I previously asked what the code below does, and mentioned that it should give interesting insight into the kind of mindset and knowledge a candidate has. Take a look at the code again:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/stat.h>


#define BUFFER_SIZE (3ULL * 1024 * 1024 * 1024) // 3GB in bytes


int main() {
    int fd;
    char *buffer;
    struct stat st;


    buffer = (char *)malloc(BUFFER_SIZE);
    if (buffer == NULL) {
        return 1;
    }


    fd = open("large_file.bin", O_WRONLY | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
    if (fd == -1) {
        return 2;
    }


    if (write(fd, buffer, BUFFER_SIZE) == -1) {
        return 3;
    }


    if (fsync(fd) == -1) {
        return 4;
    }


    if (close(fd) == -1) {
        return 5;
    }


    if (stat("large_file.bin", &st) == -1) {
        return 6;
    }


    printf("File size: %.2f GB\n", (double)st.st_size / (1024 * 1024 * 1024));


    free(buffer);
    return 0;
}

This program will output: File size: 2.00 GB

And it will write 2 GB of zeros to the file:

~$ head  large_file.bin  | hexdump -C
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
7ffff000

The question is why? And the answer is quite simple. Linux has a limitation of about 2 GB for writes to the disk. Any write call that attempts to write more than that will only write that much, and you’ll have to call the system again. This is not an error, mind. The write call is free to write less than the size of the buffer you passed to it.

Windows has the same limit, but it is honest about it

In Windows, all write calls accept a 32-bit int as the size of the buffer, so this limitation is clearly communicated in the API. Windows will also ensure that for files, a WriteFile call that completes successfully writes the entire buffer to the disk.

And why am I writing 2 GB of zeros? In the code above, I’m using malloc(), not calloc(), so I wouldn’t expect the values to be zero. Because this is a large allocation, malloc() calls the OS to provide us with the buffer directly, and the OS is contractually obligated to provide us with zeroed pages.

Here is a pretty simple C program, running on Linux. Can you tell me what you expect its output to be?

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/stat.h>


#define BUFFER_SIZE (3ULL * 1024 * 1024 * 1024) // 3GB in bytes


int main() {
    int fd;
    char *buffer;
    struct stat st;


    buffer = (char *)malloc(BUFFER_SIZE);
    if (buffer == NULL) {
        return 1;
    }


    fd = open("large_file.bin", O_WRONLY | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
    if (fd == -1) {
        return 2;
    }


    if (write(fd, buffer, BUFFER_SIZE) == -1) {
        return 3;
    }


    if (fsync(fd) == -1) {
        return 4;
    }


    if (close(fd) == -1) {
        return 5;
    }


    if (stat("large_file.bin", &st) == -1) {
        return 6;
    }


    printf("File size: %.2f GB\n", (double)st.st_size / (1024 * 1024 * 1024));


    free(buffer);
    return 0;
}

And what happens when I run:

head  large_file.bin  | hexdump -C

This shows both surprising behavior and serves as a good opening for discussion on a whole bunch of issues. In an interview setting, that can give us a lot of insight into the sort of knowledge a candidate has.

The problem was that this took time - many days or multiple weeks - for us to observe that. But we had the charts to prove that this was pretty consistent. If the RavenDB service was restarted (we did not have to restart the machine), the situation would instantly fix itself and then slowly degrade over time.

The scenario in question was performance degradation over time. The metric in question was the average request latency, and we could track a small but consistent rise in this number over the course of days and weeks. The load on the server remained pretty much constant, but the latency of the requests grew.

That the customer didn’t notice that is an interesting story on its own. RavenDB will automatically prioritize the fastest node in the cluster to be the “customer-facing” one, and it alleviated the issue to such an extent that the metrics the customer usually monitors were fine. The RavenDB Cloud team looks at the entire system, so we started the investigation long before the problem warranted users’ attention.

I hate these sorts of issues because they are really hard to figure out and subject to basically every caveat under the sun. In this case, we basically had exactly nothing to go on. The workload was pretty consistent, and I/O, memory, and CPU usage were acceptable. There was no starting point to look at.

Those are also big machines, with hundreds of GB of RAM and running heavy workloads. These machines have great disks and a lot of CPU power to spare. What is going on here?

