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MySQL Creator Contemplates RAM-only Databases
Aavidwriter writes "Peter Wayner asks Michael 'Monty' Widenius of MySQL, 'When will RAM prices make disk drives obsolete for database developers?' From Monty's answers, it sounds like hard drives may be nothing but backups before long." From experience, I'd wager that RAM failure rates are less than hard drive failure rates, so it might also mean more stability from that perspective.
But also a very strong Memory Manager. We've all seen a poorly written program corrupt memory.
If you have a database that is stored in RAM and periodically written out to the hard disk (for backup reasons) then you get better performance than if you have a database that is reading and writing most of the time.
UPS would prevent the data loss, the database could be written to disk when the power fails.
I guess the issue with databases is not only speed and reliability, but a totally different ballgame called 'user-perception'. Even now, tape drives are used to archive databases; despite the fact that less than 1 in 1000 of the tape media get used for actually retrieving the data during a crash. NAS devices and the like have changed this, but the temptation remains to use tape.
I guess the RAM vs disk debate is on similar lines - but there are some vital differences:
1. Disks (esp. IDE) have become a commodity item and can be accessed by different system architectures easily.
2. IDE and SCSI standards have stood the test of time - 13 and 20 years respectively, unlike RAM wihch has evolved from Parity, non-EDO, EDO, DRAM, SDRAM, DDR-RAM, RAMBUS RAM etc., and suffers several patent and copyright encumberances.
3. Although RAM prices are driving down, the h/w to interface speciality RAM banks is proprietary and hence cost-prohibitive, and comes with attendant long-term supportability risks - think Palladium, or even Server mobos over the last 10 years. TCO for RAM based systems could thus be much higher than disk-based systems.
Overall, except for apps that need super-high speeds, and users that can risk proprietary stuff, disk-based databases shall remain.
My 0.02
If you keep throwing chairs, one day you'll break windows....
Many databases contemplate database sizes in the tens to tens of thousands of gigabytes. For smaller databases, RAM is an easy thing, and even for a small number of gigabytes it might be reasonable. For larger databases RAM would be unthinkable. The fact that a database developer doesn't know what databases are used for is disturbing.
Most modern databases also make very effective use of RAM as a cache in order to speed up queries. I don't know about MySQL since I don't use it. My guess, however, is that it does not, since that would eliminate the need for this stupid measure.
As far as reliability, RAM is more vulnerable to transient things like cosmic radiation. ECC memory will take care of most single-bit problems (there are lots of them...), but all it can do for multi-bit failures is determine that yes, your data is screwed.
Also, swapping out a bad hard disk in a RAID configuration is relatively simple and has a recovery process. Suppose your RAM stick fails; what is your recourse? You've permanently lost that data, and systems with hot-swappable RAM are much more costly than ones with similar capabilities for hard drives.
Finally, consider the problem of catastrophic system failure. If the power goes out, your RAM dies but your hard disk is still safe. If it is worse (say your facility burns down) then it is much easier to recover data from the charred remnants of a hard disk than from the charred remnants of a DRAM chip.
The idea of replacing disks with DRAMs has been around for quite a while now. But disks continue to get (a bit) faster and (much) larger. Every time the morons want to replace it they get shot down. More sensible people focus on using the resources available in ways such as caches that make systems faster and more reliable.
Surely a well tuned database server stores uses quite a lot of RAM for buffering?
Well, a professional database like Oracle manages its own cache, but MySQL really only relies only on the OS-level cache. The problem with that approach is that the database knows a lot more about what you're doing, so it can make much smarter decisions about what to cache, what to age out, what to prefetch, etc. On an Oracle server, you want to lock the buffer in memory, and leave as little as possible over for the OS filesystem cache. You see, if a block doesn't exist in the cache, it has to be fetched from disk into the database cache, and if it does, the db will go straight to its own cache. Another caching layer inbetween provided by the OS is just wasted.
I don't think Monty understands any of this; in the article he seems to say that ACID, rather than being a fundamental principle of relational databases, is just something you need to do because disks are slower than RAM. The only reason that you might not want full ACID and use semaphores instead, as he suggests, is because you are only updating one record in one table in a single transaction!
Further, if he is thinking in terms of a few Gb of data, then he is a little out of touch with modern database usage. SouthWest airlines do not have a database that stores 10 bytes of data for every seat on every flight, and I have a hard time figuring out why they would want to - the database of seats would be tied into the customer records, the freight/baggage handling database, the billing records, the accounting system. That is the point of relational database, that you can query data based on data that exists elsewhere. Monty thinks in terms of databases that have only a few tables, because that's all you can do in MySQL. He says that database programmers are forced to be very careful about the consistency of their data - well those using MySQL are, but those using Oracle (or any other database with real transactions and real integrity constraints) find it's all taken care of transparently.
...have nothing to do with the medium the data is stored in! What you're trying to guard against is concurrent access of resources by transactions which in cases can cause incorrect or inconsistent results in a RDBMS. I think this article is a bit obvious for most people who've had any training in how databases actually work and I think Monty was actually pretty gracious for taking the time to give the interviewer a smidgeon of clue.
First, as other have said, a properly designed RAM subsystem can be battery backed up. In terms of getting the data out, loss of power to the RAM is no more catastrophic than loss of power to the CPU, the router, the computer running the middleware, or whatever. Because RAM is a purely semiconductor approach, any battery backup system can be simple and reliable.
In fact, it should not be too difficult to design a system which, in the event of power fail, dumps data to backup disk drives. To get to that state, the main system has already failed to do a clean shutdown, so this is a last resort issue.
The next thing is error detection and correction. It's true that single bit ECC is limited, but it also takes only limited resources (7 additional bits for a 32-bit subsystem, 8 for 64). Memory subsystems could have extra columns so that if bit errors start to multiply in one column, it can be switched out for a new one. Just as with any error detection and correction strategy, single bit detection in columns can be combined with further error correction down rows, using dedicated hardware to encode and decode on the fly. Just good old basic electronics.
In the worst case, it should be possible to build an extremely reliable memory system for a bit penalty of 50% - no worse than mirroring two hard drives. It won't be quite as fast as writing direct to motherboard RAM, but we don't want to do that anyway (we want to be able to break the link on power fail, save to disk, then later on restore from disk. And we want the subsystem in its own cabinet along with the batteries. No one in their right minds is suggesting having a couple of C cells taped to this thing and held on with croc clips.)
I'd even venture to suggest that most MySQL databases are not in the terabyte range, and that most databases aren't in the gigabyte range even if they are mission critical in SMEs.
Conclusion? As usual we have the people trying to boast "My database is far too big and complicated for MySQL! So MySQL sucks! My database is too (etc.) to run in RAM! So running DBs from RAM sucks!" and ignoring the fact that there are many web databases where transactional integrity is not an issue, and the market for a RAM store for databases in the low Gbyte range might actually be rather substantial.
Panurge has posted for the last time. Thanks for the positive moderations.