[Sheepdog] Block write size

[Liu Yuan]

On 12/16/2011 03:12 PM, John Ryan wrote:

> 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?
> 


Sheepdog uses system call pread/pwrite that do the object rw. So in your
case, sheep daedom just finds the targed replica and sends the requests
to the nodes. The sheep daemons running on those nodes will parse the
requests and call pread/pwrite finally.

So this operations (read/modify/write) for partial rw rely on the
underlying file system, I guess normally as you described above.

Thanks,
Yuan