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Gepf Forms Z894 Appeal Advice

morning so can I ask how many of you are.familiar with GPFS thank you and how.many of familiar with raid erase and use.them thank you.gives me understand everybody yep just.want to make sure yeah.my name is Veera I'm from IBM Almaden.Research Center in San Jose and I'm.gonna talk to you today about a feature.that we recently added to IBM gpfs it's.a parallel file system that we use.that's used in several HPC systems and.we built this native raid layer for.working on a hundred thousand kind of.disk hundreds and thousands of disks.petascale system and obviously we are.talking about hard disks here we still.can't you know even supercomputers can't.afford hundreds and thousands of solid.state disks and given that these are.hard disks so let's look at hard disk.technology what's happening and yeah the.disk drives are bigger than they used to.be ten years ago yes they are faster.than they used to be ten years ago but.they're not growing at the same rate at.which they were growing ten years ago in.other words the curves are flatter than.they were ten years ago so what this.means is that to get the same.performance rate of increase that we.wear ten years ago we need to use more.or these devices more of these hard.disks more of more components so even.though the the curves are become flatter.all those inflection points that you see.here in the wrong direction be that as.it may all the government programs that.are using supercomputers they're not.saying that yeah we can't increase you.know we they still want the system.performance to continue to increase at.the same rate that matches with their.algorithms and all the processing they.want to do.so what this means is there are too many.moving parts in the system you know.given that that these densities are not.increasing at the same rate so here is a.chart that shows compares two systems an.ascii purple system a supercomputer that.was in lawrence livermore with the.target six system sometime next year so.if you really see the number of parts.are about 10 to 20 times at 40 times.what it was in the past so the even.though these number of components are.increased in these disparities have.evolved but still what's happening is.the reliability has not caught up so.there are not more number of parts but.their reliability has not increased so.what that means is the system that is.using these components have to deal with.the problems that are there so the.question is with these parts with these.many moving parts how do you build a.system a supercomputer or an acceptable.performance and an acceptable cost.that's the challenge we had when we.started working on this project so with.that said that's the problem and here's.the outline of my talk I'm going to give.you a quick overview of GPFS parallel.computing braid and then move on to the.challenges with traditional way of.building things using traditional rate.and traditional dis price and some of.you probably will relate to the problems.that I'm going to share with you and I'm.going to talk about the solutions we.added how we address these problems in.gpfs native rate so GPFS originated at.IBM Almaden Research Center and it is.continued even though it has moved on to.become a product it's IBM researchers.continue to continue to be involved in.delivering new features into the product.and ZP FS as I said is used in a wide.variety of supercomputer applications.aerospace weather modeling defends.National Labs a lot of places use gpfs.and it is also used in scaleable nass.environments but this talk is mainly.focused on supercomputers so a professor.ken batcher from Kent University he he.humorously mentioned that a.supercomputer is a device for turning IO.bone compute bone problems into IO bone.problems now what this says basically is.that storage even though it's a small.part in a big supercomputer which is.primarily designed for computing it.should not be a bottleneck for.supercomputers that's the takeaway so.there are two models I've shown in the.in the bottom here - one is a work flow.model where the supercomputers do a.bunch of computations with the data.they've read and then they write write.out the disks because the work flow is.changing they execute a barrier where.all the super or all the nodes in the.cluster right out to the disk and then.when they move on to the next phase of.the operations they read whatever they.need to read for the next phase and move.on to do the computations that is one.model and the second model is below that.where it's a checkpointing kind of a.model where there's a it's a continuous.stream of computations going on but once.in awhile the supercomputer needs to.check point its memory to the disks so.that it can do crashed activity now if.you see in this in these what what the.point of these two pictures is to show.that whenever there is a Oh happening.computation is not happening all those.processes all those compute nodes that.are primarily there for computation.they're not being used which means.that's where it relates to the the first.code is half-humorous but the point is.we don't want storage to be the.bottleneck and all in other words any.time the view of the supercomputer or.folks is that any time doing.IO this time.from computing so how do supercomputers.access these disks they use paddle file.systems there are a couple of them and.gpfs is one such and why do they use.these paddle file systems