linux-kernel - Re: Terrible performance of sequential O_DIRECT 4k writes in SAN environment. ~3 times slower then Solars 10 with the same HBA/Storage.

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Message-ID: <20140108152610.GA5863@infradead.org>
Date:	Wed, 8 Jan 2014 07:26:10 -0800
From:	Christoph Hellwig <hch@...radead.org>
To:	Sergey Meirovich <rathamahata@...il.com>
Cc:	Christoph Hellwig <hch@...radead.org>, Jan Kara <jack@...e.cz>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Gluk <git.user@...il.com>
Subject: Re: Terrible performance of sequential O_DIRECT 4k writes in SAN
 environment. ~3 times slower then Solars 10 with the same HBA/Storage.

On Wed, Jan 08, 2014 at 04:43:07PM +0200, Sergey Meirovich wrote:
> Results are almost the same:
>     14.68Mb/sec  3758.02 Requests/sec
>

On my laptop SSD I get the following results (sometimes up to 200MB/s,
sometimes down to 100MB/s, always in the 40k to 50k IOps range):

time elapsed (sec.):	5
bandwidth (MiB/s):	160.00
IOps:			40960.00

The IOps are more than the hardware is physically capable of, but given
that you didn't specify O_SYNC this seems sensible given that we never
have to flush the disk cache.

Could it be that your array has WCE=0?  In Linux we'll never enable the
cache automatically, but Solaris does at least when using ZFS.  Try
running:

   sdparm --set=WCE /dev/sdX

and try again.
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