I'm curious about keeping source code around reliably and securely for several years. From my research/experience:
Optical media, such as burned DVD-R's lose bits of data over time. After a couple years, I don't get all the files off that I put on them. Read errors, etc.
Hard drives are mechanical and subject to failure/obsolescence with expensive data recovery fees, that hardly keep your data private (you send it away to some company).
Magnetic tape storage: see #2.
Online storage is subject to the whim of some data storage center, the security or lack of security there, and the possibility that the company folds, etc. Plus it's expensive, and you can't guarantee that they aren't peeking in.
I've found over time that I've lost source code to old projects I've done due to these problems. Are there any other solutions?
Summary of answers:
1. Use multiple methods for redundancy.
2. Print out your source code either as text or barcode.
3. RAID arrays are better for local storage.
4. Open sourcing your project will make it last forever.
5. Encryption is the answer to security.
6. Magnetic tape storage is durable.
7. Distributed/guaranteed online storage is cheap and reliable.
8. Use source control to maintain history, and backup the repo.
The best answer is "in multiple places". If I were concerned about keeping my source code for as long as possible I would do:
1) Backup to some optical media on a regular basis, say burn it to DVD once a month and archive it offsite.
2) Back it up to multiple hard drives on my local machines
3) Back it up to Amazon's S3 service. They have guarantees, it's a distributed system so no single points of failure and you can easily encrypt your data so they can't "peek" at it.
With those three steps your chances of losing data are effectively zero. There is no such thing as too many backups for VERY important data.
Based on your level of paranoia, I'd recommend a printer and a safe.
More seriously, a RAID array isn't so expensive anymore, and so long as you continue to use and monitor it, a properly set-up array is virtually guaranteed never to lose data.
Any data you want to keep should be stored in multiple places on multiple formats. While the odds of any one failing may be significant, the odds of all of them failing are pretty small.
I think you'd be surprised how reasonably priced online storage is these days. Amazon S3 (simple storage solution) is $0.10 per gigabyte per month, with upload costs of $0.10 per GB and download costing $0.17 per GB maximum.
Therefore, if you stored 20GB for a month, uploaded 20GB and downloaded 20GB it would cost you $8.40 (slightly more expensive in the European data center at $9).
That's cheap enough to store your data in both US and EU data centers AND on dvd - the chances of losing all three are slim, to say the least.
There are also front-ends available, such as JungleDisk.
http://aws.amazon.com
http://www.jungledisk.com/
http://www.google.co.uk/search?q=amazon%20s3%20clients
The best way to back up your projects is to make them open source and famous. That way there will always be people with a copy of it and able to send it to you.
After that, just care of the magnetic/optical media, continued renewal of it and multiple copies (online as well, remember you can encrypt it) on multiple media (including, why not, RAID sets)
If you want to archive something for a long time, I would go with a tape drive. They may not hold a whole lot, but they are reliable and pretty much the storage medium of choice for data archiving. I've never personally experienced dataloss on a tape drive, however.
Don't forget to use Subversion (http://subversion.tigris.org/). I subversion my whole life (it's awesome).
The best home-usable solution I've seen was printing out the backups using a 2D barcode - the data density was fairly high, it could be re-scanned fairly easily (presuming a sheet-feeding scanner), and it moved the problem from the digital domain back into the physical one - which is fairly easily met by something like a safe deposit box, or a company like Iron Mountain.
The other answer is 'all of the above'. Redundancy always helps.
For my projects, I use a combination of 1, 2, & 4. If it's really important data, you need to have multiple copies in multiple places. My important data is replicated to 3-4 locations every night.
If you want a simpler solution, I recommend you get an online storage account from a well known provider which has an insured reliability guarantee. If you are worried about security, only upload data inside TrueCrypt encrypted archives. As far as cost, it will probably be pricey... But if it's really that important the cost is nothing.
For regulatory mandated archival of electronic data, we keep the data on a RAID and on backup tapes in two separate locations (one of which is Iron Mountain). We also replace the tapes and RAID every few years.
If you need to keep it "forever" probably the safest way is to print out the code and stick that in a plastic envelope to keep it safe from the elements. I can't tell you how much code I've lost to a backup means which are no longer reachable.... I don't have a paper card reader to read my old cobol deck, no drive for my 5 1/4" floppies, or my 3 1/2" floppies. but yet the print out that I made of my first big project still sits readable...even after my once 3 year old decided that it would make a good coloring book.