because they.give a single system image there is so.much storage out there and you get a.single system image and they provide.POSIX semantics which is makes it easier.for programming and also these parallel.file systems gives high throughput and.load balancing and so how do they do.that which is what you'll see in the.next slide.so what GPFS does is that so let's take.an example of so gpfs stripes both data.and metadata on all disks across several.disks across many disks and let's take.an example of I'm sorry I'm having.trouble with this thing so I'm gonna use.this thing so let's take an example of a.large file in one compute node now what.happens is if that file is the compar.going to go back and make sure these are.clear so on the top is the compute nodes.and the bottom is an are the i/o nodes.which are talking to the disks now the.ratio here is an illustrative purpose in.reality there are several the ratio is.very high between compute nodes to IO.nodes they're connected over a high high.high speed fabric and let's take a file.that's in a you know in one compute node.and it's writing out this large file.what gpfs does is it takes that file and.it stripes it across all the disks in.the array it's an animation I have.trouble with animations all the time so.this is a this is a second example of a.single file but are being operated by.all the nodes in the cluster they are.operating in different parts of the.cluster all of the file or it could be.that those green.Bob blobs are independent different.files in either case what GPFS does is.it again stripes it across all the disks.so the takeaway of this is it achieve.this high throughput by striping across.all the disks it has that's the takeaway.from this that's what I want you to.remember so in reality these disk drives.are logical arrays but then they.underlying they use rain disk arrays.right what gpfs is thinking of a disk is.actually a raid array so moving on to.the raid arrays I'm sure most of you are.familiar with this but just to make sure.we are all on the same page so you start.off with a bunch of disks.just this is used for data and parity.and then you assign some spare disks so.that when a disk fails the RAID.controller can start rebuilding onto the.spare space and if while during that we.build if a second disk fails starts.rebuilding into the second spare spare.disk so there are two types of rebuild.one is a degraded rebuild as there is a.critical rebuild so the reason why it is.critical the second type of rebuild as.critical is because another failure.the raid array cannot tolerate another.failure the day that the array is in the.critical state and in the above case its.degraded it can tolerate one more disk.failure but it is still degraded why why.is the degrade it is because it's.performance is degraded in both cases.the reason is while the client is.continuing to read and write these are.the raid array is going to do a rebuild.in the background and if you can pay.attention to this arrows this is this.arrow is a thicker arrow compared to.this one that's because the bandwidth to.do the rebuild operation is twice as.much during critical because you're.writing to two disks to spare disks.compared to one spare disk but the point.to remember is this is a background.activity and you great controllers.typically give you some knobs to control.the rate of this background activity but.I'm sure as administrators when the data.when the array is in a critical state.when it can't tolerate any more disk.failures administrators would like the.RAID controller to rebuild everything as.fast as possible so that the data is.protected before the next failure and.rate not only handles disk failures but.also handles media failures media errors.on individual disk drives again it.reconstructs and writes it or overrides.the data and similar to the file system.raid also stripes its data across all.the discs in the array so here is a view.of the simplified storage stack that I.just showed you one is on the top is the.application and then there is the file.system underneath this is a raid and.then the physical disks so depending on.what the budget of the project is you're.going to either have Saturday's SAS.disks maybe a mix of SSDs but it all.depends on the budget the performance of.the disks are determined by our budget.and but under but whether but the fact.is performance is important at every.layer the raid has to perform the v.system has to perform in order for the.application to see the performance so.any slowdown anywhere will show up in.the file system performance so how do.what is the building block that's used.typically to build these supercomputers.right so here is the building block so.you take for a set of compute nodes as I.said the ratio is high here it is a very.low ratio just for the sake of.illustration for a set of compute nodes.there are a set of ion owns a small set.of i/o nodes and each attached to a RAID.controller to entail and each of the.RAID controller talks to a bunch of.disks pross possibly multipath now.be I'm sorry.now what happens if this is the building.block how do you build a cluster using.this building block is you take this one.building block and then you replicate.several of these buildings go connect.all of them together and network now you.have a huge system that's how you get to.hundreds and thousands of disks but the.problem is if let us say in a big.population of disks there are going to.be failures very very often because