After a long while, we got a good handle on what is actually going on. When RavenDB starts, it creates memory maps of the file it is working with. Over time, as needed, RavenDB will map, unmap, and remap as needed. A process that has been running for a long while, with many databases and indexes operating, will have a lot of work done in terms of memory mapping.

In Linux, you can inspect those details by running:

$ cat /proc/22003/smaps


600a33834000-600a3383b000 r--p 00000000 08:30 214585                     /data/ravendb/Raven.Server
Size:                 28 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Rss:                  28 kB
Pss:                  26 kB
Shared_Clean:          4 kB
Shared_Dirty:          0 kB
Private_Clean:        24 kB
Private_Dirty:         0 kB
Referenced:           28 kB
Anonymous:             0 kB
LazyFree:              0 kB
AnonHugePages:         0 kB
ShmemPmdMapped:        0 kB
FilePmdMapped:         0 kB
Shared_Hugetlb:        0 kB
Private_Hugetlb:       0 kB
Swap:                  0 kB
SwapPss:               0 kB
Locked:                0 kB
THPeligible:    0
VmFlags: rd mr mw me dw
600a3383b000-600a33847000 r-xp 00006000 08:30 214585                     /data/ravendb/Raven.Server
Size:                 48 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Rss:                  48 kB
Pss:                  46 kB
Shared_Clean:          4 kB
Shared_Dirty:          0 kB
Private_Clean:        44 kB
Private_Dirty:         0 kB
Referenced:           48 kB
Anonymous:             0 kB
LazyFree:              0 kB
AnonHugePages:         0 kB
ShmemPmdMapped:        0 kB
FilePmdMapped:         0 kB
Shared_Hugetlb:        0 kB
Private_Hugetlb:       0 kB
Swap:                  0 kB
SwapPss:               0 kB
Locked:                0 kB
THPeligible:    0
VmFlags: rd ex mr mw me dw

Here you can see the first page of entries from this file. Just starting up RavenDB (with no databases created) will generate close to 2,000 entries. The smaps virtual file can be really invaluable for figuring out certain types of problems. In the snippet above, you can see that we have some executable memory ranges mapped, for example.

The problem is that over time, memory becomes fragmented, and we may end up with an smaps file that contains tens of thousands (or even hundreds of thousands) of entries.

Here is the result of running perf top on the system, you can see that the top three items that hogs most of the resources are related to smaps accounting.

This file provides such useful information that we monitor it on a regular basis. It turns out that this can have… interesting effects. Consider that while we are running the scan through all the memory mapping, we may need to change the memory mapping for the process. That leads to contention on the kernel locks that protect the mapping, of course.

It’s expensive to generate the smaps file

Reading from /proc/[pid]/smaps is not a simple file read. It involves the kernel gathering detailed memory statistics (e.g., memory regions, page size, resident/anonymous/shared memory usage) for each virtual memory area (VMA) of the process. For large processes with many memory mappings, this can be computationally expensive as the kernel has to gather the required information every time /proc/[pid]/smaps is accessed.

When /proc/[pid]/smaps is read, the kernel needs to access memory-related structures. This may involve taking locks on certain parts of the process’s memory management system. If this is done too often or across many large processes, it could lead to contention or slow down the process itself, especially if other processes are accessing or modifying memory at the same time.

If the number of memory mappings is high, and the frequency with which we monitor is short… I hope you can see where this is going. We effectively spent so much time running over this file that we blocked other operations.

This wasn’t an issue when we just started the process, because the number of memory mappings was small, but as we worked on the system and the number of memory mappings grew… we eventually started hitting contention.

The solution was two-fold. We made sure that there is only ever a single thread that would read the information from the smaps (previously it might have been triggered from multiple locations).  We added some throttling to ensure that we aren’t hammering the kernel with requests for this file too often (returning cached information if needed) and we switched from using smaps to using smaps_rollup instead. The rollup version provides much better performance, since it deals with summary data only.

With those changes in place, we deployed to production and waited. The result was flat latency numbers and another item that the Cloud team could strike off the board successfully.

