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SQLite improves performance with memory-mapped I/O (sqlite.org)
185 points by moonboots 1644 days ago | hide | past | web | 80 comments | favorite

I like to take every possible opportunity to sing the praises of SQLite, which is one of the most widely deployed RDBMS's in the world. Not only is it open source, but it's public domain, and it's an incredibly stably developed and well-tested piece of software.

Please note that it's not for every application. It is not a client/server architecture. It simply a tightly optimized piece of C code that interacts with a file (or RAM) to create a lightning fast approximation of a SQL server that is ACID compliant.

One thing I've used it for the most, is for small, ad-hoc internal web applications. With the webserver as its "single user", it is incredibly speedy and reliable and holds a shockingly large amount of data (still works pretty well when data gets up into the Terabytes).

> it's an incredibly stably developed and well-tested piece of software.

It also has a policy of safe by default, fast at your risk (mmap is itself an example, it is disabled by default), which is the only sane attitude when working with databases.

Contrast this to "modern" databases such as MongoDB, which is crazy fast out of the box, but then you start enabling durability, synchronous updates, etc... it becomes slower than standard DBs. This makes you win benchmarks, but in the long term causes a lot of headache and drives users away.

   it becomes slower than standard DBs
Sorry but either you're ignorant or being deliberately disingenuous. Either way this statement is completely untrue for many use cases. Can you not imagine situations where a document database would be orders of magnitude faster than ANY SQL one ?

Think about a document with hundreds of embedded documents which equate to joins in SQL.

You can use SQL as a document store, you don't have to use joins.

MongoDB queues inserts into memory before flushing it to disk. This means it's not durable in case of a power outage before the data is flushed. Turn on the safety of flushing-first and it's more or less doing the same stuff your sql software is doing.

Is is, in fact, much much slower than both SQLite and Postgres. It's even slower than a fully indexed hstore in postgres! (which is pretty much identical to mongo's data model, except consistent, durable, with transactions, etc.)

That doesn't make sense. Relational databases use relational algebra (which includes joins) to model data. The mere fact that you model it that way doesn't necessarily have to affect the physical representation. True, there is no such thing as an actual RDBMS on the market, but that's hardly Dr. Codd's fault. :-)

> which is one of the most widely deployed RDBMS's in the world

Being inside Chrome, Android, Firefox, Skype and other places (http://www.sqlite.org/famous.html), I'd argue that SQLite is by far the most widely deployed RDBMS in the world :-)

And, BTW, I love it too.

It is interesting how SQLite is so well known but de SCM built on top of it, Fossil, and by the same author did not catch so much.

Sqlite filled a desperate need with no contenders, while Fossil, released in 2006, was going up against git and hg (both released a year earlier). Though Fossil is much more than a SCM (wiki, bug tracker, blog) and while not mainstream, is still used by tcl.tk and others.

SQLite itself is hosted on fossil, as is Tcl/Tk as you mention (and it's various satellite projects), and I know NetBSD was flirting with it too (no formal decision made yet on migrating from CVS afaik). I use fossil for all of my personal projects. It's more than capable, and a joy to use. Everybody should try it for a week. http://www.fossil-scm.org/

It is also an excellent tool to have in your command line toolkit. Rather than | wc | uniq | sort etc, pipe to file and bring into SQLite for more extensive querying.

I've a different but related use case. I want to make a 5 GB file in a specific format, which requires that the data be sorted. I want it to run on my laptop machine, which has 4 GB of RAM. I dump the unsorted data to a sqlite database, tell it to use up to 1 GB of cache, create an index on the sort field, and pull back the sorted data, which I can stream directly to the file.

What about simply using the original NoSQL system? http://www.strozzi.it/cgi-bin/CSA/tw7/I/en_US/nosql/Home%20P...

I'm using Python, which has had sqlite as a standard module since Python 2.5. Therefore, I don't need to install any third-party module.

NoSQL provides command-line programs. sqlite is embedded in the same process. Error reporting across exec boundaries is much more complicated.