it's.a lot of disks are there now statistics.comes into play now if there is one.array that is rebuilding you would think.that the performance of only that arrays.affected so what's a big deal but that's.not that that's not the case the entire.file system performance is affected even.if one array is rebuilding the slowest.component affects the overall system.that's because these are not real.you know you they're not workers they're.not these these arrays are not workers.that can be an animal and anonymized.they basically have some data if that.array holds the data you cannot go to.another array and get the data you need.to go to that array to get the data.right so that's why the bar the.performance is a bottleneck if these are.any random place if it's alright.operations you could do some workarounds.in the file system to avoid that slow.array and go to elsewhere if that's only.possible for a right and for and even.then you're gonna have one arrays less.of performance and for how long are you.going to do that and you know you've got.at some point come back to that slow.array and access it for the capacity it.offers so what we decided to do was so.so this is the highlight of one the.first problem that I told you is a.rebuild problem is the with hundreds and.thousands of disks even if you assume an.mtbf of a million hours for a disk drive.you're going to see a failure you know.one one disk is going to fail every ten.hours in this hundred thousand disks.system right so which means rebuild is.happening on a daily basis on this ten.thousand disk system.and we can and if people is going to be.happening every day.if one array is going to always.rebuilding somewhere in the system the.performance of the entire file system is.affected all the time so this is one.problem with the traditional rate and.some of you might have experienced this.too and the of course this is this is.what is spoken and laden the industry.that discs are getting bigger and bigger.and they are taking longer to rebuild.and a couple of years ago there was this.question even raised its rate dead and.some folks saying no a rate is really.not dead and the problem that they are.saying is this is the problem this.rebuilt problem is what they are talking.about now is that the only problem not.really now when there are so many disk.drives the first order problem is these.disk failures but then there are the.second-order and third-order kind of.problems start to show up which is.basically you know you start seeing.corruption issues from the disk right.and that is another issue and I'll go.into the details of these a little bit.later but there are two issues in the.traditional RAID controller what that.mean what I'm trying to say about the.silent data corruption is that discs.tell you that here is the data that you.wrote but it is not exactly what your.honor.right that's what I mean by corruption.and I'll show you some more information.on that so to address these problems.what we decided to do is we said we're.going to change this picture where we.are not using any external raid.controllers instead we implement a.native raid layer inside the file system.so that directly does the right now the.main cause the main reason why we were.trying to look at this kind of a option.was that it was motivated by cost but.then it turned out that there are these.other problems that I told you about the.rebel problem could also be effectively.addressed here and also the data.integrity problem can be addressed.really well at this layer of the file.system.so the stack looks from this to this.because of the native rate is integrated.into the file system and the problems it.solves are the you know it addresses the.slow rebuild performance and the way we.address it is we use the be clustered.rate and I'll talk to you more about.what is D clustered rate it is already.implemented by some other products even.IBM other there are the XIV M XIV also.implements it and add a network raid.layer but this is at the disk block.trade raid layer and we also solve the.problem of data integrity on disk drives.by using end-to-end checks and and.version numbers so now going to the.first solution of these high-performance.rebuild so here is a this is again an.animation chart I hope it comes out.right so what we are showing for this is.for illustrative purposes.so that you can get the idea of D.clustered raid.so there are this each one colored pair.is a raid 1 array mirrored array so you.have three such arrays and 1 spare disk.dedicated so this animation is trying to.show how you take this traditional three.arrays each of which are is a raid 1.array and D cluster them right so you.take the first array and you spread it.across the seven disk so on both the.bottom left and right.you have seven disks so we're not taking.up any more capacity except what we're.doing is D clustering you're taking.those to the strips I forgot to tell you.that so I told you that raid also.stripes and this the column is an entire.disk and the rows are stripes the raid.stripes so if it was a white so this.this one will be the the first blue copy.will be the primary copy and the second.is the mirrored copy right and then so.we take those two stripes and stripe it.across.all the seven discs of the D clustered.array so that's what we do for the first.one and we do the same thing for the.other two right now we start a D cluster.the spare space also de ello is this.parrot and with this what you have is a.D