From: Roman Mamedov <rm@romanrm.net>
To: Doug Dumitru <doug@easyco.com>
Cc: Marc Roos <M.Roos@f1-outsourcing.eu>,
linux-raid <linux-raid@vger.kernel.org>
Subject: Re: Raid5 performance issue
Date: Sat, 24 Dec 2016 11:46:35 +0500 [thread overview]
Message-ID: <20161224114635.46c340f1@natsu> (raw)
In-Reply-To: <CAFx4rwQY3zF3cz-WV+L72mtmJodrYjO-kP1MUhC4YeX=zLJWmA@mail.gmail.com>
On Fri, 23 Dec 2016 11:24:13 -0800
Doug Dumitru <doug@easyco.com> wrote:
> With hard drives, I suspect your single disk tests are taking
> advantage of the disks' on-controller cache and is doing read-ahead
> and thus streaming. With the array in place, you are probably doing
> 512K reads (check the array chunk size) so the disks will see bursts
> of 512K reads with big gaps. The gaps are large enough that the
> rotation has gone too far and the caching makes you wait a rotation.
> This is just a guess.
You can compensate for that via setting a higher read-ahead for the array
itself, such as
blockdev --setra 8192 /dev/mdX
(or more)
--
With respect,
Roman
next prev parent reply other threads:[~2016-12-24 6:46 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <H0000071000db2da.1482500583.sx.f1-outsourcing.eu*@MHS>
2016-12-23 19:24 ` Raid5 performance issue Doug Dumitru
2016-12-24 6:46 ` Roman Mamedov [this message]
[not found] <"H0000071000db2da.1482500583.sx.f1-outsourcing.eu*"@MHS>
2016-12-23 19:27 ` Peter Grandi
2016-12-23 13:43 Marc Roos
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