NoSQL requires standard unix command-line programs, and for at least one case "This program has only been tested with GNU sort" This makes portability to other OSes, specifically MS Windows but perhaps also OpenBSD, harder.

Hence there's no "simple" about using NoSQL for my use case.

That's pretty interesting. Can you give an example?

Just make a quick, loose, all-text-fields schema and dump your data as a CSV: http://www.sqlite.org/cvstrac/wiki?p=ImportingFiles Then you can run SQL queries against it using the SQLite commandline tool: http://www.sqlite.org/sqlite.html

I love using SQLite for websites. Unlike MySQL, SQL is a well-written and predictable database, there's no daemon, users or permissions to worry about, and the runtime is built into Python!

You have to be careful though, as SQLite doesn't support concurrent read/writes, so it blocks, which makes it rather difficult to scale up for websites, or at least it has in my experience with it.

Yes. In the Go language one can use channels to funnel the data to/from a single process that does the SQLite reads/writes. It's trivial to batch up data in transactions as it comes in from users (the current channel length is the number of rows to include in the transaction).

Could you explain this a bit more, or point me to a tutorial? That actually sounds really promising.

This is how you'd perpetually write data from all users using a single process, batching the data into transactions for better performance; there's not much new here that isn't covered in most tutorials: On app init create a channel into which data will be put as it comes in from users, and start a goroutine that is passed the channel. In the goroutine have an endless for loop, inside of which you wait for data to arrive in the channel, using a select statement. When data arrives start a SQLite transaction, then iterate the length of the channel in a for loop, writing rows to the SQLite table. (The length of the channel is the number of data values currently in it, which will vary depending on how fast you're putting data into it from elsewhere in the app.) After that loop you commit the transaction.

It supports concurrent reads/single writer.

One potential pitfall of SQLite is that datatypes declared in your CREATE TABLE statements have purely indicative value. You can in effect store strings or booleans in an integer column. I'd welcome a 'strict mode' which would do away with weak typing and behave more like traditional SQL databases.

I have to agree wholeheartedly. I find SQlite a great choice for web apps and small DBD websites as it's so much easier to deploy and more than fast-enought for most projects. Plus it is easy to upgrade if the need arises as the queries are for the most-part identical to SQL.

Yes, and I don't understand why Wordpress does not support it officially and we need to use MySQL instead.

The theoretical answer is because SQLite is not the right tool for the sort of things Wordpress does. SQLite supports more concurrency than one might expect from an embedded db - it's still an embedded db, though. Writing, for instance involves a database-global lock.

The practical answer is - Wordpress is written in a MySQL-specific way. It doesn't officially support, say, Postgres or Oracle either.

> The theoretical answer is because SQLite is not the right tool for the sort of things Wordpress does.

Which probably isn't true.

SQLite is bad at concurrent writes. Your average wordpress install is almost only reads. And with 3.7.0's WAL, readers and writers don't block one another anymore.

The trouble is that there are many, many Wordpress sites out there that are not an average Wordpress install. People (disturbingly) write e-commerce sites on top of Wordpress. SQLite is not the right tool for the sort of things Wordpress does not because you couldn't write a blogging tool using SQLite but because Wordpress is crazy.

Given how easy it is to install Wordpress such that it will fall over at the slightest breeze of traffic, and it appears to be installed that way by default, it's hard to see SQLite making things any worse. Maybe it wouldn't handle crazy situations out of the box, but WP is already in that situation anyway.

If one could write blogs and ecommerce sites using MyISAM (shrudders) then why wouldn't SQLite work? MyISAM had -- at least in theory, I have not done any benchmarks -- even worse concurrency than SQLite.

I hope I've completely forgotten any details of the convoluted locking behaviours and tuning options of MyISAM (which, in my head at least, is read as 'miasma') but they were there to at least attempt to support certain types of concurrency. And they were table-level whereas the SQLite write lock is db-wide.

Could you reasonably write blogging software that uses SQLite? Sure. It's just that Wordpress isn't that blogging software - it's written to be paired with MySQL, for good or ill.