clustered granary right so this shows.an example of the rebuild problem just.to show you illustrate again so take for.example this disc is failed what happens.in the traditional rebel you read from.the mirror and write it to the spare.disk that way so that the raid is no.longer having faults it is fully.replicated it can tolerate one more.failure so that's what is the rebuild.operation of a mirror and raid looks.like now what happens if a disk fails in.a D cluster array is consider the same.disc that has failed the fifth disc in.the D cluster that has failed now what.happens is you start whatever data was.not there was lost because of the disk.failure is now replicated using the.mirrored copy that is stored on all the.other disks in the cluster away now if.you take this example going back to this.look at the performance so we are.reading from this one disk and writing.to this one disk that's the throughput.used now whereas in the case of D.clustered array the graph shows that all.the disks in the array are not only read.are also written right so this is where.what what we get is what D clustered.rebill gives is two things you know it.gives you if you want to rebuild fast.you can do that because you have all the.throughput or if you want to provide.less disruption to the user you can do.that too so it gives you two two options.and now this this is an example.ample of a rate six again an.illustrative purpose using four discs.because my screen doesn't fit the bit of.the real array so you take the on the.left side you saw that what we what I.show is traditional arrays each color is.a rate six array by itself on the right.side is a D clustered this is how it'll.look if your D cluster now what happens.if two discs fail in both cases now the.table below tries to show the difference.actually now here what happens is when.two discs fail in the green array the.green array suffers two faults in every.stripe of that array whereas in this.case only the red array suffers two.faults all the other arrays have.suffered only one fault this is an.interesting side effect of D clustered.Iranian actually this is the main reason.for doing this D clustering it's not.only throughput also the fact that not.all when two discs fail not all the data.not all the affected stripes have two.faults in them so what this means is.there's only about in this example in a.rate six example about ten percent of.the data is only critical right.whereas in two fault happens all the.stripes all the seven stripes in this.example have two faults whereas here.there's only one stripe that has two.fault now this the effect is very.similar actually better when the pre.fault tolerant array about only one.percent of the data has is affected it.becomes critical whereas the rest of it.is not critical so what this means is I.can quickly rebuild only the stripes.that have two faults and the rest of the.stripes with one fault I can take my own.sweet time to rebuild.so that way during this stripe.rebuilding the small percentage of.stripes that are critical when I am.rebuilding those alone at that time.alone my.application workload will we'll see a.performance degradation severe.performance degradation because I want.to rebuild as fast as possible whereas.during the rest of the time when I'm.rebuilding the single Falls I can kick.back and relax because I can tolerate.more failures so that is the main.advantage of the cluster away now.this is a power 775 disk enclosure that.we thought this is the main motivation.we we did this and this is the disk.enclosure that goes with the power 775.processor rack of IBM and what this disk.enclosure the number of disk lies this.disk enclosure can hold is 384 disks and.I don't know about you but that's a hell.a lot of disk rice my view and and what.you see in the middle is the blower's so.it can have displace in the front and in.the back and the disk drives are mounted.on a carrier so there are four disks in.each carrier and compared to other.models where you pull the entire disk.drawer out to service a failed disk.where you move the all the destroys.together and this discourage don't like.that you know moving it while while they.are running or do you attach a carrier.through a cable right cables also don't.like movement very well so this one.doesn't do either you just if you are a.service a disk drive we power off all.the disk drives and we indicate which.there are some LEDs in for each for each.of the discs right there is an LED that.indicates which disk needs replacement.and we before replacement we power of.all the disk drives this the service.person pulls the cat carrier out looks.which this needs replacement because.there's a light there replaces that puts.it back in we power up all the disk.drives and it go from there and we keep.track of the question will be you know.you know if somebody if you told me this.model I.very very worried about oh you're gonna.take in order to be placed one destroy.you're gonna take for displays offline.and are you gonna rebuild all those for.dis price not really so we keep track of.let's say one the first describe is bad.anyway that's bad we have rebuilt at.long time I'll go on to space space all.the rest of the three drives have data.in them now.if Reed is coming during this time we.can reconstruct and give the use of the.data if a right is happening we have to.keep track of what are those rights that.we didn't write to this location and.then when the disk the carrier is.inserted