been initialized by a Blobstore-based application is referred to as "a Blobstore." A Blobstore owns the entire underlying device which is made up of a private Blobstore metadata region and the collection of blobs as managed by the application.

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Atomicity

For all Blobstore operations regarding atomicity, there is a dependency on the underlying device to guarantee atomic operations of at least one page size. Atomicity here can refer to multiple operations:

• Data Writes: For the case of data writes, the unit of atomicity is one page. Therefore if a write operation of greater than one page is underway and the system suffers a power failure, the data on media will be consistent at a page size granularity (if a single page were in the middle of being updated when power was lost, the data at that page location will be as it was prior to the start of the write operation following power restoration.)
• Blob Metadata Updates: Each blob has its own set of metadata (xattrs, size, etc). For performance reasons, a copy of this metadata is kept in RAM and only synchronized with the on-disk version when the application makes an explicit call to do so, or when the Blobstore is unloaded. Therefore, setting of an xattr, for example is not consistent until the call to synchronize it (covered later) which is, however, performed atomically.
• Blobstore Metadata Updates: Blobstore itself has its own metadata which, like per blob metadata, has a copy in both RAM and on-disk. Unlike the per blob metadata, however, the Blobstore metadata region is not made consistent via a blob synchronization call, it is only synchronized when the Blobstore is properly unloaded via API. Therefore, if the Blobstore metadata is updated (blob creation, deletion, resize, etc.) and not unloaded properly, it will need to perform some extra steps the next time it is loaded which will take a bit more time than it would have if shutdown cleanly, but there will be no inconsistencies.

Callbacks

Blobstore is callback driven; in the event that any Blobstore API is unable to make forward progress it will not block but instead return control at that point and make a call to the callback function provided in the API, along with arguments, when the original call is completed. The callback will be made on the same thread that the call was made from, more on threads later. Some API, however, offer no callback arguments; in these cases the calls are fully synchronous. Examples of asynchronous calls that utilize callbacks include those that involve disk IO, for example, where some amount of polling is required before the IO is completed.

Backend Support

Blobstore requires a backing storage device that can be integrated using the bdev layer, or by directly integrating a device driver to Blobstore. The blobstore performs operations on a backing block device by calling function pointers supplied to it at initialization time. For convenience, an implementation of these function pointers that route I/O to the bdev layer is available in bdev_blob.c. Alternatively, for example, the SPDK NVMe driver may be directly integrated bypassing a small amount of bdev layer overhead. These options will be discussed further in the upcoming section on examples.

Metadata Operations

Because Blobstore is designed to be lock-free, metadata operations need to be isolated to a single thread to avoid taking locks on in memory data structures that maintain data on the layout of definitions of blobs (along with other data). In Blobstore this is implemented as the metadata thread and is defined to be the thread on which the application makes metadata related calls on. It is up to the application to setup a separate thread to make these calls on and to assure that it does not mix relevant IO operations with metadata operations even if they are on separate threads. This will be discussed further in the Design Considerations section.

Threads

An application using Blobstore with the SPDK NVMe driver, for example, can support a variety of thread scenarios. The simplest would be a single threaded application where the application, the Blobstore code and the NVMe driver share a single core. In this case, the single thread would be used to submit both metadata operations as well as IO operations and it would be up to the application to assure that only one metadata operation is issued at a time and not intermingled with affected IO operations.

Channels

Channels are an SPDK-wide abstraction and with Blobstore the best way to think about them is that they are required in order to do IO. The application will perform IO to the channel and channels are best thought of as being associated 1:1 with a thread.

Blob Identifiers

When an application creates a blob, it does not provide a name as is the case with many other similar storage systems, instead it is returned a unique identifier by the Blobstore that it needs to use on subsequent APIs to perform operations on the Blobstore.

