| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Konstantin Knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru> |
| Cc: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, Sean Chittenden <seanc(at)joyent(dot)com> |
| Subject: | Re: WAL prefetch |
| Date: | 2018-06-14 13:25:03 |
| Message-ID: | CA+TgmoaMka+EtKo6Cntg6XnYCrMdMjXpS-e32EXu_Rsp0oTGfw@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thu, Jun 14, 2018 at 9:23 AM, Konstantin Knizhnik
<k(dot)knizhnik(at)postgrespro(dot)ru> wrote:
> Speed of random HDD access is limited by speed of disk head movement.
> By running several IO requests in parallel we just increase probability of
> head movement, so actually parallel access to HDD may even decrease IO speed
> rather than increase it.
> In theory, given several concurrent IO requests, driver can execute them in
> optimal order, trying to minimize head movement. But if there are really
> access to random pages,
> then probability that we can win something by such optimization is very
> small.
You might be right, but I feel like I've heard previous reports of
significant speedups from prefetching on HDDs. Perhaps I am
mis-remembering.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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