back again in a few minutes we.go back and write it to the home.location so we don't do an entire.rebuild operation but just a selectively.rebuild of those data that where that.was not written when the carrier was in.service so this is the photograph of a.prototype of a machine in our lab so.just to give you the scale the disk.drive the enclosure goes all the way to.the back of the picture yes that's right.so this this picture again shows the.disk drives in the front and in the back.think of it as a top view of the.enclosure so each of this column is a.carrier it has four disks and we.dedicate a few of them for SSD for.metadata that offers higher performance.for metadata operations and we take all.the disks in one row one disk from each.carrier and make a D cluster array out.of it so it's a 47 disk D clustered.array so it's a 47 this be clustered.array we don't do just wait six we do.one fault-tolerant more you know we.tolerate three three faults by using a.you know these it's a it's 11 wide.stripe with a data blocks and three.parity blocks three parity eight eight.data strips and three parity strips in a.one rate stripe and we also support for.waiver application and as I said it is.distributed across 47 disk array now.what this means is if one disk dies.which is the most common case the one.disk dies in an array we will rebuild.very slowly this is counter-intuitive.from what you may be used to you know.you set the rebuild at one speed and all.types of rebuild run at that same speed.that's what rate control is typically do.but here we give you two knobs one for.what we call the normal rebuild which is.the normal case where one does dice this.is going to die every day so we call it.a normal rebuild it's no longer an error.scenario because it's in the population.of hundred thousand disks this they're.going to die every day and the system is.going to rebuild and it's a normal.rebuild because the performance of the.client workload is not affected its.affected by you know five percent maybe.but not but not by 30 percent and when.it is three three three disks die in the.same array not in the entire population.of 100,000 this we are talking about.that we are talking about three disks in.a population of 48 disks this is a very.very rare event in that case one percent.of that 48 this data will have three.faults and during that one percent.rebuild which is going to take the order.of minutes not in the order of hours not.in the order of days we quickly rebuild.that by the client workload will be.affected because it's a critical at that.point we don't want at all away we.cannot tolerate one more disk failure so.we'll rebuild at a very high rate but.that's just one percent and that high.critical rebuild will last for five.minutes maybe five ten minutes at the.most in the order of minutes and you're.out of rebuild and you know you as a.system administrator might get a call.you know somebody would notice it in.five minutes and tell you that hey my.system is you know slowed down.considerably and you log in and you will.check what's going on by the time you do.it the system would have be out of.critical rebuilt and you would say well.it's all fine now so that's the idea so.we not only support three fault-tolerant.we also support rate six.and to a replica three web application.so bottom line is we used the native red.layer and gpfs uses be clustered rate to.address the problems in the traditional.rebuild the traditional raider is so.moving on to the next solution that we.added which is this data integrity.solution so you know so imagine you're.the user and you have a disk that has a.data block a and you try to read this.data a from the disk and if the disk.returns you the content a then.everything is good but if the disk gives.you B then we call it a corruption right.so in those cases that's when the disk.drive things actually the content is B.but actually what you wrote is a now why.it gives there could be lots of factors.but bottom line from our perspective you.know it's better not to give me data you.know instead of giving me bad data right.and just to remind you that all the.widely-publicized five-nines number you.know how reliable the system is how.available the system is does not.consider data integrity issues the.assumption there is the data it's 100%.data integrity is hundred percent that's.the assumption there so that number that.figure the five nines figure doesn't.include data integrity problems now.these undetected disk errors are known.in the literature by variety of names.some call it silent errors some call it.phantom errors etc but they're not the.same as media errors mediator the disk.tells you I cannot give you the data.that's great it's not great it's bad.that it's not giving you the data but I.really at least it tells you it's.harnessed about it whereas in the other.case not that it's dishonest about it it.just doesn't know it's not aware that it.is bad data.and there are papers in the literature.that talk about experiences in the field.where discs have these corruption issues.and some of us at Almaden Research Lab.took those data so the problem is.because this is silent you don't have a.rate how often this happens there are no.statistics on this corruption issue.because it's silent suddenly one day.some crash and crash happens and you.maybe you reboot the system or you know.you think the operating you the blame.goes on all layers because you don't.know what caused the problem so there.are