From mboxrd@z Thu Jan 1 00:00:00 1970 From: Neil Brown Subject: Re: md-raid and block sizes Date: Tue, 28 Dec 2010 22:45:13 +1100 Message-ID: <20101228224513.31e71638@notabene.brown> References: <1293451228.3266.9.camel@Waves.darkmere> <1293525980.23780.2.camel@Waves.darkmere> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: hansbkk@gmail.com Cc: "D.S. Ljungmark" , doug@easyco.com, linux-raid@vger.kernel.org List-Id: linux-raid.ids On Tue, 28 Dec 2010 18:29:26 +0700 hansbkk@gmail.com wrote: > This doesn't actually relate to the blocksize issue, but a caveat - > I've heard that these "green" drives are not suitable for use in a > RAID. > > The specific issue is apparently that these drives spin down very > frequently, but most RAID implementations keep spinning them back up > again just as frequently (perhaps unnecessarily?), thus causing undue > wear and tear on the drives' mechanics and ultimately premature > failure. > After a brief period of no writes, md will update the bitmap and/or the superblock to record that the array is clean (it may update the bitmap at other times too, but that is not relevant here). If the auto-spindown time of the drive is less than the delay-before-marking the-array-clean of md, then you could get extra spin-ups. The delay for updating the superblock is in sysfs in the md/safe_mode_timeout file which defaults to 0.2 seconds (200msec). The delay for updating the bitmap is set by an mdadm option (--bitmap-delay or something like that) when adding a bitmap to an array, and I think is available in sysfs in md/bitmap/something in recent kernels. The actual delay before a write is between 2 and 3 times this number. I think it defaults to 5 seconds (hence 10 to 15 second delay). So if the drive spins down sooner than 15 seconds after the last IO, there could be a problem but tuning md can git rid of that problem. If the drive spin-down time is longer than 15 seconds, they should be no unnecessary spin-ups. If anyone has any data on default spin-down times of these "green" drives I would be keen to hear about it. Thanks, NeilBrown