MyISAM has reader/writer table locks[1] which meant that nobody could read while someone was writing to a table.

SQLite can be ran in two modes 1) database reader/writer lock or 2) in WAL mode[2] where readers do not block writers and writers do not block readers. Writers block each other though so there can only be one concurrent writer. WAL mode provides concurrency superior to what MyISAM did.

  1. The locks could sometimes be bypassed by inserts, but not reliably so.
  2. http://www.sqlite.org/wal.html

Sure but WAL in SQLite is 3 years old. Wordpress is 10 years old. MySQL with MyISAM is a lot older than that. I personally happen to think almost everything about Wordpress is wrong. It's just hard to blame them they didn't start or subsequently adopt SQLite. I doubt it's ever been as much as on their radar.

I believe you can approximate table-level locks in sqlite by splitting your database into multiple files. Transactions across multiple databases are atomic (provided you don't use WAL), so you're still in a fairly safe place. However, I'm not sure how practical it is to split a database into lots of files; or whether the sqlite engine deals efficiently with queries that span many databases, for instance - I've never actually done this.

But if you really need a particular table to be writable without locking the rest of the database, it's probably a fairly decent workaround.

Agreed, and I do not think the lack of SQLite support in Wordpress has anything to do with what SQLite can or cannot do. Only that it was written by LAMP developers who do not care for databases other than MySQL.

I think WordPress just tries to cut complexity by using MySQL only so it doesn't have to deal with support for arbitrary RDBMS's.

More likely, the code is poorly written and heavily reliant on MySQL.

This is more or less the correct answer for Wordpress-the-ecosystem.

Wordpress-the-base-install has a thin abstraction layer over the database in the $wpdb object.

But plugins and themes can, and regularly do, use PHP's MySQL functions to access and manipulate the DB directly.

WordPress doesn't support anything except MySQL because all the queries are specifically written for it. Developing for a single database enables the queries to be optimized for that database.

(That's the official WP line anyway, I'd love Postgres support myself.)

Fun SQLite story, I had a project that needed to do reasonably large scale data processing (gigabytes of data) on a pretty bare boned machine in a pretty bare boned environment at a customer location. I had a fairly up-to-date perl install and notepad. For the process the data needed to look at any single element in the data and find elements similar to it. I thought long and hard about various complex data structures and caching bits to disk to make portions of the problem set fit into memory, and various ways of searching the data. It was okay if the process took a couple days to finish.

It suddenly hit me, I wonder if the sysadmins had installed DBD::SQLite...yes they had!

Why mess with all that crap if I could just use SQLite. Reasonably fast indexing, expressive searching tools, I could dump the data onto disk and use an in memory SQLite db at the same time, caching solved. It turned weeks of agonizing work (producing fragile code) and turned it into a quick 2 week coding romp. The project was a huge success and spread to a couple other departments.

One day they asked if I could advise a group to build a more "formal" implementation of the system as they were finding the results valuable. A half dozen developers and a year later they had succeeded in building a friendlier web interface running on a proper RDBMS (Oracle 10something) with some hadoop goodness running the processing stuff on 8 or 9 dedicated machines.

In the meantime, and largely because SQLite let me query into the data in more and more useful ways that I hadn't foreseen, I had extended the original prototype significantly and it was outperforming their larger scale solution on a 3 year old, very unimpressive, desktop with the same install of perl and sqlite. On top of the raw calculations, it was also performing some automated results analysis and spitting out ready to present artifacts that could be stuck in your presentation tool of choice. Guess which ones the users actually used?

Soon I had a users group, regular monthly releases and was running a semi-regular training class on this cooky command line tool (since most users had never seen a command line). I've since finished that job up and moved on, and last I heard my little script using SQLite was in heavy use, the enterprise dev team was still struggling getting the performance of their solution up to the levels of my original script, and hadn't even touched the enhancements yet.

SQLite was rock-iron solid the entire time. I had a few things I wished it could do, but I usually found a way to manage. I wasn't using it for its feature, it existing was the feature. SQLite sprinkled magic on my one-off hack job and turned into a thing that people ended up using.

