Validating data transfers between HDFS and Cloud Storage

When you're copying or moving data between distinct storage systems such as multiple Apache Hadoop Distributed File System (HDFS) clusters or between HDFS and Cloud Storage, it's a good idea to perform some type of validation to guarantee data integrity. This validation is essential to be sure data wasn't altered during transfer.

While various mechanisms already ensure point-to-point data integrity in transit (such as TLS for all communication with Cloud Storage), explicit end-to-end data integrity validation adds protection for cases that may go undetected by typical in-transit mechanisms. This can help you detect potential data corruption caused, for example, by noisy network links, memory errors on server computers and routers along the path, or software bugs (such as in a library that customers use).

For Cloud Storage, this validation happens automatically client-side with commands like gsutil cp and rsync. Those commands compute local file checksums, which are then validated against the checksums computed by Cloud Storage at the end of each operation. If the checksums do not match, gsutil deletes the invalid copies and prints a warning message. This mismatch rarely happens, and if it does, you can retry the operation.

Now there's also a way to automatically perform end-to-end, client-side validation in Apache Hadoop across heterogeneous Hadoop-compatible file systems like HDFS and Cloud Storage. This article describes how the new feature lets you efficiently and accurately compare file checksums.

How HDFS performs file checksums

HDFS uses CRC32C, a 32-bit cyclic redundancy check (CRC) based on the Castagnoli polynomial, to maintain data integrity in different contexts:

  • At rest, Hadoop DataNodes continuously verify data against stored CRCs to detect and repair bit-rot.
  • In transit, the DataNodes send known CRCs along with the corresponding bulk data, and HDFS client libraries cooperatively compute per-chunk CRCs to compare against the CRCs received from the DataNodes.
  • For HDFS administrative purposes, block-level checksums are used for low-level manual integrity checks of individual block files on DataNodes.
  • For arbitrary application-layer use cases, the FileSystem interface defines getFileChecksum, and the HDFS implementation uses its stored fine-grained CRCs to define a file-level checksum.

For most day-to-day uses, the CRCs are used transparently with respect to the application layer. The only CRCs used are the per-chunk CRC32Cs, which are already precomputed and stored in metadata files alongside block data. The chunk size is defined by dfs.bytes-per-checksum and has a default value of 512 bytes.

Shortcomings of Hadoop's default file checksum type

By default when using Hadoop, all API-exposed checksums take the form of an MD5 of a concatenation of chunk CRC32Cs, either at the block level through the low-level DataTransferProtocol, or at the file level through the top-level FileSystem interface. A file-level checksum is defined as the MD5 of the concatenation of all the block checksums, each of which is an MD5 of a concatenation of chunk CRCs, and is therefore referred to as an MD5MD5CRC32FileChecksum. This is effectively an on-demand, three-layer Merkle tree.

This definition of the file-level checksum is sensitive to the implementation and data-layout details of HDFS, namely the chunk size (default 512 bytes) and the block size (default 128 MB). So this default file checksum isn't suitable in any of the following situations:

  • Two different copies of the same files in HDFS, but with different per-file block sizes configured.
  • Two different instances of HDFS with different block or chunk sizes configured.
  • A combination of HDFS and non-HDFS Hadoop-compatible file systems (HCFS) such as Cloud Storage.

The following diagram shows how the same file can end up with different checksums depending on the file system's configuration:

Checksum mismatch

You can display the default checksum for a file in HDFS by using the Hadoop fs -checksum command:

hadoop fs -checksum hdfs:///user/bob/data.bin

In an HDFS cluster that has a block size of 64 MB (dfs.block.size=67108864), this command displays a result like the following:

hdfs:///user/bob/data.bin   MD5-of-131072MD5-of-512CRC32C   000002000000000000020000e9378baa1b8599e50cca212ccec2f8b7

For the same file in another cluster that has a block size of 128 MB (dfs.block.size=134217728), you see a different output:

hdfs:///user/bob/data.bin   MD5-of-0MD5-of-512CRC32C    000002000000000000000000d3a7bae0b5200ed6707803a3911959f9

You can see in these examples that the two checksums differ for the same file.

How Hadoop's new composite CRC file checksum works

A new checksum type, tracked in HDFS-13056, was released in Apache Hadoop 3.1.1 to address these shortcomings. The new type, configured by dfs.checksum.combine.mode=COMPOSITE_CRC, defines new composite block CRCs and composite file CRCs as the mathematically composed CRC across the stored chunk CRCs. Using this type replaces using an MD5 of the component CRCs in order to calculate a single CRC that represents the entire block or file and is independent of the lower-level granularity of chunk CRCs.

CRC composition has many benefits — it is efficient; it allows the resulting checksums to be completely chunk/block agnostic; and it allows comparison between striped and replicated files, between different HDFS instances, and between HDFS and other external storage systems. (You can learn more details about the CRC algorithm in this PDF download.)

The following diagram provides a look at how a file's checksum is consistent after transfer across heterogeneous file system configurations:

Checksum match

This feature is minimally invasive: it can be added in place to be compatible with existing block metadata, and doesn't need to change the normal path of chunk verification. This also means even large preexisting HDFS deployments can adopt this feature to retroactively sync data. For more details, you can download the full design PDF document.

Using the new composite CRC checksum type

To use the new composite CRC checksum type within Hadoop, set the dfs.checksum.combine.mode property to COMPOSITE_CRC (instead of the default value MD5MD5CRC). When a file is copied from one location to another, the chunk-level checksum type (that is, the property dfs.checksum.type that defaults to CRC32C) must also match in both locations.

You can display the new checksum type for a file in HDFS by passing the -Ddfs.checksum.combine.mode=COMPOSITE_CRC argument to the Hadoop fs -checksum command:

hadoop fs -Ddfs.checksum.combine.mode=COMPOSITE_CRC -checksum hdfs:///user/bob/data.bin

Regardless of the block size configuration of the HDFS cluster, you see the same output, like the following:

hdfs:///user/bob/data.bin   COMPOSITE-CRC32C    c517d290

For Cloud Storage, you must also explicitly set the Cloud Storage connector property fs.gs.checksum.type to CRC32C. This property otherwise defaults to NONE, causing file checksums to be disabled by default. This default behavior by the Cloud Storage connector is a preventive measure to avoid an issue with distcp, where an exception is raised if the checksum types mismatch instead of failing gracefully. The command looks like the following:

hadoop fs -Ddfs.checksum.combine.mode=COMPOSITE_CRC -Dfs.gs.checksum.type=CRC32C -checksum gs://[BUCKET]/user/bob/data.bin

The command displays the same output as in the previous example on HDFS:

gs://[BUCKET]/user/bob/data.bin COMPOSITE-CRC32C    c517d290

You can see that the composite CRC checksums returned by the previous commands all match, regardless of block size, as well as between HDFS and Cloud Storage. By using composite CRC checksums, you can now guarantee that data integrity is preserved when transferring files between all types of Hadoop cluster configurations.

If you are running distcp, as in the following example, the validation is performed automatically:

hadoop distcp -Ddfs.checksum.combine.mode=COMPOSITE_CRC -Dfs.gs.checksum.type=CRC32C hdfs:///user/bob/* gs://[BUCKET]/user/bob/

If distcp detects a file checksum mismatch between the source and destination during the copy, then the operation will fail and return a warning.

Accessing the feature

The new composite CRC checksum feature is available in Apache Hadoop 3.1.1 (see release notes), and backports to versions 2.7, 2.8 and 2.9 are in the works. It has been included by default in subminor versions of Cloud Dataproc 1.3 since late 2018.

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