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SergioS-1551 asked SumanthMarigowda-MSFT answered

Blob block limits


I am reviewing the scalability of the BLOB service and have the following doubt:

What is a block in a block blob? I am assuming that a block blob is an object

The table have an "Maximum number of blocks in a block blob or append blob (50.000)"

Can you help me clarify this limitation?

Thanks in advance


Sergio S

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1 Answer

SumanthMarigowda-MSFT avatar image
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SumanthMarigowda-MSFT answered

@SergioS-1551 Welcome to Microsoft Q&A Forum, Thank you for posting your query here!

Block blobs are optimized for uploading large amounts of data efficiently. Block blobs are composed of blocks, each of which is identified by a block ID. A block blob can include up to 50,000 blocks. Each block in a block blob can be a different size, up to the maximum size permitted for the service version in use. To create or modify a block blob, write a set of blocks via the Put Block operation and then commit the blocks to a blob with the Put Block List operation.

Block blobs store text and binary data. Block blobs are made up of blocks of data that can be managed individually. Block blobs can store up to about 190.7 TiB.

Blobs that are less than a certain size (determined by service version) can be uploaded in their entirety with a single write operation via Put Blob.


Storage clients default to a 128 MiB maximum single blob upload, settable in the Azure Storage client library for .NET version 11 by using the SingleBlobUploadThresholdInBytes property of the BlobRequestOptions object. When a block blob upload is larger than the value in this property, storage clients break the file into blocks. You can set the number of threads used to upload the blocks in parallel on a per-request basis using the ParallelOperationThreadCount property of the BlobRequestOptions object.

When you upload a block to a blob in your storage account, it is associated with the specified block blob, but it does not become part of the blob until you commit a list of blocks that includes the new block's ID. New blocks remain in an uncommitted state until they are specifically committed or discarded. There can be a maximum of 100,000 uncommitted blocks. Writing a block does not update the last modified time of an existing blob.

Block IDs are strings of equal length within a blob. Block client code usually uses base-64 encoding to normalize strings into equal lengths. When using base-64 encoding, the pre-encoded string must be 64 bytes or less. Block ID values can be duplicated in different blobs.

If you write a block for a blob that does not exist, a new block blob is created, with a length of zero bytes. This blob will appear in blob lists that include uncommitted blobs. If you don't commit any block to this blob, it and its uncommitted blocks will be discarded one week after the last successful block upload. All uncommitted blocks are also discarded when a new blob of the same name is created using a single step (rather than the two-step block upload-then-commit process).

Please let us know if you have any further queries. I’m happy to assist you further.

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