DBD::SQLite remains one of my greatest successes - so glad you like it.

So many projects I have prototyped using SQLite (although for most I later upgrade to PostgreSQL). Such a great little tool, and building DBD::SQLite the way I did (including all the source) made it so easy for anyone to install.

(once again why HN is awesome)

Thank you thank you thank you thank you

from the bottom of my heart.

DBD::SQLite has been one of the most important libraries for perl that I've ever used. Including the SQLite libraries instead of making it another dependency to install someplace makes it so unbelievably useful.

Plus it's surprisingly easy to move code over to using a more grown up database later.

I've had similar story - direct port of "sqlite" to "postgres" (localhost connection) made postgresql originally 6 times slower. But then I've made a connection pool connecting N, N2, or N3 connections where N is the number of logical CPU's. The trick was to use pg_export_snapshot, and make all connections to go through the same place.

This way I matched SQLite, but then I've made it x3-x4 faster by using COPY (SELECT *....) in BINARY format (took some time to write binary parser) - and what was loading for 3-4secs from SQLite, loads for 0.8secs from PostgreSQL 9.2 (Windows 7 / 32GB / 6 core / 12 cpu machine).

e.g. - the point is - that if you care, you could go and optimize a lot. This is not a fair story though, since I've used real enums, floats and ints on PostgreSQL which saved quite a lot of space. The db was bigger than SQLite even with that, but somehow faster... though faster only when used through multiple connections.

I could've go the opposite way, and find ways to make SQLite shine in this respect.

One lesson from the game development industry is that it takes 5 years to develop the best looking games at the end of the hardware cycle. And the hardware did not change even bit for the whole time.

It takes a lot of learning, experimenting, trying out strange things until you get good. And even then, someone comes and makes your solution look stupid :) (I hope someone comes and would do that for me, so I just sit, enjoy and smile at his/her code - several times I've seen it).

Great story. I suspect there are about a million single user applications out there running oracle and paying huge license fees for db's under a million rows that would have been orders of magnitude easier to set up and faster using SQLite.

One thing to ponder about single user apps running on Oracle: many environments won't let the developer manhandle the database, so now they need to divert or hire a dedicated DBA person to configure and do the dB stuff. SQLite literally makes this entire problem go away.

To add another war story: back in 2004 I was in the position of having to write software for a very, very underpowered barcode terminal running ucLinux (the intermec ck1, for those interested. Only a year later, it was discontinued (grrr) due to the EU finalizing the RoHS norm).

I needed to synchronize product data from our web application with that scanner in order to display useful info instead of just scanned bar codes.

Thankfully I found a patch for SQLite to work with the greatly reduced stack size in ucLinux (the site is still around here: http://www.menie.org/georges/uClinux/sqlite.html)

Now, after 9 years, one user is still holding on to their device and we're still synchronizing product data to that SQLite database. We never had crashes or data losses (even though recovery would be trivial) and ever since then, every subsequent hardware solution we were using always relied on SQLite for storage.

I don't even want to start thinking about how much pain it would have been to write my own database for that slow ck1 monster. Seeing SQLite run on the device in my serial terminal was butch a lt of fun and a great relief as it just saved me countless hours of work.

Me, I love SQLite :-)

I love to use SQLite for projects which process data locally and do not have concurrent writes. SQLite is faster than other database alternatives I tried. I guess one of the main reasons is that there's no context switch on every query. Data set sizes are usually tens of million keys and tens of gigabytes of data. Works like a charm. Btw. SQLite was over 10 times faster than some of the slowest SQLite servers I tried. I have 20+ projects using SQLite successfully. Other tested options were MS SQL, PostgreSQL, MySQL, MongoDB and CodernityDB. MongoDB performance isn't so great, when every commit must be persisted.

Using the Go language you can pretty easily get around the concurrent write issue, using channels. Data for concurrent users is written to the same channel. A routine that reads from the channel does all the writing to the SQLite database.

