From mboxrd@z Thu Jan  1 00:00:00 1970
From: NeilBrown <neilb@suse.de>
Subject: Re: Reason for md raid1 max_sectors_kb limited to 127?
Date: Mon, 7 May 2012 21:18:46 +1000
Message-ID: <20120507211846.789d5808@notabene.brown>
References: <4FA798D7.4070506@profitbricks.com>
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To: Sebastian Riemer <sebastian.riemer@profitbricks.com>
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On Mon, 07 May 2012 11:41:43 +0200 Sebastian Riemer
<sebastian.riemer@profitbricks.com> wrote:

> Hi list,
>=20
> I'm wondering why max_sectors_kb is set to 127 with md raid1.
> This value is at 512 on normal HDDs.
>=20
> If I do a file copy on ext4 on a normal HDD then I can see with
> blktrace/blkparse that 128 KiB chunks are read and 512 KiB chunks are
> written.
>=20
> With md raid it looks like this: 124 KiB, 4 KiB, 124 KiB, 4 KiB, ... .
> This looks very inefficient.
>=20
> So, is there a particular reason that max_sectors_kb is set to 127?
>=20
> Cheers,
> Sebastian
>=20

You didn't say which kernel you are running.

However md/raid1 bases all those settings on the minimum or maximum (as
appropriate) of the setting of the underlying devices, using blk_stack_limi=
ts
(in block/blk-settings.c).

So the likely answer is that one of your HDDs has a smaller max_sectors_kb?

NeilBrown

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