Re: [Beowulf] Big storage

[Jeffrey B. Layton]

Bruce Allen wrote:

Hi Jeff,

OK, I see the point. You are not worried about multiple unreadable sectors making it impossible to reconstruct lost data. You are worried about 'whole disk' failure.

Well, no actually. I'm worried about unrecoverable reads on the
remaining disks during reconstruction. :) Is that what you are referring
to?

I definitely agree that this is a possible problem. In fact we operate all of our UWM data archives (about 300 TB) as RAID-6 to reduce the probability of this. The idea of a second disk failing in a RAID-5 array during rebuild does not make for a good night's sleep!

Did you see Garth's comments? Even using a number of 500TB drives
greatly increases the probability of a URE during reconstruction. RAID-6
helps you sleep, but not as much as you think :) Scares the cr** out of me.
I'm looking to build a home server and I think I'm going to do RAID-61
to give myself some extra protection. I just have to figure out how to power
all of them and find a case where they can fit and a motherboard with enough
SATA connectors :)

Enjoy!

Jeff

Cheers,
    Bruce

On Fri, 24 Aug 2007, Jeffrey B. Layton wrote:

Bruce,

I urge you to read Garth's comments. Your description of what
RAID controllers do is very good when there are no failed drives.
If a drive fails though, you can't scan the disks looking for bad
sectors.

During a reconstruction, the RAID controller is reconstructing
the data based on the remaining drives and the parity.
Unfortunately, the controller is likely to be block based so it has
to rebuild every block of the failed disk. But if the controller is
doing a reconstruction and hits a URE, then the reconstruction
process just stops and the controller cries uncle. This means you
have to restore the failed array from a backup. This means the
entire volume.

With drives getting larger and larger all the time, the window of
vulnerability during reconstruction (where a second drive failure
will fail the entire volume) has grown because it takes longer and
longer to reconstruct so much data. This is why people are moving
to RAID-6. But RAID-6 is expensive in terms of capacity and performance
(Note: it has worse write performance than RAID-5). It gives the
ability to tolerate a second drive failure, but it may not reduce the
window of vulnerability during reconstruction because it takes longer
to reconstruct.

Here's an article where Garth talks about this (it's at the end):

http://www.eweek.com/article2/0,1895,2168821,00.asp

I wanted to note one quick thing from the article:

"The probability of the disk failing to read back data is the same as
it was long ago, so today you can expect at least one failed read every
10TB to 100TB. But the reconstruction of a failed 500GB disk in an
11-disk array has to read 5TB, so there can be an unacceptably large
chance of failure to rebuild every one of the 1 billion sectors on the
failed disk."

So if a reconstruction fails, you have to copy 5TB of data from the
backup to the volume. If you do this from tape - you're going to wait
a long time. You can do it from a disk backup but it still may take
some time to move 5TB across the wire depending upon how you
everything connected.

Jeff

Hi Jeff,

For this reason, in a RAID system with a lot of disks it is important to scan the disks looking for unreadable (UNC = uncorrectable) data blocks on a regular basis. If these are found, then the missing data at that Logical Block Address (LBA) has to be reconstructed from the *other* disks and re-written onto the failed disk.

In a well-designed (hardware or software) RAID implementation, you can reconstruct the missing data by only reading a handful of logical blocks from the redundant disks. It is not necessary to read the entire disk surface just to get a few 512 byte sectors of data. So a failure for different data somewhere else on a disk should not (in principle) prevent reconstruction of the lost/missing data. In a poorly-designed RAID implementation, you have to read the ENTIRE disk surface to get data from a few sectors. In this case, another uncorrectable disk sector can be crippling.

Most good hardware RAID cards have an option for continous disk scanning. For example ARECA called this 'consistency checking'. It should be done on a regular basis.

You can use smartmontools to do this also, by cayring out regular read scans of the disk surface and then forcing a RAID consistency check/rebuild if there is a read failure at some disk block.

Note that continous scanning is also needed for ECC memory to prevent correctable single-bit errors from becomming uncorrectable double-bit errors. In this RAM/memory context it is called 'memory scrubbing'

Cheers,
    Bruce

On Thu, 23 Aug 2007, Jeffrey B. Layton wrote:

This isn't really directed at Jeff, but it seemed like a good segway
for a comment. Everyone - please read some recent article by
Garth Gibson about large capacity disks and large number of
disks in a RAID group. Just to cut to the chase, given the
Unrecoverable Read Error (URE) rate and large disks, during
a rebuild you are almost guaranteed to hit a URE. When that
happens, the rebuild stops and you have to restore everything
from a backup. RAID-6 can help, but given enough disks and
large enough disks, the same thing can happen (plus RAID-6
rebuilds take longer since there are more computations involved).

Jeff

P.S. I guess I should disclose that my day job is at Panasas. But
regardless, I would recommend reading some of Garth's comments.
Maybe I can also get one of his presentations to pass around.

P.P.S. If you don't know Garth, he's one of the fathers of RAID.

Hello Jakob,
A couple of things...
1. ClusterFS has an easy to understand calculation on why raid 6 is
necessary for the amount of disks you're considering. You do need to
plan for multi-disk failure, especially with the rebuild time of 1TB
disks.

http://manual.lustre.org/manual/LustreManual16_HTML/DynamicHTML-10-1.html#wp1037512

2. Avoid tape if you can. At this scale, the administrative time and
costs far outweigh the benefits. Of course if you need to move your
data to a secure vault that's another thing. If you really want to do

tape, some people choose to do disk > disk > tape. This eliminates the

read interrupts on the primary storage and provides some added
redundancy.

3. We do use Nexsan's satabeasts for storage similar to this. Without
commenting on costs, the jackrabbit is technologically superior.

Thanks,
                jeff

On 8/23/07, Jakob Oestergaard <[EMAIL PROTECTED]> wrote:

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

   acquisition
 + 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

"no".

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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