[Beowulf] Big storage

Jakob Oestergaard jakob at unthought.net
Thu Aug 23 06:37:05 PDT 2007

On Thu, Aug 23, 2007 at 07:56:15AM -0400, Joe Landman wrote:
> Greetings Jakob:

Hi Joe,

Thanks for answering!

> up front disclaimer: we design/build/market/support such things.

That does not disqualify you  :)

> > I'm looking at getting some big storage. Of all the parameters, getting as low
> > dollars/(month*GB) is by far the most important. The price of acquiring and
> > maintaining the storage solution is the number one concern.
> Should I presume density, reliability, and performance also factor in
> somewhere as 2,3,4 (somehow) on the concern list?

I expect that the major components of the total cost of running this beast will
be something like

 + power
 + cooling
 + payroll (disk-replacing admins :)

Real-estate is a concern as well, of course. The rent isn't free. It would be
nice to pack this in as few racks as possible.  Reliability, well... I expect
frequent drive failures, and I would expect that we'd run some form of RAID to
mitigate this. If the rest of the hardware is just reasonably well designed,
the most frequently failing components should be redundant and hot-swap
replacable (fans and PSUs).

It's acceptable that a head-node fails for a short period of time. The entire
system will not depend on all head nodes functioning simultaneously.

> > The setup will probably have a number of "head nodes" which receive a large
> > amount of data over standard gigabit from a large amount of remote sources.
> > Data is read infrequently from the head nodes by remote systems. The primary
> > load on the system will be data writes.
> Ok, so you are write dominated.  Could you describe (guesses are fine)
> what the writes will look like?  Large sequential data, small random
> data (seek, write, close)?

I would expect something like 100-1000 simultaneous streaming writes to just as
many files (one file per writer). The files will be everything from a few
hundred MiB to many GiB.

I guess that on most filesystems these streaming sequential writes will result
in something close to "random writes" to the block layer. However, we can be
very generous with write buffering.

> > The head nodes need not see the same unified storage; so I am not required to
> > have one big shared filesystem. If beneficial, each of the head nodes could
> > have their own local storage.
> There are some interesting designs with a variety of systems, including
> GFS/Lustre/... on those head nodes, and a big pool of drives behind
> them.  These designs will add to the overall cost, and increase complexity.

Simple is nice :)

> > The storage pool will start out at around 100TiB and will grow to ~1PiB within
> > a year or two (too early to tell). It would be nice to use as few racks as
> > possible, and as little power as possible  :)
> Ok, so density and power are important.  This is good.  Coupled with the
>  low management cost and low acquisition cost, we have about 3/4 of what
> we need.  Just need a little more description of the writes.

I hope the above helped.

> Also, do you intend to back this up? 

That is a *very* good question.

> How important is resiliency of the
> system?  Can you tolerate a failed unit (assume the units have hot
> spares, RAID6, etc). 

Yes. Single head nodes may fail. They must be fairly quick to get back on line
(having a replacement box I would expect no more than an hour of downtime).

> When you look at storage of this size, you have to
> start planning for the eventual (and likely) failure of a chassis (or
> some number of them), and think about with a RAIN configuration.

Yep. I don't know how likely a "many-disk" failure would be... If I have a full
replacement chassis, I would guess that I could simply pull out all the disks
from a failed system, move them to the replacement chassis and be up and
running again in "short" time.

If a PSU decides to fry everything connected to it including the disks, then
yes, I can see the point in RAIN or a full backup.

It's a business decision if a full node loss would be acceptable. I honestly
don't know that, but it is definitely interesting to consider both "yes" and

> Either
> that, or invest into massive low level redundancy (which should be scope
> limited to the box it is on anyway).

Yes; I had something like RAID-5 or so in mind on the nodes.

> > It *might* be possible to offload older files to tape; does anyone have
> > experience with HSM on Linux?  Does it work?  Could it be worthwhile to
> > investigate?
> Hmmm...  First I would suggest avoiding tape, you should likely be
> looking at disk to disk for backup, and use slower nearline mechanisms.

Why would you avoid tape?

Let's say there was software which allowed me to offload data to tape in a
reasonable manner. Considering the running costs of disk versus tape, tape
would win hands down on power, cooling and replacements.

Sure, the random seek time of a tape library sucks golf balls through a garden
hose, but assuming that one could live with that, are there more important
reasons to avoid tape?

> > One setup I was looking at, is simply using SunFire X4500 systems (you can put
> > 48 standard 3.5" SATA drives in each 4U system). Assuming I can buy them with
> > 1T SATA drives shortly, I could start out with 3 systems (12U) and grow the
> > entire setup to 1P with 22 systems in little over two full racks.
> > 
> > Any better ideas?  Is there a way to get this more dense without paying an arm
> > and a leg?  Has anyone tried something like this with HSM?
> Yes, but I don't want to turn this into a commercial, so I will be
> succinct.  Scalable Informatics (my company) has a similar product,
> which does have a good price and price per gigabyte, while providing
> excellent performance.  Details (white paper, benchmarks, presentations)
> at the http://jackrabbit.scalableinformatics.com web site.

Yep, I was just looking at that actually.

The hardware looks similar in concept to the SunFire, but as I see it you guys
have thought about a number of services atop of that (RAIN etc.)

Very interesting!


 / jakob

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