When you state "back up source code", I hope you include in your meaning the backing up of your version control system too.
Backing your current source code (to multiple places) is definitely critical, but backing up your history of changes as preseved by your VCS is paramount in my opinion. It may seem trivial especially when we are always "living in the present, looking towards the future". However, there have been way too many times when we have wanted to look backward to investigate an issue, review the chain of changes, see who did what, whether we can rollback to a previous build/version. All the more important if you practise heavy branching and merging. Archiving a single trunk will not do.
Your version control system may come with documentation and suggestions on backup strategies.
One way would be to periodically recycle your storage media, i.e. read data off the decaying medium and write it to a fresh one. There exist programs to assist you with this, e.g. dvdisaster. In the end, nothing lasts forever. Just pick the least annoying solution.
As for #2: you can store data in encrypted form to prevent data recovery experts from making sense of it.
I think Option 2 works well enough if you have the write backup mechanisms in place. They need not be expensive ones involving a third-party, either (except for disaster recovery).
A RAID 5 configured server would do the trick. If a hard drive fails, replace it. It is HIGHLY unlikely that all the hard drives will fail at the same time. Even a mirrored RAID 1 drive would be good enough in some cases.
If option 2 still seems like a crappy solution, the only other thing I can think of is to print out hard-copies of the source code, which has many more problems than any of the above solutions.
Online storage is subject to the whim of some data storage center, the security or lack of security there, and the possibility that the company folds, etc. Plus it's expensive,
Not necessarily expensive (see rsync.net for example), nor insecure. You can certainly encrypt your stuff too.
and you can't guarantee that they aren't peeking in.
True, but there's probably much more interesting stuff to peek at than your source-code. ;-)
More seriously, a RAID array isn't so expensive anymore
RAID is not backup.
I was just talking with a guy who is an expert in microfilm. While it is an old technology, for long term storage it is one of the most enduring forms of data storage if properly maintained. It doesn't require sophisticated equipment (magifying lens and a light) to read altough storing it may take some work.
Then again, as was previously mentioned, if you are only talking in the spans of a few years instead of decades printing it off to paper and storing it in a controlled environment is probable the best way. If you want to get really creative you could laminate every sheet!
Drobo for local backup
DVD for short-term local archiving
Amazon S3 for off-site,long-term archiving
Related
I am working on an application, where we are writing lots and lots of key value pairs. On production the database size will run into hundreds of Terabytes, even multiple Petabytes. The keys are 20 bytes and the value is maximum 128 KB, and very rarely smaller than 4 KB. Right now we are using MongoDB. The performance is not very good, because obviously there is a lot of overhead going on here. MongoDB writes to the file system, which writes to the LVM, which further writes to a RAID 6 array.
Since our requirement is very basic, I think using a general purpose database system is hitting the performance. I was thinking of implementing a simple database system, where we could put the documents (or 'values') directly to the raw drive (actually the RAID array), and store the keys (and a pointer to where the value lives on the raw drive) in a fast in-memory database backed by an SSD. This will also speed-up the reads, as all there would not be no fragmentation (as opposed to using a filesystem.)
Although a document is rarely deleted, we would still have to maintain a pool of free space available on the device (something that the filesystem would have provided).
My question is, will this really provide any significant improvements? Also, are there any document storage systems that do something like this? Or anything similar, that we can use as a starting poing?
Apache Cassandra jumps to mind. It's the current elect NoSQL solution where massive scaling is concerned. It sees production usage at several large companies with massive scaling requirements. Having worked a little with it, I can say that it requires a little bit of time to rethink your data model to fit how it arranges its storage engine. The famously citied article "WTF is a supercolumn" gives a sound introduction to this. Caveat: Cassandra really only makes sense when you plan on storing huge datasets and distribution with no single point of failure is a mission critical requirement. With the way you've explained your data, it sounds like a fit.
Also, have you looked into redis at all, at least for saving key references? Your memory requirements far outstrip what a single instance would be able to handle but Redis can also be configured to shard. It isn't its primary use case but it sees production use at both Craigslist and Groupon
Also, have you done everything possible to optimize mongo, especially investigating how you could improve indexing? Mongo does save out to disk, but should be relatively performant when optimized to keep the hottest portion of the set in memory if able.
