HDFS

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HDFS - Design & Limitations

HDFS is designed for storing very large files with streaming data access patterns, running on clusters of commodity hardware. Let’s understand the design of HDFS

It is designed for very large files. “Very large” in this context means files that are hundreds of megabytes, gigabytes, or terabytes in size.

It is designed for streaming data access. It is built around the idea that the most efficient data processing pattern is a write-once, read-many-times pattern. A dataset is typically generated or copied from the source, and then various analyses are performed on that dataset over time. Each analysis will involve a large proportion, if not all, of the dataset, so the time to read the whole dataset is more important than the latency in reading the first record

It is designed for commodity hardware. Hadoop doesn’t require expensive, highly reliable hardware. It’s designed to run on the commonly available hardware that can be obtained from multiple vendors. HDFS is designed to carry on working without a noticeable interruption to the user in case of hardware failure.

It is also worth knowing the applications for which HDFS does not work so well.

HDFS does not work well for Low-latency data access. Applications that require low-latency access to data, in the tens of milliseconds range, will not work well with HDFS. HDFS is optimized for delivering high throughput and this may be at the expense of latency.

HDFS is not a good fit if we have a lot of small files. Because the namenode holds filesystem metadata in memory, the limit to the number of files in a filesystem is governed by the amount of memory on the namenode

If we have multiple writers and arbitrary file modifications, HDFS will not a good fit. Files in HDFS are modified by a single writer at any time.

Writes are always made at the end of the file, in the append-only fashion.

There is no support for modifications at arbitrary offsets in the file.