For a new feature in RavenDB, I needed to associate each transaction with a source ID. The underlying idea is that I can aggregate transactions from multiple sources in a single location, but I need to be able to distinguish between transactions from A and B.

Luckily, I had the foresight to reserve space in the Transaction Header, I had a whole 16 bytes available for me. Separately, each Voron database (the underlying storage engine that we use) has a unique Guid identifier. And a Guid is 16 bytes… so everything is pretty awesome.

There was just one issue. I needed to be able to read transactions as part of the recovery of the database, but we stored the database ID inside the database itself. I figured out that I could also put a copy of the database ID in the global file header and was able to move forward.

This is part of a much larger change, so I was going full steam ahead when I realized something pretty awful. That database Guid that I was relying on was already being used as the physical identifier of the storage as part of the way RavenDB distributes data. The reason it matters is that under certain circumstances, we may need to change that.

If we change the database ID, we lose the association with the transactions for that database, leading to a whole big mess. I started sketching out a design for figuring out that the database ID has changed, re-writing all the transactions in storage, and… a colleague said: why don’t we use another ID?

It hit me like a ton of bricks. I was using the existing database Guid because it was already there, so it seemed natural to want to reuse it. But there was no benefit in doing that. Instead, it added a lot more complexity because I was adding (many) additional responsibilities to the value that it didn’t have before.

Creating a Guid is pretty easy, after all, and I was able to dedicate one I called Journal ID to this purpose. The existing Database ID is still there, and it is completely unrelated to it. Changing the Database ID has no impact on the Journal ID, so the problem space is radically simplified.

I had to throw away heaps of complexity because of a single comment. I used the Database ID because it was there, never considering having a dedicated value for it. That single suggestion led to a better, simpler design and faster delivery.

It is funny how you can sometimes be so focused on the problem at hand, when a step back will give you a much wider view and a better path to the solution.

We ran into a memory issue recently in RavenDB, which had a pretty interesting root cause. Take a look at the following code and see if you can spot what is going on:

ConcurrentQueue<Buffer> _buffers = new();


void FlushUntil(long maxTransactionId)
{
    List<Buffer> toFlush = new();
    while(_buffers.TryPeek(out buffer) && 
        buffer.TransactionId <= maxTransactionId)
    {
        if(_buffers.TryDequeue(out buffer))
        {
            toFlush.Add(buffer);
        }
    }


    FlushToDisk(toFlush);
}

The code handles flushing data to disk based on the maximum transaction ID. Can you see the memory leak?

If we have a lot of load on the system, this will run just fine. The problem is when the load is over. If we stop writing new items to the system, it will keep a lot of stuff in memory, even though there is no reason for it to do so.

The reason for that is the call to TryPeek(). You can read the source directly, but the basic idea is that when you peek, you have to guard against concurrent TryTake(). If you are not careful, you may encounter something called a torn read.

Let’s try to explain it in detail. Suppose we store a large struct in the queue and call TryPeek() and TryTake() concurrently. The TryPeek() starts copying the struct to the caller at the same time that TryTake() does the same and zeros the value. So it is possible that TryPeek() would get an invalid value.

To handle that, if you are using TryPeek(), the queue will not zero out the values. This means that until that queue segment is completely full and a new one is generated, we’ll retain references to those buffers, leading to an interesting memory leak.

RavenDB is a transactional database, we care deeply about ACID. The D in ACID stands for durability, which means that to acknowledge a transaction, we must write it to a persistent medium. Writing to disk is expensive, writing to the disk and ensuring durability is even more expensive.

After seeing some weird performance numbers on a test machine, I decided to run an experiment to understand exactly how durable writes affect disk performance.

A few words about the term durable writes. Disks are slow, so we use buffering & caches to avoid going to the disk. But a write to a buffer isn’t durable. A failure could cause it to never hit a persistent medium. So we need to tell the disk in some way that we are willing to wait until it can ensure that this write is actually durable.

This is typically done using either fsync or O_DIRECT | O_DSYNC flags. So this is what we are testing in this post.

I wanted to test things out without any of my own code, so I ran the following benchmark.

I pre-allocated a file and then ran the following commands.