not much statistics but so we took.some data that was published and we.modeled we abstracted the IO and we.basically modeled and we we've estimated.that about it even in a thousand disk.system will it will experience the data.corruption once a once every five years.now that's an estimate but you can.imagine what is the number if it is a.hundred thousand this system so we just.cannot afford to not to ignore this.problem that's the bottom line and what.are the types of errors you know why do.we see this corruption there are a.variety of factors.you know describes our complex you know.they're not as simple as we would like.to think they are you know we in school.in textbooks we read that tracks a.circular which is true if you look at it.if I look at it but if I am the disc.drive head I'm looking at the track it's.actually a very curvy winding track that.I am following and it's very possible.that I have with good all intentions I'm.trying to go to one track but I land in.a different track due to vibration.issues due to thermal issues so many.issues are there in a disk drive and not.only in a disk drive and there are so.many other layers above it corruption.could happen anywhere the system above.it needs to be able to deal with it.so when it comes to describes itself.what are the various problems in which a.right can go and you know basically the.describe and it writes it doesn't do any.checking off did I write the data.correctly only when the time of reading.it goes back to figure out and if you.can find a problem it will tell you.there's a problem but while writing do.do let's say ahead issue or hardware.issue or a firmware issue the data.instead of landing in one location could.land in an H and other location we call.it a far off track right in which a.different block is totally corrupted.that's one type of corruption the second.corruption is the the data is very close.to the existing track the disk really.meant to go to that track it went there.it was writing it but there was some.vibration the the data landed very close.to it it's an ear of crack right we call.that and then there is the third case.where the disk totally doesn't write to.the disk because the head is too too.high or due to some firmware issue what.hardware issue god knows what it doesn't.write at all and then when it comes time.to reading that's when you will see the.effects and then even if the data was.properly stored on the right track right.location there could be problems while.reading right so that is a separate.class of read errors and these problems.can be either transient in nature.because of some vibration happening or.it could be it's a persistent problem.because of a hardware problem let's say.so typically how do systems address this.you know they attach a checks and.trailer to the data and they write the.checksum and the date data together so.that when reading back I can make sure.the data I read matches the checksum.that I stored with it so that I can.detect any corruption this will detect.corruption in the sense of it will.detect if it is a far off track off.track right you know instead of writing.to this track I went far away and wrote.to a different track now when I read it.that location I'm going to get a totally.different problems like I would check.some maybe then I can detect it.but this does not detect this method.does not detect drop rates if I totally.forgot to write if the disk drive forgot.to write the data to the location then.what will happen is the next time you go.to read you will get the old data stale.data and the stale data will match the.the checksum that is there in it and you.might think that hey the data and the.checksum match must be right not really.it's old data so what that means is we.need a way to validate the checks and.trailer itself the checksum validate the.data but then who's going to validate.the checksum itself you need a.validation which means that validation.has to be not stored with the same data.again because actually that's acceptable.to drop writes it has to be stored at.another location or a different destroy.or a different different mechanism you.can't be co-located in other words the.checksum and the validation of the.checksum cannot be co-located it has to.be located separately so that's what we.do so we use version numbers and.checksums we store the the version.number in the metadata so that and we.log it so that there's no performance.penalty for it and what we do is at the.time of reading we need read the data.make sure the checksum matches but.before you in doing that checksum.verification we make sure the checksum.block is valid using this validation.information and we do this end to end.what we mean by that is what I mean by.that is so we in a write operation a.write operation comes from the compute.node to the i/o node that that is.checksum it's a network checksum you.know which we don't rely on the TCP.checks and we also add on top of top of.that our don't checksum layer and we.verify that the data between the.transaction between the two nodes is is.correct and then when you go to the.right to the de-risk we write the.validation information and write the.validation to the metadata same way.while reading we read the checksum block.validate it and then again use check.sums to send it to the client node.it's an end-to-end solution so what we.what do we store in the trailer is not.just checksum alone it's not a blind.checksum it's a very in fully aware.trailer that