From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michael Reed Subject: md question re: max_hw_sectors_kb Date: Tue, 03 May 2011 15:20:46 -0500 Message-ID: <4DC0639E.1070707@sgi.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org Cc: neilb@suse.de, Jeremy Higdon , Hannes Reinecke List-Id: linux-raid.ids Resending to linux-raid. Hi Neil, My name is Mike Reed. I work at SGI. I've been looking at a performance issue with md writes and have tracked it down to the md device not having a high enough initial max_hw_sectors_kb setting. There is code in blk_queue_make_request() which lowers the default value from INT_MAX to BLK_SAFE_MAX_SECTORS, which is 255. This is generally lower than all the underlying devices with which I use md. As md appears to be a stacking driver, i.e., it calls disk_stack_limits() for each member of a volume, it would seem reasonable for md to use the, INT_MAX setting for max_hw_sectors_kb instead of BLK_SAFE_MAX_SECTORS. I have tried this, and have observed that md correctly limits the md device's max_sectors_kb to the value of the underlying devices in my mirror volume. Is this the correct way to address this issue? Applies to linux-2.6.39-rc4. Signed-off-by: Michael Reed --- drivers/md/md.c 2011-04-18 21:26:00.000000000 -0700 +++ drivers/md/md.c.new 2011-04-21 15:53:11.452536201 -0700 @@ -4328,6 +4328,7 @@ static int md_alloc(dev_t dev, char *nam mddev->queue->queuedata = mddev; blk_queue_make_request(mddev->queue, md_make_request); + blk_queue_max_sectors(mddev->queue, INT_MAX); disk = alloc_disk(1 << shift); if (!disk) {