Normal writes (buffered) with different sizes (256 KB, 512 KB, etc).

dd if=/dev/zero of=/data/test bs=256K count=1024
dd if=/dev/zero of=/data/test bs=512K count=1024

Durable writes (force the disk to acknowledge them) with different sizes:

dd if=/dev/zero of=/data/test bs=256k count=1024 oflag=direct,sync
dd if=/dev/zero of=/data/test bs=256k count=1024 oflag=direct,sync

The code above opens the file using:

openat(AT_FDCWD, "/data/test", O_WRONLY|O_CREAT|O_TRUNC|O_SYNC|O_DIRECT, 0666) = 3

I got myself an i4i.xlarge instance on AWS and started running some tests. That machine has a local NVMe drive of about 858 GB, 32 GB of RAM, and 4 cores. Let’s see what kind of performance I can get out of it.

Write sizeTotal writesBuffered writes

What you can see here is that writes are really fast when buffered. But when I hit a certain size (above 1 GB or so), we probably start having to write to the disk itself (which is NVMe, remember). Our top speed is about 550 MB/s at this point, regardless of the size of the buffers I’m passing to the write() syscall.

I’m writing here using cached I/O, which is something that as a database vendor, I don’t really care about. What happens when we run with direct & sync I/O, the way I would with a real database? Here are the numbers for the i4i.xlarge instance for durable writes.

Write sizeTotal writesDurable writes

In other words, when using direct I/O, the smaller the write, the more time it takes. Remember that we are talking about forcing the disk to write the data, and we need to wait for it to complete before moving to the next one.

For 16 KB writes, buffered writes achieve a throughput of 553 MB/s vs. 126 MB/s for durable writes. This makes sense, since those writes are cached, so the OS is probably sending big batches to the disk. The numbers we have here clearly show that bigger batches are better.

My next test was to see what would happen when I try to write things in parallel. In this test, we run 4 processes that write to the disk using direct I/O and measure their output.

I assume that I’m maxing out the throughput on the drive, so the total rate across all commands should be equivalent to the rate I would get from a single command.

To run this in parallel I’m using a really simple mechanism - just spawn processes that would do the same work. Here is the command template I’m using:

parallel -j 4 --tagstring 'Task {}' dd if=/dev/zero of=/data/test bs=16M count=128 seek={} oflag=direct,sync ::: 0 1024 2048 3072

This would write to 4 different portions of the same file, but I also tested that on separate files. The idea is to generate a sufficient volume of writes to stress the disk drive.

Write sizeTotal writesDurable & Parallel writes

I also decided to write some low-level C code to test out how this works with threads and a single program. You can find the code here.  I basically spawn NUM_THREADS threads, and each will open a file using O_SYNC | O_DIRECT and write to the file WRITE_COUNT times with a buffer of size BUFFER_SIZE.

This code just opens a lot of files and tries to write to them using direct I/O with 8 KB buffers. In total, I’m writing 16 GB (128 MB x 128 threads) to the disk. I’m getting a rate of about 320 MB/sec when using this approach.

As before, increasing the buffer size seems to help here. I also tested a version where we write using buffered I/O and call fsync every now and then, but I got similar results.

The interim conclusion that I can draw from this experiment is that NVMes are pretty cool, but once you hit their limits you can really feel it. There is another aspect to consider though, I’m running this on a disk that is literally called ephemeral storage. I need to repeat those tests on real hardware to verify whether the cloud disk simply ignores the command to persist properly and always uses the cache.

That is supported by the fact that using both direct I/O on small data sizes didn’t have a big impact (and I expected it should). Given that the point of direct I/O in this case is to force the disk to properly persist (so it would be durable in the case of a crash), while at the same time an ephemeral disk is wiped if the host machine is restarted, that gives me good reason to believe that these numbers are because the hardware “lies” to me.

In fact, if I were in charge of those disks, lying about the durability of writes would be the first thing I would do. Those disks are local to the host machine, so we have two failure modes that we need to consider:

• The VM crashed - in which case the disk is perfectly fine and “durable”.
• The host crashed - in which case the disk is considered lost entirely.

Therefore, there is no point in trying to achieve durability, so we can’t trust those numbers.

