RE: RAID HDDs spin up sequence

[<brian.foster@emc.com>]

> -----Original Message-----
> From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
> owner@vger.kernel.org] On Behalf Of Piergiorgio Sartor
> Sent: Monday, January 31, 2011 5:24 PM
> To: Roberto Spadim
> Cc: Piergiorgio Sartor; linux-raid@vger.kernel.org
> Subject: Re: RAID HDDs spin up sequence
> 
> On Mon, Jan 31, 2011 at 07:09:24PM -0200, Roberto Spadim wrote:
> > you psu must be dimensioned to work with everythink at full work load
> > (it´s a real production NAS right?! not a test)
> > your SAS/IDE/SATA controller and HDD manual should be checked
> > how hdd wake up? one command (read/write) over sata/sas/ide channel
> wake it up?
> > on linux raid we have a read algorithm and a write algorithm
> > if a raid1 write occur all disks will wake up
> > if a raid1 (raid0 or another) read occur only the disk will wake up
> >
> > but check you SATA/IDE/SATA controller, how it wake up your disk, and
> > how you hdd wake up
> 
> Hi, thanks for the answer, unfortunately I was
> hoping to have made myself clear enough.
> 
> First of all, it is a RAID-6, so let's say that's
> already decided by requirements. With SATA HDDs.
> 
> Second, the question was exactly about how the HDDs
> are waked up. This is a SW issue, trying with normal
> setups, i.e. a couple of disks, it is possible to
> send them to sleep (hdparm -y /dev/hdX) and the wake
> them up by a simple access.
> I had no opportunity to check this with a RAID-5/6,
> so I was asking if anyone knows.
> 
> Finally, in order to be power efficient, the PSU,
> assuming something like an 80 Plus Gold, should work
> at not less than 20% of the nominal power, otherwise
> (according to some reviews), the efficiency drops far
> below the 80%~90% declared by the 80 Plus standard
> (which is measured at 20%, 50% and 100% of the maximum
> specified power).
> It seem it gets easily around 40%~50%.
> So, the PSU must be somehow under dimensioned for the
> spin up of 10 HDDs, which seem to require a possible
> 30W*10=300W (some nasty HDDs seem to require 30W, in
> this situation) only for the storage.
> 
> If the HDDs spin up one after the other, then the peak
> consumption is only 30W, which might allow a lower
> power PSU, in contrast with the requirement to provide
> 300W alone for the spin up.
> 
> So, back to the original question, if a 10 HDDs RAID-6
> is in standby, how do the single HDD will be waked up,
> in case of access? Of course, a quite larger access,
> i.e. some GiB of data.

We have a similar situation with our Iomega NAS products. We had a fairly crude locking mechanism implemented at the SCSI level that suits our needs to support staggered spin up. As indicated previously, we find that a 1 or 2 second delay between spin ups modulates the current draw enough such that we don't run into problems. We use this in conjunction with MD without much of a problem.

I'm not sure that the code as implemented is appropriate for mainline inclusion (and I'm not going to post it directly), but FYI the patches are included in the open source tarball that is made available on the support section of the iomega.com website (hint: check out the ix12 support & downloads section).

Brian

> 
> Thanks again,
> 
> bye,
> 
> --
> 
> piergiorgio
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