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In this session, we will review the Red Hat Ceph Storage architecture and explain the purpose of CRUSH. __u64 draw; We can see, the bucket root rgw1’s id is -1, type = 10 means root, alg = 4 means straw type. static int bucket_straw_choose(struct crush_bucket_straw *bucket, CRUSH. Ceph uses the CRUSH algorithm, that decides in which OSD the particular data should be stored. (in my cluster, pool “neo” id is 29, name of object to write is “neo-obj”), In object_locator_to_pg, the first calculation begins: ceph_str_hash hashes object name into a uint32_t type value as so-called ps(placement seed). A healthy Red Hat Ceph Storage deployment depends on a properly configured CRUSH map. (welcome to my csdn blog~~~) Analysis method. Understand CRUSH data placement algorithm and how it is used to determine data placement. At the core of the CRUSH algoritm is the CRUSH map. Ceph Clients: By distributing CRUSH maps to Ceph clients, CRUSH empowers Ceph clients to communicate with OSDs directly. CEPH CRUSH algorithm source code analysis. We’d be remiss if we didn’t mention that the docs are getting a lot better, too. The algorithm of CRUSH dictates how data is stored and retrieved through the Ceph cluster. The CRUSH algorithm then defines the placement group for storing an object and thereafter calculates which Ceph OSD Daemon should store the placement group. By default, Ceph will retry 19 times if it can’t find a suitable replica location. In this function: crush_hash32_2(CRUSH_HASH_RJENKINS1,ceph_stable_mod(pg.ps(), pgp_num, pgp_num_mask),pg.pool()); ps mod pgp_num_mask and the result(i.e. simulate CRUSH algorithm with online data in a live Ceph cluster. This algorithm ensures that all the data is properly distributed across the cluster and data quickly without any constraints. Ceph pseudo-randomly distributes files, meaning that they appear to be filed indiscriminately. When the weight is < 1, it is less likely that CRUSH will select the Ceph OSD Daemon to act as a primary. Ceph is a full-featured, yet evolving, software-defined storage (SDS) solution. As explained on the previous section, each time a client wants to perform an IO operation it has to calculate the placement location. Ceph clients and Ceph OSDs both use the CRUSH map and the CRUSH algorithm. The CRUSH map algorithm is one of the jewels in Ceph’s crown, and provides a mostly deterministic way for clients to locate and distribute data on disks across the cluster. ; What function crush_ln() in the given code is actually computing? The formula is more known as the CRUSH algorithm is the one that determines the placement of an object within the cluster. We met an issue of read performance issues (17% degradation) when working on ceph object storage performance evaluation, and found the root cause is unbalanced pg distribution among all osd disks, due to current CRUSH algorithm. This weight is an arbitrary value (generally the size of the disk in TB or something) and controls how much data the system tries to allocate to the OSD. This command integrates the host daisy in the cluster and gives it the same weight (1,0) as all the other nodes. Having OSS in Ceph was also important. A brief overview of the Ceph project and what it can do. The command for that is: ceph osd crush remove osd.4 Optimized CRUSH algorithm brings about 13% performance improvement for 128KB read case, 12% improvement for 10MB read case, and no sacrifice for write performance. It’s now documented that this is insufficient for some clusters. Ceph’s CRUSH algorithm liberates storage clusters from the scalability and performance limitations imposed by centralized data table mapping. This command integrates the host daisy in the cluster and gives it the same weight (1,0) as all the other nodes. the pps is x. int x, int r) Ceph stores data as objects within logical storage pools. In this function there are several important variables: scratch[3 * result_max] and a, b, c points to 0, 1/3, 2/3 locations of scratch array. Ceph is open source software designed to provide highly scalable object-, block- and file-based storage under a unified system. It's very popular because of its robust design and scaling capabilities, and it has a thriving open source community. Ceph continuously re-balances data across the cluster-delivering consistent performance and massive scaling. P.S. then we enter the GDB interface. 4. assuming that daisy is the hostname of the new server. optimized CRUSH algorithm, introduced a new bucket type called “linear” with new hash methods and an adaptive module. Ceph stores data as objects within logical storage pools. In order to achieve scalability, rebalancing and recovery capabilities, Ceph shards the pools into placement groups. I tried: extract crush + client code from kernel code. -O0 is important, it means shutdown the compiler optimization, if not, when using GDB following the program, most variables are optimized out. 1. As the documentation states, CRUSH is meant to pseudo-randomly distribute data across the cluster in a uniform manner. Let’s see some parameters in runtime. In that case you will see 1024 PGs but all of those PGs will map to the same set of OSDs. When you increase pg-num you are splitting PGs, when you increase