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From: Cyrill Gorcunov via Tarantool-patches <tarantool-patches@dev.tarantool.org>
To: Serge Petrenko <sergepetrenko@tarantool.org>
Cc: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>,
	tml <tarantool-patches@dev.tarantool.org>
Subject: Re: [Tarantool-patches] [PATCH v8 2/2] relay: provide information about downstream lag
Date: Thu, 17 Jun 2021 09:55:14 +0300
Message-ID: <YMrx0pyW4XyAMWzM@grain> (raw)
In-Reply-To: <73be2a27-dacf-c750-7bb6-da777b9c2f5a@tarantool.org>

On Tue, Jun 15, 2021 at 01:03:59PM +0300, Serge Petrenko wrote:
> > 
> > In other words this is a lag in seconds between the main node writes
> > data to own WAL and the replica `n` get this data replicated to own
> > WAL journal.
> 
> This is not true. You describe `upstream.lag` in this paragraph.
> Downstream lag is the time difference between the WAL write on master
> side and the receipt of an ack (confirmation of a WAL write on replica)
> for this transaction. Also on master side.

No. The upstream.lag is calulated as

applier_read_tx_row
  ...
  applier->lag = ev_now(loop()) - row->tm;

IOW it is a time between data is written to txn initiator to WAL and
the packet _received_ by replica, and there is no guarantee that the
packet will be *replicated* at all. In turn for downstream we compute
time delta when data is already writtent to a replica's WAL.

	Cyrill

  reply	other threads:[~2021-06-17  6:55 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-07 15:55 [Tarantool-patches] [PATCH v8 0/2] relay: provide downstream lag information Cyrill Gorcunov via Tarantool-patches
2021-06-07 15:55 ` [Tarantool-patches] [PATCH v8 1/2] applier: send transaction's first row WAL time in the applier_writer_f Cyrill Gorcunov via Tarantool-patches
2021-06-07 19:20   ` Vladislav Shpilevoy via Tarantool-patches
2021-06-15  9:36   ` Serge Petrenko via Tarantool-patches
2021-06-16 13:32     ` Cyrill Gorcunov via Tarantool-patches
2021-06-17  9:16       ` Serge Petrenko via Tarantool-patches
2021-06-07 15:55 ` [Tarantool-patches] [PATCH v8 2/2] relay: provide information about downstream lag Cyrill Gorcunov via Tarantool-patches
2021-06-07 19:21   ` Vladislav Shpilevoy via Tarantool-patches
2021-06-08  8:40     ` Cyrill Gorcunov via Tarantool-patches
2021-06-08  9:58       ` Cyrill Gorcunov via Tarantool-patches
2021-06-08 18:15       ` Vladislav Shpilevoy via Tarantool-patches
2021-06-15 10:03   ` Serge Petrenko via Tarantool-patches
2021-06-17  6:55     ` Cyrill Gorcunov via Tarantool-patches [this message]
2021-06-17  9:01       ` Serge Petrenko via Tarantool-patches
2021-06-17  9:58         ` Cyrill Gorcunov via Tarantool-patches
2021-06-07 19:20 ` [Tarantool-patches] [PATCH v8 0/2] relay: provide downstream lag information Vladislav Shpilevoy via Tarantool-patches
2021-06-07 20:00   ` Cyrill Gorcunov via Tarantool-patches

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