Design Considerations

Initialization Options

When the Blobstore is initialized, there are multiple configuration options to consider. The options and their defaults are:

• Cluster Size: By default, this value is 1MB. The cluster size is required to be a multiple of page size and should be selected based on the application’s usage model in terms of allocation. Recall that blobs are made up of clusters so when a blob is allocated/deallocated or changes in size, disk LBAs will be manipulated in groups of cluster size. If the application is expecting to deal with mainly very large (always multiple GB) blobs then it may make sense to change the cluster size to 1GB for example.
• Number of Metadata Pages: By default, Blobstore will assume there can be as many clusters as there are metadata pages which is the worst case scenario in terms of metadata usage and can be overridden here however the space efficiency is not significant.
• Maximum Simultaneous Metadata Operations: Determines how many internally pre-allocated memory structures are set aside for performing metadata operations. It is unlikely that changes to this value (default 32) would be desirable.
• Maximum Simultaneous Operations Per Channel: Determines how many internally pre-allocated memory structures are set aside for channel operations. Changes to this value would be application dependent and best determined by both a knowledge of the typical usage model, an understanding of the types of SSDs being used and empirical data. The default is 512.
• Blobstore Type: This field is a character array to be used by applications that need to identify whether the Blobstore found here is appropriate to claim or not. The default is NULL and unless the application is being deployed in an environment where multiple applications using the same disks are at risk of inadvertently using the wrong Blobstore, there is no need to set this value. It can, however, be set to any valid set of characters.

Sub-page Sized Operations

Blobstore is only capable of doing page sized read/write operations. If the application requires finer granularity it will have to accommodate that itself.

Threads

As mentioned earlier, Blobstore can share a single thread with an application or the application can define any number of threads, within resource constraints, that makes sense. The basic considerations that must be followed are:

• Metadata operations (API with MD in the name) should be isolated from each other as there is no internal locking on the memory structures affected by these API.
• Metadata operations should be isolated from conflicting IO operations (an example of a conflicting IO would be one that is reading/writing to an area of a blob that a metadata operation is deallocating).
• Asynchronous callbacks will always take place on the calling thread.
• No assumptions about IO ordering can be made regardless of how many or which threads were involved in the issuing.

Data Buffer Memory

As with all SPDK based applications, Blobstore requires memory used for data buffers to be allocated with SPDK API.

Error Handling

Asynchronous Blobstore callbacks all include an error number that should be checked; non-zero values indicate and error. Synchronous calls will typically return an error value if applicable.

Asynchronous API

Asynchronous callbacks will return control not immediately, but at the point in execution where no more forward progress can be made without blocking. Therefore, no assumptions can be made about the progress of an asynchronous call until the callback has completed.

Xattrs

Setting and removing of xattrs in Blobstore is a metadata operation, xattrs are stored in per blob metadata. Therefore, xattrs are not persisted until a blob synchronization call is made and completed. Having a step process for persisting per blob metadata allows for applications to perform batches of xattr updates, for example, with only one more expensive call to synchronize and persist the values.

Synchronizing Metadata

As described earlier, there are two types of metadata in Blobstore, per blob and one global metadata for the Blobstore itself. Only the per blob metadata can be explicitly synchronized via API. The global metadata will be inconsistent during run-time and only synchronized on proper shutdown. The implication, however, of an improper shutdown is only a performance penalty on the next startup as the global metadata will need to be rebuilt based on a parsing of the per blob metadata. For consistent start times, it is important to always close down the Blobstore properly via API.

Iterating Blobs

Multiple examples of how to iterate through the blobs are included in the sample code and tools. Worthy to note, however, if walking through the existing blobs via the iter API, if your application finds the blob its looking for it will either need to explicitly close it (because was opened internally by the Blobstore) or complete walking the full list.

The Super Blob

The super blob is simply a single blob ID that can be stored as part of the global metadata to act as sort of a "root" blob. The application may choose to use this blob to store any information that it needs or finds relevant in understanding any kind of structure for what is on the Blobstore.

Examples

There are multiple examples of Blobstore usage in the repo:

• Hello World: Actually named hello_blob.c this is a very basic example of a single threaded application that does nothing more than demonstrate the very basic API. Although Blobstore is optimized for NVMe, this example uses a RAM disk (malloc) back-end so that it can be executed easily in any development environment. The malloc back-end is a bdev module thus this example uses not only the SPDK Framework but the bdev layer as well.

• CLI: The blobcli.c example is command line utility intended to not only serve as example code but as a test and development tool for Blobstore itself. It is also a simple single threaded application that relies on both the SPDK Framework and the bdev layer but offers multiple modes of operation to accomplish some real-world tasks. In command mode, it accepts single-shot commands which can be a little time consuming if there are many commands to get through as each one will take a few seconds waiting for DPDK initialization. It therefore has a shell mode that allows the developer to get to a blob> prompt and then very quickly interact with Blobstore with simple commands that include the ability to import/export blobs from/to regular files. Lastly there is a scripting mode to automate a series of tasks, again, handy for development and/or test type activities.