Is it possible to cache this data if its not too transient?
I would totally caution you against rolling your own with this. Just a fair warning. That's not a knock at you or anyone else, its just that I've personally had to maintain custom "data indexes" written by in house developers who got in way over their heads before. At my job we have a massive on disk key-value store that is a major performance bottleneck in our system that was written by a developer who has since separated from the company. It's frustrating to be stuck such a solution among the exciting NoSQL opportunities of today. Projects like the ones I cited above take advantage of the whole strength of the open source community to proof and optimize their use. That isn't something you will be able to attain working on your own solution unless you make a massive investment of time, effort and promotion. At the very least I'd encourage you to look at all your nosql options and maybe find a project you can contribute to rather than rolling your own. Writing a database server itself is definitely a nontrivial task that needs a huge team, especially with the requirements you've given (but should you end up doing so, I wish you luck! =) )
Late answer, but for future reference I think Spider does this
We are building a system that will need to serve loads of small requests from day one. By "loads" I mean ~5,000 queries per second. For each query we need to retrieve ~20 records from noSQL database. There will be two batch reads - 3-4 records at first and then 16-17 reads instantly after that (based on the result of first read). That would be ~100,000 objects to read per second.
Until now we were thinking about using DynamoDB for this as it's really easy to start with.
Storage is not something I would be worried about as the objects will be really tiny.
What I am worried about is cost of reads. DynamoDB costs $0.0113 per hour per 100 eventually consistent (which is fine for us) reads per second. That is $11,3 per hour for us provided that all objects are up to 1KB in size. And that would be $5424 per month based on 16 hours/day average usage.
So... $5424 per month.
I would consider other options but I am worried about maintenance issues, costs etc. I have never worked with such setups before so your advice would be really valuable.
What would be the most cost-effective (yet still hassle-free) solution for such read/write intensive application?
From your description above, I'm assuming that your 5,000 queries per second are entirely read operations. This is essentially what we'd call a data warehouse use case. What are your availability requirements? Does it have to be hosted on AWS and friends, or can you buy your own hardware to run in-house? What does your data look like? What does the logic which consumes this data look like?
You might get the sense that there really isn't enough information here to answer the question definitively, but I can at least offer some advice.
First, if your data is relatively small and your queries are simple, save yourself some hassle and make sure you're querying from RAM instead of disk. Any modern RDBMS with support for in-memory caching/tablespaces will do the trick. Postgres and MySQL both have features for this. In the case of Postgres make sure you've tuned the memory parameters appropriately as the out-of-the-box configuration is designed to run on pretty meager hardware. If you must use a NoSQL option, depending on the structure of your data Redis is probably a good choice (it's also primarily in-memory). However in order to say which flavor of NoSQL might be the best fit we'd need to know more about the structure of the data that you're querying, and what queries you're running.
If the queries boil down to SELECT * FROM table WHERE primary_key = {CONSTANT} - don't bother messing with NoSQL - just use an RDBMS and learn how to tune the dang thing. This is doubly true if you can run it on your own hardware. If the connection count is high, use read slaves to balance the load.
Long-after-the-fact Edit (5/7/2013):
Something I should've mentioned before: EC2 is a really really crappy place to measure performance of self-managed database nodes. Unless you're paying out the nose, your I/O perf will be terrible. Your choices are to either pay big money for provisioned IOPS, RAID together a bunch of EBS volumes, or rely on ephemeral storage whilst syncing a WAL off to S3 or similar. All of these options are expensive and difficult to maintain. All of these options have varying degrees of performance.
I discovered this for a recent project, so I switched to Rackspace. The performance increased tremendously there, but I noticed that I was paying a lot for CPU and RAM resources when really I just need fast I/O. Now I host with Digital Ocean. All of DO's storage is SSD. Their CPU performance is kind of crappy in comparison to other offerings, but I'm incredibly I/O bound so I Just Don't Care. After dropping Postgres' random_page_cost to 2, I'm humming along quite nicely.
Moral of the story: profile, tune, repeat. Ask yourself what-if questions and constantly validate your assumptions.
Another long-after-the-fact-edit (11/23/2013): As an example of what I'm describing here, check out the following article for an example of using MySQL 5.7 with the InnoDB memcached plugin to achieve 1M QPS: http://dimitrik.free.fr/blog/archives/11-01-2013_11-30-2013.html#2013-11-22
By "loads" I mean ~5,000 queries per second.