The next step is to run it on a real machine. The economics of benchmarks on cloud instances are weird. For a one-off scenario, the cloud is a godsend. But if you want to run benchmarks on a regular basis, it is far more economical to just buy a physical machine. Within a month or two, you’ll already see a return on the money spent.

We got a machine in the office called Kaiju (a Japanese term for enormous monsters, think: Godzilla) that has:

• 32 cores
• 188 GB RAM
• 2 TB NVMe for the system disk
• 4 TB NVMe for the data disk

I ran the same commands on that machine as well and got really interesting results.

Write sizeTotal writesBuffered writes

We are faster than the cloud instance, and we don’t have a drop-off point when we hit a certain size. We are also seeing higher performance when we throw bigger buffers at the system.

But when we test with small buffers, the performance is also great. That is amazing, but what about durable writes with direct I/O?

I tested the same scenario with both buffered and durable writes:

ModeBufferedDurable

Wow, that is an interesting result. Because it means that when we use direct I/O with 1 MB buffers, we lose about 600 MB/sec compared to buffered I/O. Note that this is actually a pretty good result. 1 GB/sec is amazing.

And if you use big buffers, then the cost of direct I/O is basically gone. What about when we go the other way around and use smaller buffers?

ModeBufferedDurable

For buffered I/O - I’m getting simply dreamy numbers, pretty much regardless of what I do 🙂.

For durable writes, the situation is clear. The bigger the buffer we write, the better we perform, and we pay for small buffers. Look at the numbers for 128 KB in the durable column for both single-threaded and parallel scenarios.

169 MB/s in the single-threaded result, but with 8 parallel processes, we didn’t reach 1.3 GB/s (which is 169x8). Instead, we achieved less than half of our expected performance.

It looks like there is a fixed cost for making a direct I/O write to the disk, regardless of the amount of data that we write.  When using 32 KB writes, we are not even breaking into the 200 MB/sec. And with 8 KB writes, we are barely breaking into the 50 MB/sec range.

Those are some really interesting results because they show a very strong preference for bigger writes over smaller writes.

I also tried using the same C code as before. As a reminder, we use direct I/O to write to 128 files in batches of 8 KB, writing a total of 128 MB per file. All of that is done concurrently to really stress the system.

When running iotop in this environment, we get:

Total DISK READ:         0.00 B/s | Total DISK WRITE:       522.56 M/s
Current DISK READ:       0.00 B/s | Current DISK WRITE:     567.13 M/s
    TID  PRIO  USER     DISK READ DISK WRITE>    COMMAND
 142851 be/4 kaiju-1     0.00 B/s    4.09 M/s ./a.out
 142901 be/4 kaiju-1     0.00 B/s    4.09 M/s ./a.out
 142902 be/4 kaiju-1     0.00 B/s    4.09 M/s ./a.out
 142903 be/4 kaiju-1     0.00 B/s    4.09 M/s ./a.out
 142904 be/4 kaiju-1     0.00 B/s    4.09 M/s ./a.out
... redacted ...

So each thread is getting about 4.09 MB/sec for writes, but we total 522 MB/sec across all writes. I wondered what would happen if I limited it to fewer threads, so I tried with 16 concurrent threads, resulting in:

Total DISK READ:         0.00 B/s | Total DISK WRITE:        89.80 M/s
Current DISK READ:       0.00 B/s | Current DISK WRITE:     110.91 M/s
    TID  PRIO  USER     DISK READ DISK WRITE>    COMMAND
 142996 be/4 kaiju-1     0.00 B/s    5.65 M/s ./a.out
 143004 be/4 kaiju-1     0.00 B/s    5.62 M/s ./a.out
 142989 be/4 kaiju-1     0.00 B/s    5.62 M/s ./a.out
... redacted ..

Here we can see that each thread is getting (slightly) more throughput, but the overall system throughput is greatly reduced.

To give some context, with 128 threads running, the process wrote 16GB in 31 seconds, but with 16 threads, it took 181 seconds to write the same amount. In other words, there is a throughput issue here. I also tested this with various levels of concurrency:

Concurrency(8 KB x 16K times - 128 MB)Throughput per threadTime / MB written

To give some context, here are two attempts to write 2GB to the disk:

ConcurrencyWriteThroughputTotal writtenTotal time

In other words, we can see the impact of concurrent writes. There is absolutely some contention at the disk level when making direct I/O writes. The impact is related to the number of writes rather than the amount of data being written.