pgp-num you are moving them, i.e. case CRUSH_BUCKET_STRAW: Ceph clients communicate directly with OSDs, eliminating a centralized object lookup and a potential performance bottleneck. At the core of the CRUSH algoritm is the CRUSH map. You can read the source code following the orders. Let’s move on. ; Is x the placement ps calculated by crush_hash32_2 function? The centerpiece of the data storage is an algorithm called CRUSH (Controlled Replication Under Scalable Hashing). With awareness of the CRUSH map and communication with their peers, OSDs can handle replication, backfilling, and recovery— allowing for dynamic failure recovery. We have developed CRUSH (Controlled Replication Un-der Scalable Hashing), a pseudo-random data distribution algorithm that efficiently and robustly distributes object replicas across a heterogeneous, structured storage cluster. The CRUSH algorithm makes Ceph self-managing and self-healing. -O0 only suits for experimental occasion, in production environment compiler optimization surely should be on. Replication, Thin provisioning, Snapshots are the key features of the Ceph storage. The formula is more known as the CRUSH algorithm is the one that determines the placement of an object within the cluster. use librados directly. CRUSH. The technique used is called CRUSH or Controlled Replication Under Scalable Hashing. I draw a flow chart to show the first period. It produces and maintains a map of all active object locations within the cluster. First of all, we should install CEPH using source code, and when doing ./configure, we’ll add compile flags like this ./configure CFLAGS='-g3 -O0' CXXFLAGS='-g3 -O0'. The self-healing capabilities of Ceph More discuss can be seen here. definition: First here is my crushrule of the target pool and my cluster hierarchy: what is worth mentioning, step emit is typically used at the end of a rule, but may also be used to pick from different trees in the same rule. Our testing cluster currently comprises three servers, with some 20TB of raw storage capacity across a dozen disks. To clarify this further, in the heart of Ceph is the CRUSH algorithm, which makes sure that OSDs and clients can calculate the location of specific chunk of data in the cluster (and connect to specific OSDs for read/write of data), without a need to read it’s position from somewhere (as opposite to a regular file systems which have pointers to the actual data location on a partition). We calculate every son bucket all over the loop and pick the biggest. In this article, we only focus on the calculation. CRUSH provides a better data management mechanism compared to older approaches, and enables massive scale by cleanly distributing the work to all the clients and OSD daemons in the cluster. A minimal system will have at least one Ceph Monitor and two Ceph OSD Daemons for data replication. - Duration: 16:00. Rendezvous hashing is more general than consistent hashing, which becomes a special case (for =) of rendezvous hashing. That map contains information about the storage nodes in the cluster. Share your ideas with me ustcxjy@gmail.com~. As we know, CRUSH core function is crush_do_rule(mapper.c line 785). CRUSH is one of the Ceph module,mainly solve the controllable, extensible, decentralized distribution of data copy. The clusters of Ceph are designed in order to run on any hardware with the help of an algorithm called CRUSH (Controlled Replication Under Scalable Hashing). We found this for ourselves and things got moving once we tweaked choose_total_tries to a value of 100, but it’s annoying that there was this much seemingly broken by default. This is not enough samples and the variations can be as high as 25%. How is pps calculated? The clusters of Ceph are designed in order to run commodity hardware with the help of an algorithm called CRUSH (Controlled Replication Under Scalable Hashing). c stores the final OSD set result. This approach may lead to massive data migration if we change the number of PGs, even reduce the availability of the system. We’ve talked about hash functions recently, they’re difficult to get right. Then let’s look into the loop: Ceph’s CRUSH algorithm efficiently maps data objects to storage devices without relying on a central directory. CRUSH. Understand CRUSH data placement algorithm and how it is used to determine data placement. pg-num is the number of PGs, pgp-num is the number of PGs that will be considered for placement, i.e. Then we go into crush_do_rule. “ceph osd crush reweight” sets the CRUSH weight of the OSD. However, Ceph requires a 10 Gb network for optimum speed, with 40 Gb being even better. CRUSH is the powerful, highly configurable algorithm Red Hat Ceph Storage uses to determine how data is stored across the many servers in a cluster. Ceph Clients and Ceph OSD Daemons both use the CRUSH algorithm to efficiently compute information about object location, instead of having to depend on a central lookup table. Ceph uses the CRUSH algorithm, that decides in which OSD the particular data should be stored. Clusters with index servers, such as the MDS in Lustre, funnel all operations through the index, which creates a single-point of failure and potential performance bottlenecks. CRUSH was designed for Ceph, an open source software designed to provide object-, block- and file-based storage under a unified system. ceph osd crush add 4 osd.4 1.0 pool=default host=daisy . 