OpenZFS 8585 - improve batching done in zil_commit()

Authored by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Approved by: Dan McDonald <danmcd@joyent.com>
Ported-by: Prakash Surya <prakash.surya@delphix.com>

Problem

The current implementation of zil_commit() can introduce significant
latency, beyond what is inherent due to the latency of the underlying
storage. The additional latency comes from two main problems:

1. When there's outstanding ZIL blocks being written (i.e. there's already a "writer thread" in progress), then any new calls to zil_commit() will block waiting for the currently oustanding ZIL blocks to complete. The blocks written for each "writer thread" is coined a "batch", and there can only ever be a single "batch" being written at a time. When a batch is being written, any new ZIL transactions will have to wait for the next batch to be written, which won't occur until the current batch finishes. As a result, the underlying storage may not be used as efficiently as possible. While "new" threads enter zil_commit() and are blocked waiting for the next batch, it's possible that the underlying storage isn't fully utilized by the current batch of ZIL blocks. In that case, it'd be better to allow these new threads to generate (and issue) a new ZIL block, such that it could be serviced by the underlying storage concurrently with the other ZIL blocks that are being serviced.
2. Any call to zil_commit() must wait for all ZIL blocks in its "batch" to complete, prior to zil_commit() returning. The size of any given batch is proportional to the number of ZIL transaction in the queue at the time that the batch starts processing the queue; which doesn't occur until the previous batch completes. Thus, if there's a lot of transactions in the queue, the batch could be composed of many ZIL blocks, and each call to zil_commit() will have to wait for all of these writes to complete (even if the thread calling zil_commit() only cared about one of the transactions in the batch).

To further complicate the situation, these two issues result in the
following side effect:

3. If a given batch takes longer to complete than normal, this results in larger batch sizes, which then take longer to complete and further drive up the latency of zil_commit(). This can occur for a number of reasons, including (but not limited to): transient changes in the workload, and storage latency irregularites.

Solution

The solution attempted by this change has the following goals:

1. no on-disk changes; maintain current on-disk format.
2. modify the "batch size" to be equal to the "ZIL block size".
3. allow new batches to be generated and issued to disk, while there's already batches being serviced by the disk.
4. allow zil_commit() to wait for as few ZIL blocks as possible.
5. use as few ZIL blocks as possible, for the same amount of ZIL transactions, without introducing significant latency to any individual ZIL transaction. i.e. use fewer, but larger, ZIL blocks.

In theory, with these goals met, the new allgorithm will allow the
following improvements:

1. new ZIL blocks can be generated and issued, while there's already oustanding ZIL blocks being serviced by the storage.
2. the latency of zil_commit() should be proportional to the underlying storage latency, rather than the incoming synchronous workload.

Porting Notes

Due to the changes made in commit 119a394ab0, the lifetime of an itx
structure differs than in OpenZFS. Specifically, the itx structure is
kept around until the data associated with the itx is considered to be
safe on disk; this is so that the itx's callback can be called after the
data is committed to stable storage. Since OpenZFS doesn't have this itx
callback mechanism, it's able to destroy the itx structure immediately
after the itx is committed to an lwb (before the lwb is written to
disk).

To support this difference, and to ensure the itx's callbacks can still
be called after the itx's data is on disk, a few changes had to be made:

• A list of itxs was added to the lwb structure. This list contains all of the itxs that have been committed to the lwb, such that the callbacks for these itxs can be called from zil_lwb_flush_vdevs_done(), after the data for the itxs is committed to disk.
• A list of itxs was added on the stack of the zil_process_commit_list() function; the "nolwb_itxs" list. In some circumstances, an itx may not be committed to an lwb (e.g. if allocating the "next" ZIL block on disk fails), so this list is used to keep track of which itxs fall into this state, such that their callbacks can be called after the ZIL's writer pipeline is "stalled".
• The logic to actually call the itx's callback was moved into the zil_itx_destroy() function. Since all consumers of zil_itx_destroy() were effectively performing the same logic (i.e. if callback is non-null, call the callback), it seemed like useful code cleanup to consolidate this logic into a single function.

Additionally, the existing Linux tracepoint infrastructure dealing with
the ZIL's probes and structures had to be updated to reflect these code
changes. Specifically:

• The "zilcw1" and "zilcw2" probes were removed, so they had to be removed from "trace_zil.h" as well.
• Some of the zilog structure's fields were removed, which affected the tracepoint definitions of the structure.
• New tracepoints had to be added for the following 3 new probes:
  • zilprocesscommit__itx
  • zilprocessnormal__itx
  • zilcommitio__error

OpenZFS-issue: https://www.illumos.org/issues/8585
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/5d95a3a
Closes #6566