Bigger writes are far more efficient. And concurrent writes allow you to get more data overall but come with a horrendous latency impact for each individual thread.

The difference between the cloud and physical instances is really interesting, and I have to assume that this is because the cloud instance isn’t actually forcing the data to the physical disk (it doesn’t make sense that it would).

I decided to test that on an m6i.2xlarge instance with a 512 GB io2 disk with 16,000 IOPS.

The idea is that an io2 disk has to be durable, so it will probably have similar behavior to physical hardware.

DiskBuffer SizeWritesDurableParallelTotalRate

In other words, we are seeing pretty much the same behavior as on the physical machine, unlike the ephemeral drive.

In conclusion, it looks like the limiting factor for direct I/O writes is the number of writes, not their size. There appears to be some benefit for concurrency in this case, but there is also some contention. The best option we got was with big writes.

Interestingly, big writes are a win, period. For example, 16 MB writes, direct I/O:

• Single-threaded - 4.4 GB/sec
• 2 threads - 2.5 GB/sec X 2 - total 5.0 GB/sec
• 4 threads - 1.4 X 4  - total 5.6 GB/sec
• 8 threads - ~590 MB/sec x 8 - total 4.6 GB/sec

Writing 16 KB, on the other hand:

• 8 threads - 11.8 MB/sec x 8 - total 93 MB/sec
• 4 threads - 12.6 MB/sec x 4- total 50.4 MB/sec
• 2 threads - 12.3 MB/sec x 2 - total 24.6 MB/sec
• 1 thread - 23.4 MB/sec

This leads me to believe that there is a bottleneck somewhere in the stack, where we need to handle the durable write, but it isn’t related to the actual amount we write. In short, fewer and bigger writes are more effective, even with concurrency.

As a database developer, that leads to some interesting questions about design. It means that I want to find some way to batch more writes to the disk, especially for durable writes, because it matters so much.

Expect to hear more about this in the future.

We got an interesting question in the RavenDB discussion group:How to do aggregation on a tree structure?

The task is to build a Work Breakdown Structure, where you have:

• Projects

• Major deliverables

• Sub-deliverables

• Work packages

The idea is to be able to track EstimatedHours and CompletedHours across the entire tree. For example, let’s say that I have the following:

• Project: Bee Keeper Chronicle App

• Major deliverable: App Design

• Sub-deliverable: Wireframes all screens

• Work Package: Login page wireframe

Users can add the EstimatedHours and CompletedHours at any level, and we want to be able to aggregate the data upward. So the Project: “Bee Keeper Chronicle App” should have a total estimated time and the number of hours that were worked on.

The question is how to model & track that in RavenDB. Here is what I think the document structure should look like:

{
    "Name": "Login page wire frame",
    "Parent": {
        "Type": "Subs",
        "Id": "subs/0000000000000000009-A"
    },
    "EsimatedHours": 8,
    "CompletedHours": 3,
    "@metadata": {
        "@collection": "WorkPackages"
    }
}


{
    "Name": "Wire frames all screens",
    "Parent": {
        "Type": "Majors",
        "Id": "major/0000000000000000008-A"
    },
    "EsimatedHours": 20,
    "CompletedHours": 7,
    "@metadata": {
        "@collection": "Subs"
    }
}


{
    "Name": "App Design",
    "Parent": {
        "Type": "Projects",
        "Id": "projects/0000000000000000011-A"
    },
    "EsimatedHours": 50,
    "CompletedHours": 12,
    "@metadata": {
        "@collection": "Majors"
    }
}


{
    "Name": "Bee Keeper Chronicle App",
    "EsimatedHours": 34,
    "CompletedHours": 21,
    "@metadata": {
        "@collection": "Projects"
    }
}

I used a Parent relationship, since that is the most flexible (it allows you to move each item to a completely different part of the tree easily). Now, we need to aggregate the values for all of those items using a Map-Reduce index.

