[Sheepdog] Block write size

[John Ryan]

Trying to get my feet wet with Sheepdog. I understand that all vdisk are
chunked at a 4MB boundary. Suppose a write of 64 bytes needs to happen at
offset 390. This maps to the first 4MB chunk. In traditionally block
devices it is a read modify write where a read of 512 (assuming this is the
sector size) bytes occurs, sector that contains the offset, the value
updated with the 64 bytes from offset 390 and then the buffer is written
out to disk. How does this map to Sheepdog case? What is the equivalent of
sector size and how many bytes are accumulated before a write is pushed out
to disk?
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