Configuration

Blobstore configuration options are described in the initialization options section under @ref blob_pg_design.

Component Detail

The information in this section is not necessarily relevant to designing an application for use with Blobstore, but understanding a little more about the internals may be interesting and is also included here for those wanting to contribute to the Blobstore effort itself.

Media Format

The Blobstore owns the entire storage device. The device is divided into clusters starting from the beginning, such that cluster 0 begins at the first logical block.

LBA 0                                   LBA N
+-----------+-----------+-----+-----------+
| Cluster 0 | Cluster 1 | ... | Cluster N |
+-----------+-----------+-----+-----------+

Cluster 0 is special and has the following format, where page 0 is the first page of the cluster:

+--------+-------------------+
| Page 0 | Page 1 ... Page N |
+--------+-------------------+
| Super  |  Metadata Region  |
| Block  |                   |
+--------+-------------------+

The super block is a single page located at the beginning of the partition. It contains basic information about the Blobstore. The metadata region is the remainder of cluster 0 and may extend to additional clusters. Refer to the latest source code for complete structural details of the super block and metadata region.

Each blob is allocated a non-contiguous set of pages inside the metadata region for its metadata. These pages form a linked list. The first page in the list will be written in place on update, while all other pages will be written to fresh locations. This requires the backing device to support an atomic write size greater than or equal to the page size to guarantee that the operation is atomic. See the section on atomicity for details.

Blob cluster layout

Each blob is an ordered list of clusters, where starting LBA of a cluster is called extent. A blob can be thin provisioned, resulting in no extent for some of the clusters. When first write operation occurs to the unallocated cluster - new extent is chosen. This information is stored in RAM and on-disk.

There are two extent representations on-disk, dependent on use_extent_table (default:true) opts used when creating a blob.

• use_extent_table=true: EXTENT_PAGE descriptor is not part of linked list of pages. It contains extents that are not run-length encoded. Each extent page is referenced by EXTENT_TABLE descriptor, which is serialized as part of linked list of pages. Extent table is run-length encoding all unallocated extent pages. Every new cluster allocation updates a single extent page, in case when extent page was previously allocated. Otherwise additionally incurs serializing whole linked list of pages for the blob.

• use_extent_table=false: EXTENT_RLE descriptor is serialized as part of linked list of pages. Extents pointing to contiguous LBA are run-length encoded, including unallocated extents represented by 0. Every new cluster allocation incurs serializing whole linked list of pages for the blob.

Sequences and Batches

Internally Blobstore uses the concepts of sequences and batches to submit IO to the underlying device in either a serial fashion or in parallel, respectively. Both are defined using the following structure:

struct spdk_bs_request_set;

These requests sets are basically bookkeeping mechanisms to help Blobstore efficiently deal with related groups of IO. They are an internal construct only and are pre-allocated on a per channel basis (channels were discussed earlier). They are removed from a channel associated linked list when the set (sequence or batch) is started and then returned to the list when completed.

Key Internal Structures

blobstore.h contains many of the key structures for the internal workings of Blobstore. Only a few notable ones are reviewed here. Note that blobstore.h is an internal header file, the header file for Blobstore that defines the public API is blob.h.

struct spdk_blob

This is an in-memory data structure that contains key elements like the blob identifier, its current state and two copies of the mutable metadata for the blob; one copy is the current metadata and the other is the last copy written to disk.

struct spdk_blob_mut_data

This is a per blob structure, included the struct spdk_blob struct that actually defines the blob itself. It has the specific information on size and makeup of the blob (ie how many clusters are allocated for this blob and which ones.)

struct spdk_blob_store

This is the main in-memory structure for the entire Blobstore. It defines the global on disk metadata region and maintains information relevant to the entire system - initialization options such as cluster size, etc.

struct spdk_bs_super_block

The super block is an on-disk structure that contains all of the relevant information that's in the in-memory Blobstore structure just discussed along with other elements one would expect to see here such as signature, version, checksum, etc.

Code Layout and Common Conventions

In general, Blobstore.c is laid out with groups of related functions blocked together with descriptive comments. For example,

/* START spdk_bs_md_delete_blob */
< relevant functions to accomplish the deletion of a blob >
/* END spdk_bs_md_delete_blob */

And for the most part the following conventions are followed throughout:

• functions beginning with an underscore are called internally only
• functions or variables with the letters cpl are related to set or callback completions