Because of the similar structure, I created the following JS function:

function processWorkBreakdownHours(doc) {
    let hours = {
        EsimatedHours: doc.EsimatedHours,
        CompletedHours: doc.CompletedHours
    };
    let results = [Object.assign({
        Scope: id(doc)
    }, hours)];


    let current = doc;
    while (current.Parent) {
        current = load(current.Parent.Id, current.Parent.Type);
        results.push(Object.assign({
            Scope: id(current)
        }, hours));
    }
    return results;
}

This will scan over the hierarchy and add the number of estimated and completed hours to all the levels. Now we just need to add this file as Additional Sources to an index and call it for all the relevant collections, like this:

map("WorkPackages",processWorkBreakdownHours);
map("Subs",processWorkBreakdownHours);
map("Majors",processWorkBreakdownHours);
map("Projects",processWorkBreakdownHours);

And the last step is to aggregate across all of them in the reduce function:

groupBy(x => x.Scope).aggregate(g => {
    return {
        Scope: g.key,
        EsimatedHours: g.values.reduce((c, val) => val.EsimatedHours + c, 0),
        CompletedHours: g.values.reduce((c, val) => val.CompletedHours + c, 0)
    };
})

You can see the full index definition here.

The end result is automatic aggregation at all levels. Change one item, and it will propagate upward.

Considerations:

I’m using load() here, which creates a reference from the parent to the child. The idea is that if we move a Work Package from one Sub-deliverable to another (in the same or a different Major & Project), this index will automatically re-index what is required and get you the right result.

However, that also means that whenever the Major document changes, we’ll have to re-index everything below it (because it might have changed the Project). There are two ways to handle that.

• Instead of using load(), we’ll use noTracking.load(), which tells RavenDB that when the referenced document changes, we should not re-index.
• Provide the relevant scopes at the document level, like this:

{
    "Name": "Login page wire frame",
    "Scope": [
       "subs/0000000000000000009-A",
       "major/0000000000000000008-A",
       "projects/0000000000000000011-A"
    ],
    "EsimatedHours": 8,
    "CompletedHours": 3,
    "@metadata": {
        "@collection": "WorkPackages"
    }
}

Note that in this case, changing the root will be more complex because you have to scan / touch everything if you move between parts of the tree.

In most cases, that is such a rare event that it shouldn’t be a consideration, but it depends largely on your context.

And there you have it, a simple Map-Reduce index that can aggregate across an entire hierarchy with ease.

When building RavenDB, we occasionally have to deal with some ridiculous numbers in both size and scale. In one of our tests, we ran into an interesting problem. Here are the performance numbers of running a particular query 3 times.

First Run: 19,924 ms

Second Run: 3,181 ms

Third Run: 1,179 ms

Those are not good numbers, so we dug into this to try to figure out what is going on. Here is the query that we are running:

from index 'IntFloatNumbers-Lucene' where Int > 0

And the key here is that this index covers 400 million documents, all of which are actually greater than 0. So this is actually a pretty complex task for the database to handle, mostly because of the internals of how Lucene works.

Remember that we provide both the first page of the results as well as its total number. So we have to go through the entire result set to find out how many items we have. That is a lot of work.

But it turns out that most of the time here isn’t actually processing the query, but dealing with the GC. Here are some entries from the GC log while the queries were running:

2024-12-12T12:39:40.4845987Z, Type: GC, thread id: 30096, duration: 2107.9972ms, index: 25, generation: 2, reason: Induced
2024-12-12T12:39:53.1359744Z, Type: GC, thread id: 30096, duration: 1650.9207ms, index: 26, generation: 2, reason: Induced
2024-12-12T12:40:07.5835527Z, Type: GC, thread id: 30096, duration: 1629.1771ms, index: 27, generation: 2, reason: Induced
2024-12-12T12:40:20.2205602Z, Type: GC, thread id: 30096, duration: 776.24ms, index: 28, generation: 2, reason: Induced

That sound you heard was me going: Ouch!

Remember that this query actually goes through 400M results. Here are the details about its Memory Usage & Object Count:

• Number of objects for GC (under LuceneIndexPersistence): 190M (~12.63GB)
• Managed Memory: 13.01GB
• Unmanaged Memory: 4.53MB

What is going on? It turns out that Lucene handles queries such as Int>0 by creating an array with all the unique values, something similar to:

string[] sortedTerms = new string[190_000_000];
long[] termPostingListOffset = new long[190_000_000];

This isn’t exactly how it works, mind. But the details don’t really matter for this story. The key here is that we have an array with a sorted list of terms, and in this case, we have a lot of terms.

