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

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Message-ID: <20111215010010.GA14805@localhost>
Date:	Thu, 15 Dec 2011 09:00:10 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	"Li, Shaohua" <shaohua.li@...el.com>
Cc:	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>,
	NeilBrown <neilb@...e.de>, linux-raid@...r.kernel.org,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: ext4 data=writeback performs worse than data=ordered now

> 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.

Thanks,
Fengguang
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