From: Antonin Houska <ah@cybertec.at>
To: pgsql-hackers@postgresql.org
Subject: Re: Slow I/O can break throttling of base backup
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Comments: In-reply-to Magnus Hagander <magnus@hagander.net>
   message dated "Thu, 15 Dec 2016 16:20:36 +0100."
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Date: Thu, 15 Dec 2016 17:21:33 +0100
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It seems to be my bug. I'll check tomorrow.

Magnus Hagander <magnus@hagander.net> wrote:

> Running pg_basebackup with a throttling of say 10M runs it into the risk =
of
> the I/O on the server actually being slower than pg_basebackup (I have
> preproduced similar issues on fake-slow disks with lower rate limits).
>=20
> What happens in this case in basebackup.c is that the value for "sleep"
> comes out negative. This means we don't sleep, which is fine.
>=20
> However, that also means we don't set throttle_last.
>=20
> That means that the next time we come around to throttle(), the value for
> "elapsed" is even bigger, which results in an even bigger negative number,
> and we're "stuck".
>=20
> AFAICT this means that a temporary I/O spike that makes reading of the di=
sk
> too slow can leave us in a situation where we never recover, and thus nev=
er
> end up sleeping ever again, effectively turning off rate limiting.
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> I wonder if the else if (sleep > 0) at the bottom of throttle() should ju=
st
> be a simple else and always run, resetting last_throttle?
>=20
> --=20
> Magnus Hagander
> Me: http://www.hagander.net/
> Work: http://www.redpill-linpro.com/
>=20

--=20
Antonin Houska
Cybertec Sch=C3=B6nig & Sch=C3=B6nig GmbH
Gr=C3=B6hrm=C3=BChlgasse 26
A-2700 Wiener Neustadt
Web: http://www.postgresql-support.de, http://www.cybertec.at


--=20
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