Constant MAX_TXN_GROUP_COUNT

pub const MAX_TXN_GROUP_COUNT: u32 = 8;

Multiple block I/O operations may be sent at once before a response is actually sent back. Block I/O ops may be sent concurrently to different vmoids, and they also may be sent to different groups at any point in time.

MAX_TXN_GROUP_COUNT “groups” are pre-allocated lanes separated on the block server. Using a group allows multiple message to be buffered at once on a single communication channel before receiving a response.

Usage of groups is identified by the GROUP_ITEM flag, and is optional.

These groups may be referred to with a “groupid”, in the range [0, MAX_TXN_GROUP_COUNT).

The protocol to communicate with a single group is as follows:

1. SEND [N - 1] messages with an allocated groupid for any value of 1 <= N. The GROUP_ITEM flag is set for these messages.
2. SEND a final Nth message with the same groupid. The GROUP_ITEM | GROUP_LAST flags are set for this message.
3. RECEIVE a single response from the Block I/O server after all N requests have completed. This response is sent once all operations either complete or a single operation fails. At this point, step (1) may begin again for the same groupid.

For READ and WRITE, N may be greater than 1. Otherwise, N == 1 (skipping step (1) in the protocol above).

Notes:

• groupids may operate on any number of vmoids at once.
• If additional requests are sent on the same groupid before step (3) has completed, then the additional request will not be processed. If GROUP_LAST is set, an error will be returned. Otherwise, the request will be silently dropped.
• Messages within a group are not guaranteed to be processed in any order relative to each other.
• All requests receive responses, except for ones with GROUP_ITEM that do not have GROUP_LAST set.

For example, the following is a valid sequence of transactions:

-> (groupid = 1, vmoid = 1, OP = Write | GroupItem, reqid = 1) -> (groupid = 1, vmoid = 2, OP = Write | GroupItem, reqid = 2) -> (groupid = 2, vmoid = 3, OP = Write | GroupItem | GroupLast, reqid = 0) <- Response sent to groupid = 2, reqid = 0 -> (groupid = 1, vmoid = 1, OP = Read | GroupItem | GroupLast, reqid = 3) <- Response sent to groupid = 1, reqid = 3 -> (groupid = 3, vmoid = 1, OP = Write | GroupItem, reqid = 4) -> (groupid = don’t care, vmoid = 1, OP = Read, reqid = 5) <- Response sent to reqid = 5 -> (groupid = 3, vmoid = 1, OP = Read | GroupItem | GroupLast, reqid = 6) <- Response sent to groupid = 3, reqid = 6

Each transaction reads or writes up to length blocks from the device, starting at dev_offset blocks, into the VMO associated with vmoid, starting at vmo_offset blocks. If the transaction is out of range, for example if length is too large or if dev_offset is beyond the end of the device, ZX_ERR_OUT_OF_RANGE is returned.