Ah that's not so much, even SQL can handle that. So you are already easily within the limits of what most modern DBs can handle. However they can only handle this with the right:
Indexes
Queries
Server Hardware
Splitting of large data (you might require a large amount of shards with relatively low data each, dependant here so I said "might")
That would be ~100,000 objects to read per second.
Now that's more of a high load scenario. Must you read these in such a fragmented manner? If so then (as I said) you may require to look into spreading the load across replicated shards.
Storage is not something I would be worried about as the objects will be really tiny.
Mongo is aggresive with disk allocation so even with small objects it will still pre-allocate a lot of space, this is something to bare in mind.
So... $5424 per month.
Oh yea the billing thrills of Amazon :\.
I would consider other options but I am worried about maintenance issues, costs etc. I have never worked with such setups before so your advice would be really valuable.
Now you hit the snag of it all. You can setup your own cluster but then you might end up paying that much in money and time (or way more) for the servers, people, admins and your own mantenance time. This is one reason why DynamoDB really shines here. For large setups who are looking to take the load and pain and stress of server management (trust me it is really painful, if your a Dev you might as well change your job title to server admin from now on) off of the company.
Considering to setup this yourself you would need:
A considerable amount of EC instances (dependant upon data and index size but I would say close to maybe 30?)
A server admin (maybe 2, maybe freelance?)
Both of which could set you back 100's of thousands of pounds a year, I would personally bet for the managed approach if it fits your needs and budget. When your need grows beyond what managed Amazon DB can give you then move to your infrastructure.
Edit
I should amend that the cost effectiveness was done with quite a few black holes for example:
I am unsure of the amount of data you have
I am unsure of writes
Both of these contribute me to place a scenario of:
Massive writes (about as much as your reading)
Massive data (lots)
Here is what I recommend in sequence.
Identify your use case and choose the correct db. We test MySQL and MongoDb regularly for all kinds of workloads (OLTP, Analytics, etc). In all cases we have tested with, MySQL outperforms MongoDb and is cheaper ($/TPS) compared to MongoDb. MongoDb has other advantages but that is another story ... since we are talking about performance here.
Try to cache your queries in RAM (by provisioning adequate RAM).
If you are bottle necked on RAM, then you can try a SSD caching solution which takes advantage of ephemeral SSD. This works if your workload is cache friendly. You can save loads of money as ephemeral SSD is typically not charged by the cloud provider.
Try PIOPS/RAID or a combination to create adequate IOPS for your application.
I often hear about eventual consistency in different speeches about NoSQL, data grids etc.
It seems that definition of eventual consistency varies in many sources (and maybe even depends on a concrete data storage).
Can anyone give a simple explanation what Eventual Consistency is in general terms, not related to any concrete data storage?
Eventual consistency:
I watch the weather report and learn that it's going to rain tomorrow.
I tell you that it's going to rain tomorrow.
Your neighbor tells his wife that it's going to be sunny tomorrow.
You tell your neighbor that it is going to rain tomorrow.
Eventually, all of the servers (you, me, your neighbor) know the truth (that it's going to rain tomorrow), but in the meantime the client (his wife) came away thinking it is going to be sunny, even though she asked after one or more of the servers (you and me) had a more up-to-date value.
As opposed to Strict Consistency / ACID compliance:
Your bank balance is $50.
You deposit $100.
Your bank balance, queried from any ATM anywhere, is $150.
Your daughter withdraws $40 with your ATM card.
Your bank balance, queried from any ATM anywhere, is $110.
At no time can your balance reflect anything other than the actual sum of all of the transactions made on your account to that exact moment.
The reason why so many NoSQL systems have eventual consistency is that virtually all of them are designed to be distributed, and with fully distributed systems there is super-linear overhead to maintaining strict consistency (meaning you can only scale so far before things start to slow down, and when they do you need to throw exponentially more hardware at the problem to keep scaling).
Eventual consistency:
Your data is replicated on multiple servers
Your clients can access any of the servers to retrieve the data
Someone writes a piece of data to one of the servers, but it wasn't yet copied to the rest
A client accesses the server with the data, and gets the most up-to-date copy
A different client (or even the same client) accesses a different server (one which didn't get the new copy yet), and gets the old copy
Basically, because it takes time to replicate the data across multiple servers, requests to read the data might go to a server with a new copy, and then go to a server with an old copy. The term "eventual" means that eventually the data will be replicated to all the servers, and thus they will all have the up-to-date copy.
