About --disable-writeback-cache options

[M-S4TO]

Question

Hello community,

I am currently using Blobfuse2 to mount Azure Blob Storage. I have come across several documents with the following comment:

azure-storage-fuse/setup/baseConfig.yaml

Lines 16 to 19 in f088b80

	# 6. By default 'writeback-cache' is enabled for libfuse3 and this may result in append/write operations to fail.
	# Either you can disable 'writeback-cache', which might hurt the performance
	# or you can configure blobfuse2 to ignore open flags given by user and make it work with ''writeback-cache'.
	# 'libfuse' sectoin below has both the configurations.

What I would like to know is whether setting the disable-writeback-cache option to true makes writes to Blob Storage bypass Blobfuse2 and directly write the data.

I am not very familiar with Unix or FUSE, so at first, I imagined that this flag was related to userland programs writing to a cache SSD mounted on a VM. However, as I looked into the code, I started to doubt my initial understanding.

Specifically, I would like to know which data's writeback cache is disabled when enabling the disable-writeback-cache option. I am also interested in how the behavior of writes to Blob Storage changes with this option enabled.

Thank you very much for your assistance.

Best regards,

[vibhansa-msft]

"write-back cache" is more inclined to kernel page caching behavior and has nothing to do with how Blobfuse upstream the data to storage container. You can use block-cache mode to upstream data to container as your write (there will be some buffering at client level and a periodic staging of the blocks). However, actual persistence will happen only when you close the file handle or fsync the data.

[M-S4TO]

Thank you very much for your detailed explanation regarding the "write-back cache" and its behavior with Blobfuse2. Your response clarified my doubts and provided the information I needed. I appreciate your assistance!

Best regards,