Those values are cached, so they aren’t actually allocated and thrown away each time we query. However, remember that the .NET GC uses a Mark & Sweep algorithm. Here is the core part of the Mark portion of the algorithm:

long _marker;
void Mark()
{
    var currentMarker = ++_marker;


    foreach (var root in GetRoots())
    {
        Mark(root);
    }


    void Mark(object o)
    {
        // already visited
        if (GetMarket(o) == currentMarker)
            return;


        foreach (var child in GetReferences(node))
        {
            Mark(child);
        }
    }
}

Basically, start from the roots (static variables, items on the stack, etc.), scan the reachable object graph, and mark all the objects in use. The code above is generic, of course (and basically pseudo-code), but let’s consider what the performance will be like when dealing with an array of 190M strings.

It has to scan the entire thing, which means it is proportional to the number of objects. And we do have quite a lot of those.

The problem was the number of managed objects, so we pulled all of those out. We moved the term storage to unmanaged memory, outside the purview of the GC. As a result, we now have the following Memory Usage & Object Count:

• Number of objects for GC (under LuceneIndexPersistence): 168K (~6.64GB)
• Managed Memory: 6.72GB
• Unmanaged Memory: 1.32GB

Looking at the GC logs, we now have:

2024-12-16T18:33:29.8143148Z, Type: GC, thread id: 8508, duration: 93.6835ms, index: 319, generation: 2, reason: Induced
2024-12-16T18:33:30.7013255Z, Type: GC, thread id: 8508, duration: 142.1781ms, index: 320, generation: 2, reason: Induced
2024-12-16T18:33:31.5691610Z, Type: GC, thread id: 8508, duration: 91.0983ms, index: 321, generation: 2, reason: Induced
2024-12-16T18:33:37.8245671Z, Type: GC, thread id: 8508, duration: 112.7643ms, index: 322, generation: 2, reason: Induced

So the GC time is now in the range of 100ms, instead of several seconds. This change helps both reduce overall GC pause times and greatly reduce the amount of CPU spent on managing garbage.

Those are still big queries, but now we can focus on executing the query, rather than managing maintenance tasks. Incidentally, those sorts of issues are one of the key reasons why we built Corax, which can process queries directly on top of persistent structures, without needing to materialize anything from the disk.

An issue was recently raised with a really scary title:

Intermittent Index corruption: VoronUnrecoverableErrorException.

Those are the kinds of issues that you know are going to be complex. Fixing such issues in the past was usually a Task Force effort and quite a challenge.

We asked for more information and started figuring out who would handle the issue (given the time of the year) when the user came back with:

After pressing the disk check issue with our hosting provider, we found out that one of the disks was reporting an error but according to our hosting, it was only because the manufacturer's guarantee expired, and not the actual disk failure. We swapped the disk anyway, and so far we are not seeing the issue.

I’m so happy that I can close that issue 🙂

I write a transactional database for a living, and the best example of why we want transactions is transferring money between accounts. It is ironic, therefore, that there is no such thing as transactions for money transfers in the real world.

If you care to know why, go back 200 years and consider how a bank would operate in an environment without instant communication. I would actually recommend doing that, it is a great case study in distributed system design. For example, did you know that the Templars used cryptography to send money almost a thousand years ago?

Recently I was reviewing my bank transactions and I found the following surprise. This screenshot is from yesterday (Dec 18), and it looks like a payment that I made is still “stuck in the tubes” two and a half weeks later.

I got in touch with the supplier in question to apologize for the delay. They didn’t understand what I was talking about. Here is what they see when they go to their bank, they got the money.

For fun, look at the number of different dates that you can see in their details.

Also, as of right now, my bank account still shows the money as pending approval (to be sent out from my bank).

I might want to recommend that they use a different database. Or maybe I should just convince the bank to approve the payment by the time of the next invoice and see if I can get a bit of that infinite money glitch.