Eventual consistency is a must if you want low latency reads, since the responding server must return its own copy of the data, and doesn't have time to consult other servers and reach a mutual agreement on the content of the data. I wrote a blog post explaining this in more detail.
Think you have an application and its replica. Then you have to add new data item to the application.
Then application synchronises the data to other replica show in below
Meanwhile new client going to get data from one replica that not update yet. In that case he cant get correct up date data. Because synchronisation get some time. In that case it haven't eventually consistency
Problem is how can we eventually consistency?
For that we use mediator application to update / create / delete data and use direct querying to read data. that help to make eventually consistency
When an application makes a change to a data item on one machine, that change has to be propagated to the other replicas. Since the change propagation is not instantaneous, there’s an interval of time during which some of the copies will have the most recent change, but others won’t. In other words, the copies will be mutually inconsistent. However, the change will eventually be propagated to all the copies, and hence the term “eventual consistency”. The term eventual consistency is simply an acknowledgement that there is an unbounded delay in propagating a change made on one machine to all the other copies. Eventual consistency is not meaningful or relevant in centralized (single copy) systems since there’s no need for propagation.
source: http://www.oracle.com/technetwork/products/nosqldb/documentation/consistency-explained-1659908.pdf
Eventual consistency means changes take time to propagate and the data might not be in the same state after every action, even for identical actions or transformations of the data. This can cause very bad things to happen when people don’t know what they are doing when interacting with such a system.
Please don’t implement business critical document data stores until you understand this concept well. Screwing up a document data store implementation is much harder to fix than a relational model because the fundamental things that are going to be screwed up simply cannot be fixed as the things that are required to fix it are just not present in the ecosystem. Refactoring the data of an inflight store is also much harder than the simple ETL transformations of a RDBMS.
Not all document stores are created equal. Some these days (MongoDB) do support transactions of a sort, but migrating datastores is likely comparable to the expense of re-implementation.
WARNING: Developers and even architects who do not know or understand the technology of a document data store and are afraid to admit that for fear of losing their jobs but have been classically trained in RDBMS and who only know ACID systems (how different can it be?) and who don’t know the technology or take the time to learn it, will miss design a document data store. They may also try and use it as a RDBMS or for things like caching. They will break down what should be atomic transactions which should operate on an entire document into “relational” pieces forgetting that replication and latency are things, or worse yet, dragging third party systems into a “transaction”. They’ll do this so their RDBMS can mirror their data lake, without regard to if it will work or not, and with no testing, because they know what they are doing. Then they will act surprised when complex objects stored in separate documents like “orders” have less “order items” than expected, or maybe none at all. But it won’t happen often, or often enough so they’ll just march forward. They may not even hit the problem in development. Then, rather than redesign things, they will throw “delays” and “retries” and “checks” in to fake a relational data model, which won’t work, but will add additional complexity for no benefit. But its too late now - the thing has been deployed and now the business is running on it. Eventually, the entire system will be thrown out and the department will be outsourced and someone else will maintain it. It still won’t work correctly, but they can fail less expensively than the current failure.
In simple English, we can say: Although your system may be in inconsistent states, the aim is always to reach consistency at some point for each piece of data.
Eventual consistency is more like a spectrum. On one end you have strong consistency and on other you have eventual consistency. In between there are levels like Snapshot, read my writes, bounded staleness. Doug Terry has a beautiful explanation in his paper on eventual consistency thru baseball
.
As per me eventual consistency is basically toleration to random data in random order every time you read from a data store. Anything better than that is a stronger consistency model. For example, a snapshot has stale data but will return same data if read again so it is predictable. Sometimes application can tolerate data which is stale for a given amount of time beyond which it demands consistent data.
If you look at meaning of consistency it relates more to uniformity or lack of deviation. So in non computer system terms it could mean toleration for unexpected variations. It could be very well explained thru ATM. An ATM could be offline hence divergent from account balance from core systems. However there is a toleration for showing different balances for a window of time. Once the ATM comes online, it can sync with core systems and reflect same balance. So an ATM could be said to be eventually consistent.
