[Tarantool-patches] [PATCH v3] wal: introduce limits on simultaneous writes

Kirill Yukhin kyukhin at tarantool.org
Fri Mar 19 18:36:06 MSK 2021


On 24 фев 22:35, Serge Petrenko via Tarantool-patches wrote:
> Since the introduction of asynchronous commit, which doesn't wait for a
> WAL write to succeed, it's quite easy to clog WAL with huge amounts
> write requests. For now, it's only possible from an applier, since it's
> the only user of async commit at the moment.
> This happens when replica is syncing with master and reads new
> transactions at a pace higher than it can write them to WAL (see docbot
> request for detailed explanation).
> To ameliorate such behavior, we need to introduce some limit on
> not-yet-finished WAL write requests. This is what this commit is trying
> to do.
> Two new counters are added to wal writer: queue_size (in bytes) and
> queue_len (in wal messages) together with configuration settings:
> `wal_queue_max_size` and `wal_queue_max_len`.
> Size and length are increased on every new submitted request, and are
> decreased once the tx receives a confirmation that a specific request
> was written.
> Actually, the limits are added to an abstract journal, but take effect
> only for wal writer.
> Once size or length reach their maximum values, applier is blocked until
> some of the write requests are finished.
> The size limit isn't strict, i.e. if there's at least one free byte, the
> whole write request fits and no blocking is involved.
> The feature is ready for `box.commit{is_async=true}`. Once it's
> implemented, it should check whether the queue is full and let the user
> decide what to do next. Either wait or roll the tx back.
> Part of #5536

I've checked your patch into 2.6, 2.7 and master.

Regards, Kirill Yukhin

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