linux-ext4 - Re: ext4 data=writeback performs worse than data=ordered now

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Message-ID: <20111215122759.7ce0b7b5@notabene.brown>
Date:	Thu, 15 Dec 2011 12:27:59 +1100
From:	NeilBrown <neilb@...e.de>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	"Li, Shaohua" <shaohua.li@...el.com>, Ted Ts'o <tytso@....edu>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	linux-raid@...r.kernel.org, Jens Axboe <axboe@...nel.dk>
Subject: Re: ext4 data=writeback performs worse than data=ordered now

On Thu, 15 Dec 2011 09:00:10 +0800 Wu Fengguang <fengguang.wu@...el.com>
wrote:

> > I found sometimes one disk hasn't any request inflight, but we can't
> > send request to the disk, because the scsi host's resource (the queue
> > depth) is used out, looks we send too many requests from other disks and
> > leave some disks starved. The resource imbalance in scsi isn't a new
> > problem, even 3.1 has such issue, so I'd think writeback introduces new
> > imbalance between the 12 disks. In fact, if I limit disk's queue depth
> > to 10, in this way the 12 disks will not impact each other in scsi
> > layer, the performance regression fully disappears for both writeback
> > and order mode.
> 
> I observe similar issue in MD. The default
> 
>         q->nr_requests = BLKDEV_MAX_RQ;
> 
> is too small for large arrays, and I end up doing
> 
>         echo 1280 > /sys/block/md0/queue/nr_requests
> 
> in my tests.

And you find this makes a difference?

That is very surprising because md devices don't use requests (and really use
the 'queue' at all) and definitely don't make use of nr_requests.

NeilBrown

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