Re: [nfsv4] New version of sparse draft(draft-hildebrand-nfsv4-read-sparse-01.txt)

[Thomas Haynes <[e-mail]>]

The harder case is for a non-clustered pNFS community, i.e., one
where the MDS and DSes need a control protocol to talk. Let's
assume that is the case here.

A hole is created when a WRITE happens at a DS that is not
contiguous with the previous (if any) WRITE.

If a DS is supposed to return the range until the next non-zero
data, then it will need to contact every other DS or perhaps
the MDS.

If the MDS is supposed to honor a GET_SPARSE_BLOCK_MAP
type of request from the client, then either each DS would have
to send hole entries to the MDS or the MDS would have to
probe each one.

What if instead of a DS returning information about other DSes,
it simply returns information about what it knows? I.e., my next
data is at address X.

Consider a file with a width of 32k. The first write it gets is at 1M
and then it gets 5 other contiguous writes. The next data is at
2M and it gets 4 other contiguous writes. The DS can't assume
that any other DS has zeros in the first 1M. Worst case, all other
N-1 DSes do have data there.

In that scenario, N-1 DSes will return 32k on the first read and
1 DS will return a hole for it only until 1M. The client will then
only send N-1 READs until it it gets to the 1M mark.