16:00. With awareness of the CRUSH map and communication with their peers, OSDs can handle replication, backfilling, and recovery— allowing for dynamic failure recovery. Nice job! Provide an overview of the requirements to create and access object storage data in Ceph Object Store. ceph osd crush add 4 osd.4 1.0 pool=default host=daisy. The CRUSH algorithm makes Ceph self-managing and self-healing. PG and PGP are important concepts. We want two copies of our data, and have designated each server as a failure boundary. And make w = a, o = b. w is used as a FIFO queue for taking a BFS traversal in CRUSH map. Ceph provides all data access methods (file, object, block) and appeals to IT administrators with its unified storage approach. Please refer to the design and Implementation part for details. Here is the instance calculation process: So bucket -16 is selected, even its straw value is a little smaller. An algorithm to fix uneven CRUSH distributions in Ceph. CRUSH is designed to facilitate the addition and removal of storage while minimizing unnecessary data movement. This particular problem will go away once the cluster grows in size or we deploy a larger cluster – this is admittedly a small testing cluster by Ceph’s standards, but we don’t think it’s an unreasonable size for anyone looking to evaluate Ceph as a solid solution. It is highly configurable and allows for maximum flexibility when designing your data architecture. . What we saw during testing was that the cluster would get hung up trying to meet our redundancy demands, and just stop replicating data. x is the pps we’ve got, rule is the crushrule’s number in memory(not ruleid, in my crushrule set, this rule’s id is 3), weight is reweight we’ve mentioned and it’s scaled up from 1 to 65536. This weight is an arbitrary value (generally the size of the disk in TB or something) and controls how much data the system tries to allocate to the OSD. We were greeted with a scary and rather unhelpful error message if we tried to use those tunables – no really, the message is stored in a variable called scary_tunables_message, and doesn’t tell you what you should do or why you shouldn’t use the tunables. Hot Network Questions Possible limitations and magic combination for contact-based teleportation tuning of … Then we get PGID. The clusters of Ceph are designed in order to run on any hardware with the help of an algorithm called CRUSH (Controlled Replication Under Scalable Hashing). The CRUSH algorithm then defines the placement group for storing an object and thereafter calculates which Ceph OSD Daemon should store the placement group. We may also share information with trusted third-party providers. This lets you demarcate failure boundaries and specify how redundancy should be managed. Each object corresponds to a file in a filesystem, which is stored on an Object Storage Device. We’ve looked into every important part of CRUSH calculation process. CRUSH stands for Controlled Replication Under Scalable Hashing. Ceph currently supports the use of one hash function, named “rjenkins1” after the author of a paper describing the methodology. Before reading this article, you need to be familiar with CEPH’s basic operations on Pools and CRUSH maps, and have a preliminary reading about the source code. We also test load line performance to demonstrate the scalability of the optimized algorithm, and the results are shown in Figure 2 and Figure 3. CRUSH rules define how a Ceph client selects buckets and the primary OSD within them to store object, and how the primary OSD selects buckets and the secondary OSDs to store replicas (or coding chunks). It replicates and rebalances data within the cluster dynamically—eliminating this tedious task for administrators, while delivering high-performance and infinite scalability. And then in librados::IoCtxImpl::operate, oid and oloc(comprising poolid) are packed into a Objecter::Op * type variable objecter_op; Through all kinds of encapsulations, we arrive at this level: _calc_target. Ceph assigns a CRUSH ruleset to a pool. As mentioned, crush_do_rule does crushrules iteratively. This value is in the range 0 to 1, and forces CRUSH to re-place (1-weight) of the data that would otherwise live on this drive. You can do that too, just tell the CRUSH map to ensure that replicas are stored in other racks, independent of the primary copy. if the step is a choose step, the function would call crush_bucket_choose to do the direct choose; if the step is a chooseleaf step, the function would run recursively until it gets leaf nodes. INPUT(object name & pool name) —> PGID —> OSD set. In certain circumstances, which aren’t entirely clearly documented, it can fail to do the job and lead to a lack of guaranteed redundancy. How CRUSH maps work. This algorithm ensures that … ``` CRUSH stands for Controlled Replication Under Scalable Hashing. It’s a pretty great solution and works well, but it’s not the easiest thing to setup and administer properly. Rendezvous hashing is more general than consistent hashing, which becomes a special case (for =) of rendezvous hashing. -g3 means MACRO infos is generated. Ceph stores data as objects