Nice! As a longtime developer using a "proper RDBMS" I was amazed at how feature packed and speedy SQLite is for having such a small footprint.

Well, SQLite might not be rocket science, but it is related to it:

http://en.wikipedia.org/wiki/SQLite#History (SQLite was created for software on missile destroyers)

I'm intrigued by disadvantage 3 from http://www.sqlite.org/mmap.html :

The operating system must have a unified buffer cache in order for the memory-mapped I/O extension to work correctly, especially in situations where two processes are accessing the same database file and one process is using memory-mapped I/O while the other is not. Not all operating systems have a unified buffer cache. In some operating systems that claim to have a unified buffer cache, the implementation is buggy and can lead to corrupt databases.

Which commonly-used operating systems lack a bug-free unified buffer cache?


Seems like one where they had trouble was OpenBSD.

Later on, the say mmapped I/O is always disabled on OpenBSD because of a buggy unified buffer cache.

We have exactly the same issue in Cyrus IMAPd.

That said, we have a test for it during configure, and either compile with full mmap support or with dodgy fake mmap support (which is shit - it just reads the entire file into real memory every time)


True, my bad!

Not sure, but there are references to proven incidents of fsync bugs in the docs. If you get that wrong, it is not hard to believe that the buffer cache could be wrong too.

My rule of thumb :

Does it can be build using SQLite ?

Yes => Use SQLite ;

No => Use PostegreSQL.

Using SQLite on small/medium projects is so better than alternatives : keeps all the ease of text files while giving the full power and reliability of a RDBMS.

Also never say "we will move from X to PostgreSQL later on" where X is something like SQLite or MySQL. It will NEVER happen.

When I was building the Stumbleupon toolbar for IE way back in the day (2009ish?), we decided that we were going to use two things that made our life extremely easy: 1) Javascript and 2) SQLite. We basically had some C++ code that exposed SQLite to the embedded JS, and ended up writing the majority of the code in JS rather than C++.

SQLite was a fantastic embedded DB that was trivial to integrate with the JS engine in Win32. I don't recall ever having to deal with any support issues around corrupted databases.

It looks like the IE toolbar is still using SQLite to this day - you can explore stumbledata.db from any SQLite explorer.

Cool! Though a bit harsh to have it always disabled on OpenBSD. If you're running only one process or core, you could still benefit from mmaped I/O, couldn't you?

It would be nice to see some benchmarks showing these speedups. It's rare for mmap to be a significant win over read/write.

The SQLite site continues to offer a wealth of information about SQLite internals. Most of the pages (ex. http://www.sqlite.org/fileio.html) are a good read for anyone interested databases or creating good system software.

Honestly, nobody should use SQLite even for testing your application. If you are going to run PSQL or MySQL, please install that DB and run tests on them. SQLite or other DBMS have a lot of incomparability. I would consider SQLite if I just need a database for some really really small project, such as a CLI that speaks to an API and I need them saved locally, then I would consider shipping my CLI with SQLite setup.

Notice how your advice is devoid of arguments, but full of opinion?

Congrats Richard on a great new release.

I'm not much for using SQLite but using mmap() is a no-brainer for fast file access and I'm surprised they hadn't used it before. I mean, its trivial to use if you know how to use fopen()...

D. Richard Hipp, the inventor and primary developer of SQLite considers the rock-solid reliability of SQLite of primary importance, which is one of the reasons everything from iOS to Skype uses it. (The fact that it has a public domain license doesn't hurt.) He lists the potential pitfalls of using this method on the page: http://www.sqlite.org/mmap.html and leaves it to users to determine if those risks are acceptable to them. I'm sure he's been thinking about this method for a while.

I first raised the issue of mmap in SQLite 2 years ago. http://www.mailinglistarchive.com/html/sqlite-dev@sqlite.org...

Richard started playing with it a couple months later http://www.mailinglistarchive.com/html/sqlite-dev@sqlite.org...

but didn't get any performance benefit at the time because he was Doing It Wrong(tm).