Eventual consistency guarantees consistency throughout the system, but not at all times. There is an inconsistency window, where a node might not have the latest value, but will still return a valid response when queried, even if that response will not be accurate. Cassandra has a ring system where your data is split up into different nodes:
Any of those nodes can act as the primary interface point for your application. So there is no single point of failure because any of those nodes can serve as your primary API point. But there is a trade-off here. Because any node can be primary, that data needs to be replicated amongst all of these nodes in order to stay up to date. So all of the other nodes needs to know what is where at all times and that means that as a trade-off for this architecture, we have eventual consistency. Because it takes time for that data to propagate throughout the ring, through every node in your system. So, as the data is written, it might be a little bit of time before you can actually read that data back you just wrote. Maybe data is written to one node, but you are reading it from a different node and that written data have not made it to that other node yet.
Let's say you back up your photos on your phone to the cloud every Sunday. If you check your photos on Friday on your cloud, you are not going to see the photos that were taken between Monday-Friday. You are still getting a response but not an updated response but if you check your cloud on Sunday night you will see all of your photos. So your data across phone and cloud services eventually reach consistency.
i had a discussion with a coworker about the architecture of a program i'm writing and i'd like some more opinions.
The Situation:
The Program should update at near-realtime (+/- 1 Minute).
It involves the movement of objects on a coordinate system.
There are some events that occur at regular intervals (i.e. creation of the objects).
Movements can change at any time through user input.
My solution was:
Build a server that runs continously and stores the data internally.
The server dumps a state-of-the-program at regular intervals to protect against powerfailures and/or crashes.
He argued that the program requires a Database and i should use cronjobs to update the data. I can store movement information by storing startpoint, endpoint and speed and update the position in the cronjob (and calculate collisions with other objects there) by calculating direction and speed.
His reasons:
Requires more CPU & Memory because it runs constantly.
Powerfailures/Crashes might destroy data.
Databases are faster.
My reasons against this are mostly:
Not very precise as events can only occur at full minutes (wouldn't be that bad though).
Requires (possibly costly) transformation of data on every run from relational data to objects.
RDBMS are a general solution for a specialized problem so a specialized solution should be more efficient.
Powerfailures (or other crashes) can leave the Data in an undefined state with only partially updated data unless (possibly costly) precautions (like transactions) are taken.
What are your opinions about that?
Which arguments can you add for any side?
Databases are not faster. How silly... How can a database be faster than writing a custom data structure and storing it in memory ?? Databases are Generalized tools to persist data to disk for you so you don't have to write all the code to do that yourself. Because they have to address the needs of numerous disparate (and sometimes inconsistent) business functions (Persistency (Durability), Transactional integrity, caching, relational integrity, atomicity, etc. etc. ) and do it in a way that protects the application developer from having to worry about it so much, by definition it is going to be slower. That doesn't necessarilly mean his conclusion is wrong however.
Each of his other objections can be addressed by writing the code to address that issue yourself... But you see where that is going... At some point, the development efforts of writing the custom code to address the issues that are important for your application outweigh the performance hit of just using a database - which already does all that stuff out of the box... How many of these issues are important ? and do you know how to write the code necessary to address them ?
From what you've described here, I'd say your solution does seem to be the better option. You say it runs once a minute, but how long does it take to run? If only a few seconds, then the transformation to relational data would likely be inconsequential, as would any other overhead. most of this would take likely 30 seconds. This is assuming, again, that the program is quite small.
However, if it is larger, and assuming that it will get larger, doing a straight dump is a better method. You might not want to do a full dump every run, but that's up to you, just remember that it could wind up taking a lot of space (same goes if you're using a database).
If you're going to dump the state, you would need to have some sort of a redundancy system in place, along with quasi-transactions. You would want to store several copies, in case something happens to the newest version. Say, the power goes out while you're storing, and you have no backups beyond this half-written one. Transactions, you would need something to tell that the file has been fully written, so if something does go wrong, you can always tell what the most recent successful save was.
Oh, and for his argument of it running constantly: if you have it set to a cronjob, or even a self-enclosed sleep statement or similar, it doesn't use any CPU time when it's not running, the same amount that it would if you're using an RDBMS.
If you're writing straight to disk, then this will be the faster method over a database, and faster retrieval, since, as you pointed out, there is no overhead.