contained that tells you.who i am who i am you know i am a block.in this stripe of that strip of this.array so that even if you take this data.due to some corruption and wrote it to a.different array totally we can detect.that this data it doesn't belong here.even though it looks consistent from.other aspects if the if the block.doesn't say it belongs to this array.then that block is corrupt so we restore.a quite a lot of information in the.trailer to make sure we catch this again.you know this is all from experience.that you know we don't want to run in to.deal with these problems again so we are.a rate controller so we have to do other.than those two problem solving you have.to do other things that the typical rate.controller does the raid layer now so as.I said when a disk is temporarily out of.service because somebody took the.carrier for servicing or if the disk was.misbehaving and we were diagnosing that.disk and we couldn't do right operations.during the diagnosis those things we.selectively rebuilt using a background.operation and when disk fails we.restored the redundancy and we do this.by you know instead of traditional.rebuild will there's only one type of.rebuild as I told you know because.there's there is this this case of some.stripes having three faults some stripes.having to fall some stripe having one.fault doesn't exist in a traditional.rebuilt traditional rate there as in de.clustered raid you know in case of pre.fault tolerant be clustered raid only.one person has three fault so we quickly.rebuild that at a critical high rate and.then we switch back to a low rate and.rebuild the two faults and then the.single faults that's what the background.rebuild does and then we also have a.rebalance tasks you know when you insert.a disk died and then you took a new disk.replaced it now that disk has.full of spare capacity right and we want.to distribute that spare space across.the array so that when it comes to again.using that spare space we can use a trip.through put on all discs so we call that.operation as a rebalance operation and.when I don't have to do either a rebuild.or rebalance everything is hunky-dory.then I'm going to make sure the data on.the disk is fine by doing a background.scrub operation where we heat the data.the parity make sure the checksums are.right you make sure they are consistent.with the data and party to make and then.fix any problems we find and then we do.it opportunistically these background.tasks we don't take up the client work.class you know we don't impact the.client workload unless it is critical.operation if it is a critical rebuild we.presume you want us to rebuild as fast.as possible and other than that it's all.a background operation and so we also.have a disk hospital layer where you.know we diagnose and treat disks if you.if you would like to think of it that.way so when a disc reports an error we.have to figure out what is the problem.is it a path problem is it media error.or is it a total disk failure right we.have to diagnose those three and then.after we have decided what really.happened we had to take corrective.actions if because the media error fix.it if it is a you know if it is a.biscuits totally unresponsive then mean.we know you may even power cycle it.through we go through the control path.of the disk enclosure the power cycle.that disk to see if it can be revived.and if we determine that the disk is.really dead because the disk itself says.so or it is totally not there then we.have to take corrective action such as.rebuild but after some point we you know.we keep maintaining health records of.these disk discs on a per disk level per.this basis and if the the disks have.been either affecting performance or.reliability then we have to take it.offline there is no point in having this.disk and we take it offline and that's.when we schedule.replacement requests that's when service.person will come and you know we that's.the service time we need to power off.the disguise as I had said before to all.of the person to pull the carrier out.and you show which discs are to be.replaced by lighting the appropriate.discs in the carrier and all that stuff.that goes to the disk replacement.so to summarize so we what I told you.was this when there are these hundreds.and thousands of discs the traditional.rebel traditional trade has the slow.rebuilt problem and has this data.integrity issues we address both of.those problems one by using the.clustered raid and second by using.version numbers and end-to-end checksums.for the data integrity problem and we.also offer cost advantage because there.is no rate controller associated this.function is now done in the i/o Knoll.and how is it possible it's because.these days these IO nodes come with.extra processing power there are.additional course anyway and so the.function can be absorbed into the i/o.node and typically when you know these.talks I'm from research and whenever we.give such talks it's at a prototype.stage and we allow to kind of say when.it will be available in the near future.and some time but fortunately and I'm.glad to say that we just shipped it last.month and it is already installed in.customer locations and like GPFS it you.know it supports a wide range of.applications and systems so that is.about the GPFS native rate thank you.very much if you have any questions.I guess my talk was very easy to.understand.okay thank you very much.

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