SQLite does a lot of excessive work; this work is justified if you're using it on an embedded processor that doesn't have a full virtual-memory based OS underneath. But on any other platform it's a waste of CPU/power/memory/time. And it's been decades since the majority of embedded applications ran on bare metal without some kind of OS.

This new mmap feature in SQLite promises "up to 2x" performance improvement, while LMDB in SQLite still yields 20x improvement over stock SQLite.

> I'm not much for using SQLite but using mmap() is a no-brainer for fast file access and I'm surprised they hadn't used it before. I mean, its trivial to use if you know how to use fopen()...

mmap() is great for reads, but much tougher for writes. this is because it is hard for the kernel to determine what it is you are trying to do. madvise() helps here, but not many projects seem to use it. using plain old write[v]() for writes can be informative enough to the kernel to "do the right thing".

> using mmap() is a no-brainer for fast file access and I'm surprised they hadn't used it before

With mmap, you map a file into process memory and read and write to it as if it were memory. When you write to this memory there isn't really a way to indicate that an error occurred because there's no function call to return an error code. Instead, the process is sent a SIGBUS to indicate that something happened. This causes control to jump from the memory access into the signal handler.

Normally this would be fine and you can handle the error there. But sqlite is a library. It lives in-process with the "real" application using it. What if the program had a signal handler there? You don't want to intercept signals that they need to receive, and you definitely don't want to mask errors that they are getting on their own mmapped files, and you don't know how to handle the errors that they should be seeing instead. And what if they have some crazy green-threading going on that you've now ripped control away from by entering the signal handler?

In this particular case, sqlite's answer is to crash when that happens, taking down the entire host application with it. That's not okay for a lot of applications, and why it defaults to being turned off.

And maybe this particular case does or doesn't apply to sqlite, but there are always cases that your application's domain-specific knowledge of how it uses disk and memory and CPU can out-perform the OS's more general optimisations.

That's not the only issue, and the particular subtleties of mmap aren't important here. What's important is that when very smart people like the authors of sqlite choose to not do something, it's generally not as much as "no-brainer" as some know-it-all types would have you believe. It's easy to read something like Varnish's "always use mmap and leave it to the OS!" manifesto and think that you now automatically know better than all of those silly plebeians that just don't get it, but usually things are more nuanced than that.

> I mean, its trivial to use if you know how to use fopen

I seriously doubt that the difficulty of implementing it is a concern

> With mmap, you map a file into process memory and read and write to it as if it were memory.

This could be an implimentation difference, but I've always accessed mmapped files as files, but I've only ever used it on *BSD and Solaris and it was very straightforward and easy.

It's just a poorly titled mis-link, clicking through describes in more detail why it's not entirely a 'no brainer':


tl;dr is "Improves performance in some cases with certain caveats/risks, is disabled by default"

> but using mmap() is a no-brainer for fast file access and I'm surprised they hadn't used it before.

Is it always? I remember benchmark sequential file reads and writes and plain old read and write were sometimes faster or there was no difference.

I think a lot of people get bit by small stdio buffers and switch to mmap, not realizing that read() based solutions can be improved greatly by better buffering strategies.

I never saw much of a reason for SQLite either until I started doing mobile development. However, it's nearly a must for doing any sort of intensive storage though with Android or iOS (unless use cases are simple enough for XML and JSON).

I just wish doing asynchronous queries with SQLite on Android was not so obtuse and resulted in a bunch of boilerplate or using third party libraries for a core part of the API.

Have to agree the problem with SQLite isn't SQLite it's the generally terrible wrappers companies put around it.

On both iOS and Android the default ones are terrible. And even many third party ones aren't the best.

On Android at least, I try to use loaderex[1] whenever possible, but it has some limitations. Best third party solution I've found so far. I was kind of fishing for someone to give some alternatives for Android and iOS, but seems like we're all kind of just in a rut :/

[1] https://github.com/commonsguy/cwac-loaderex

It's a no-brainer for "fast", but you had better brain pretty hard if your goal is "reliable".

I had assumed SQLite had "reliable" down :)

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