Summary: A database is a good idea if you have a lot of idle processor time or historical records, but if resources are a legitimate concern, then it can become too much overhead and a dump with precautions taken is better.
mySQL can now model spatial data.
http://dev.mysql.com/doc/refman/4.1/en/gis-introduction.html
http://dev.mysql.com/doc/refman/5.1/en/spatial-extensions.html
You could use the database to keep track of world locations, user locations, items locations ect.
Many file storage systems use hashes to avoid duplication of the same file content data (among other reasons), e.g., Git and Dropbox both use SHA256. The file names and dates can be different, but as long as the content gets the same hash generated, it never gets stored more than once.
It seems this would be a sensible thing to do in a OS file system in order to save space. Are there any file systems for Windows or *nix that do this, or is there a good reason why none of them do?
This would, for the most part, eliminate the need for duplicate file finder utilities, because at that point the only space you would be saving would be for the file entry in the file system, which for most users is not enough to matter.
Edit: Arguably this could go on serverfault, but I feel developers are more likely to understand the issues and trade-offs involved.
ZFS supports deduplication since last month: http://blogs.oracle.com/bonwick/en_US/entry/zfs_dedup
Though I wouldn't call this a "common" filesystem (afaik, it is currently only supported by *BSD), it is definitely one worth looking at.
It would save space, but the time cost is prohibitive. The products you mention are already io bound, so the computational cost of hashing is not a bottleneck. If you hashed at the filesystem level, all io operations which are already slow will get worse.
NTFS has single instance storage.
NetApp has supported deduplication (that's what its called in the storage industry) in the WAFL filesystem (yeah, not your common filesystem) for a few years now. This is one of the most important features found in the enterprise filesystems today (and NetApp stands out because they support this on their primary storage also as compared to other similar products which support it only on their backup or secondary storage; they are too slow for primary storage).
The amount of data which is duplicate in a large enterprise with thousands of users is staggering. A lot of those users store the same documents, source code, etc. across their home directories. Reports of 50-70% data deduplicated have been seen often, saving lots of space and tons of money for large enterprises.
All of this means that if you create any common filesystem on a LUN exported by a NetApp filer, then you get deduplication for free, no matter what the filesystem created in that LUN. Cheers. Find out how it works here and here.
btrfs supports online de-duplication of data at the block level. I'd recommend duperemove as an external tool is needed.
It would require a fair amount of work to make this work in a file system. First of all, a user might be creating a copy of a file, planning to edit one copy, while the other remains intact -- so when you eliminate the duplication, the hard link you created that way would have to give COW semantics.
Second, the permissions on a file are often based on the directory into which that file's name is placed. You'd have to ensure that when you create your hidden hard link, that the permissions were correctly applied based on the link, not just the location of the actual content.
Third, users are likely to be upset if they make (say) three copies of a file on physically separate media to ensure against data loss from hardware failure, then find out that there was really only one copy of the file, so when that hardware failed, all three copies disappeared.
This strikes me as a bit like a second-system effect -- a solution to a problem long after the problem ceased to exist (or at least matter). With hard drives current running less than $100US/terabyte, I find it hard to believe that this would save most people a whole dollar worth of hard drive space. At that point, it's hard to imagine most people caring much.
There are file systems that do deduplication, which is sort of like this, but still noticeably different. In particular, deduplication is typically done on a basis of relatively small blocks of a file, not on complete files. Under such a system, a "file" basically just becomes a collection of pointers to de-duplicated blocks. Along with the data, each block will typically have some metadata for the block itself, that's separate from the metadata for the file(s) that refer to that block (e.g., it'll typically include at least a reference count). Any block that has a reference count greater than 1 will be treated as copy on write. That is, any attempt at writing to that block will typically create a copy, write to the copy, then store the copy of the block to the pool (so if the result comes out the same as some other block, deduplication will coalesce it with the existing block with the same content).
Many of the same considerations still apply though--most people don't have enough duplication to start with for deduplication to help a lot.
At the same time, especially on servers, deduplication at a block level can serve a real purpose. One really common case is dealing with multiple VM images, each running one of only a few choices of operating systems. If we look at the VM image as a whole, each is usually unique, so file-level deduplication would do no good. But they still frequently have a large chunk of data devoted to storing the operating system for that VM, and it's pretty common to have many VMs running only a few operating systems. With block-level deduplication, we can eliminate most of that redundancy. For a cloud server system like AWS or Azure, this can produce really serious savings.