within logical storage pools and calculates which placement group should contain the object, and further calculates which Ceph OSD Daemon should store the placement group using the CRUSH algorithm. DorianDotSlash 1,256,494 views. It’s difficult to discern the intent just from looking at the code, but our gut feeling is that the mixing function isn’t being used quite as intended. Having OSS in Ceph was also important. The code is given below: Questions: Why there is a need of taking log of hash value? CRUSH is designed to facilitate the addition and removal of storage while minimizing unnecessary data movement. This avoids the need for an index server to coordinate reads and writes. As an example, each server in your cluster has 2 disks, and there are 20 servers in a rack. Ceph clients communicate directly with OSDs, eliminating a centralized object lookup and a potential performance bottleneck. (welcome to my csdn blog~~~). -- no access to crushmap. This algorithm ensures that all the data is properly distributed across the cluster and data quickly without any constraints. call: Here the weight is the OSD weight we set scales up by 65536(i.e. A CRUSH map is the heart of Ceph's storage system. But for obj->PG mapping, Ceph still uses the traditional hash, which is pgid = hash (obj_name) % pg_num. I.2. Perhaps you’ve seen this for yourself if you’ve deployed a Ceph cluster – now you know why. As soon as you write into Ceph, all the objects get equally spead accross the entire cluster, understanding machines and disks.. Why Ceph calculate PG ID by object hash rather than CRUSH algorithm? All rights reserved. A typical application is when clients need to agree on which sites (or proxies) objects are assigned to. indep means erasure code storage. 4. I am confused by the comment 2^44*log2(input+1). CRUSH … Now we are at do_rule: … In certain circumstances, which aren’t entirely clearly documented, it can fail to do the job and lead to a lack of guaranteed redundancy. I am confused by the comment 2^44*log2(input+1). Ceph's Controlled Replication Under Scalable Hashing, or CRUSH, algorithm decides where to store data in the Ceph object store. 37 * 65536 = 2424832). This isn’t unreasonable, but it’s exceedingly frustrating that this wasn’t mentioned anywhere. It's designed to guarantee fast access to Ceph storage. It was quite reassuring to see a lot of discussion about Ceph at Linuxconf recently, and we’re certainly looking forward to the continued progress in future. int high = 0; In the event of component failure in a failure zone, CRUSH senses which component has failed and determines the effect on the cluster. Ceph provides all data access methods (file, object, block) and appeals to IT administrators with its unified storage approach. Ceph using CRUSH algorithm for PG->OSD mapping and it works fine for increasing/decreasing of OSD nodes. Anyhow, straw value is positive relation to the OSD weight. return bucket_straw_choose((struct crush_bucket_straw *)in, x, r); It's very popular because of its robust design and scaling capabilities, and it has a thriving open source community. For this blog post, I will review the Red Hat Ceph Storage architecture and explain the purpose of CRUSH. More detailed can be seen at offical site. If you’ve been thinking “I’d use a hash function to evenly distribute the data once the constraints have been resolved”, you’d be right! -- difficult. CRUSH Rules. BTW, firstn means replica storage, CRUSH need to select n osds to store these n replicas. We met an issue of read performance issues (17% degradation) when working on ceph object storage performance evaluation, and found the root cause is unbalanced pg distribution among all osd disks, due to current CRUSH algorithm. A typical application is when clients need to agree on which sites (or proxies) objects are assigned to. And straw2 is being developped. Ceph Clients and Ceph OSD Daemons both use the CRUSH algorithm to efficiently compute information about object location, instead of having to depend on a central lookup table. This layer of indirection allows Ceph to rebalance dynamically when new Ceph OSD Daemons and the underlying OSD devices come online. P.S. Next, we’ll look into 3 functions below. SATA drives are sufficient for good Ceph performance. You can see the rules in memory: step 2 would run crush_choose_firstn to choose 1 rack-type bucket from root rgw1. Introduce the course, Ceph Storage, and related Ceph systems. This is the most important function in CRUSH. In the true spirit of SDS solutions, Ceph can work with commodity… Because CRUSH allows clients to communicate directly with storage devices without the need for a central index server to manage data object locations, Ceph clusters can store and retrieve data very quickly and scale up or down quite easily. Before reading this article, you need to be familiar with CEPH’s basic operations on Pools and CRUSH maps, and have a preliminary reading about the source code. Ceph’s CRUSH algorithm determines the distribution and configuration of all OSDs in a given node. Ceph is a full-featured, yet evolving, software-defined storage (SDS) solution. “ceph osd reweight” sets an override weight on the OSD. ceph crush map - replication.

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