Deuxfleurs/garagehq.deuxfleurs.fr
26
2
Fork 1
Code Issues 5 Pull requests 3 Projects Releases Wiki Activity

Adjust spelling - this is a blog not a research paper

Browse source
This commit is contained in:
maximilien 2022-09-29 09:55:46 +02:00
parent 990dd55948
commit 4f57f6c742
1 changed files with 86 additions and 86 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

164
content/blog/2022-perf/index.md
View file

@ -8,8 +8,8 @@ date=2022-09-26
their theoretical trade-offs for Garage. In particular, we pondered the impacts
of data structures, networking methods, and scheduling algorithms.
Garage worked well enough for our production
cluster at Deuxfleurs, but we also knew that people started to discover some
unexpected behaviors. We thus started a round of benchmarks and performance
cluster at Deuxfleurs, but we also knew that people started to experience some
unexpected behaviors, which motivated us to start a round of benchmarks and performance
measurements to see how Garage behaves compared to our expectations.
This post presents some of our first results, which cover
3 aspects of performance: efficient I/O, "myriads of objects", and resiliency,
@ -21,35 +21,35 @@ reflecting the high-level properties we are seeking.*
## ⚠️ Disclaimer
The results presented in this blog post must be taken with a critical grain of salt due to some
The results presented in this blog post must be taken with a (critical) grain of salt due to some
limitations that are inherent to any benchmarking endeavor. We try to reference them as
exhaustively as possible in this first section, but other limitations might exist.
exhaustively as possible here, but other limitations might exist.
Most of our tests were made on simulated networks, which by definition cannot represent all the
diversity of real networks (dynamic drop, jitter, latency, all of which could be
Most of our tests were made on _simulated_ networks, which by definition cannot represent all the
diversity of _real_ networks (dynamic drop, jitter, latency, all of which could be
correlated with throughput or any other external event). We also limited
ourselves to very small workloads that are not representative of a production
cluster. Furthermore, we only benchmarked some very specific aspects of Garage:
our results are thus not an evaluation of the performance of Garage as a whole.
our results are not an evaluation of the performance of Garage as a whole.
For some benchmarks, we used Minio as a reference. It must be noted that we did
not try to optimize its configuration as we have done for Garage, and more
generally, we have way less knowledge on Minio than on Garage, which can lead
generally, we have significantly less knowledge of Minio's internals compared to Garage, which could lead
to underrated performance measurements for Minio. It must also be noted that
Garage and Minio are systems with different feature sets. For instance, Minio supports
erasure coding for higher data density, which Garage doesn't, Minio implements
erasure coding for higher data density and Garage doesn't, Minio implements
way more S3 endpoints than Garage, etc. Such features necessarily have a cost
that you must keep in mind when reading the plots we will present. You should consider
results on Minio as a way to contextualize our results on Garage, to see that our improvements
are not artificial in the light of existing object storage implementations.
Minio's results as a way to contextualize Garage's numbers, to justify that our improvements
are not simply artificial in the light of existing object storage implementations.
The impact of the testing environment is also not evaluated (kernel patches,
configuration, parameters, filesystem, hardware configuration, etc.). Some of
these parameters could favor one configuration or software product over another.
Especially, it must be noted that most of the tests were done on a
consumer-grade PC with only an SSD, which is different from most
consumer-grade PC with only a SSD, which is different from most
production setups. Finally, our results are also provided without statistical
tests to check their significance, and might thus have insufficient significance
tests to validate their significance, and might have insufficient ground
to be claimed as reliable.
When reading this post, please keep in mind that **we are not making any
@ -75,16 +75,16 @@ geo-distributed topology. We used the Grid5000 testbed only during our
preliminary tests to identify issues when running Garage on many powerful
servers. We then reproduced these issues in a controlled environment
outside of Grid5000, so don't be
surprised then if Grid5000 is not mentioned often on our plots.
surprised then if Grid5000 is not always mentioned on our plots.
To reproduce some environments locally, we have a small set of Python scripts
called [`mknet`](https://git.deuxfleurs.fr/Deuxfleurs/mknet) tailored to our
needs[^ref1]. Most of the following tests were thus run locally with `mknet` on a
needs[^ref1]. Most of the following tests were run locally with `mknet` on a
single computer: a Dell Inspiron 27" 7775 AIO, with a Ryzen 5 1400, 16GB of
RAM, a 512GB SSD. In terms of software, NixOS 22.05 with the 5.15.50 kernel is
RAM and a 512GB SSD. In terms of software, NixOS 22.05 with the 5.15.50 kernel is
used with an ext4 encrypted filesystem. The `vm.dirty_background_ratio` and
`vm.dirty_ratio` has been reduced to `2` and `1` respectively as, with default
values, the system tends to freeze when it is under heavy I/O load.
`vm.dirty_ratio` have been reduced to `2` and `1` respectively: with default
values, the system tends to freeze under heavy I/O load.
## Efficient I/O
@ -93,7 +93,7 @@ across the network, and the faster these two functions can be accomplished,
the more efficient the system as a whole will be. For this analysis, we focus on
2 aspects of performance. First, since many applications can start processing a file
before receiving it completely, we will evaluate the time-to-first-byte (TTFB)
on GetObject requests, i.e. the duration between the moment a request is sent
on `GetObject` requests, i.e. the duration between the moment a request is sent
and the moment where the first bytes of the returned object are received by the client.
Second, we will evaluate generic throughput, to understand how well
Garage can leverage the underlying machine's performance.
@ -101,18 +101,18 @@ Garage can leverage the underlying machine's performance.
**Time-to-First-Byte** - One specificity of Garage is that we implemented S3
web endpoints, with the idea to make it a platform of choice to publish
static websites. When publishing a website, TTFB can be directly observed
by the end user, as it will impact the perceived reactivity of the website.
by the end user, as it will impact the perceived reactivity of the page being loaded.
Up to version 0.7.3, time-to-first-byte on Garage used to be relatively high.
This can be explained by the fact that Garage was not able to handle data internally
at a smaller granularity level than entire data blocks, which are 1MB chunks of a given object
at a smaller granularity level than entire data blocks, which are up to 1MB chunks of a given object
(a size which [can be configured](https://garagehq.deuxfleurs.fr/documentation/reference-manual/configuration/#block-size)).
Let us take the example of a 4.5MB object, which Garage will split into 4 blocks of
1MB and 1 block of 0.5MB. With the old design, when you were sending a `GET`
request, the first block had to be fully retrieved by the gateway node from the
storage node before starting to send any data to the client.
Let us take the example of a 4.5MB object, which Garage will split by default into four 1MB blocks and one 0.5MB block.
With the old design, when you were sending a `GET`
request, the first block had to be _fully_ retrieved by the gateway node from the
storage node before it starts to send any data to the client.
With Garage v0.8, we integrated a data streaming logic that allows the gateway
With Garage v0.8, we added a data streaming logic that allows the gateway
to send the beginning of a block without having to wait for the full block to be received from
the storage node. We can visually represent the difference as follow:
@ -120,13 +120,13 @@ the storage node. We can visually represent the difference as follow:
<img src="schema-streaming.png" alt="A schema depicting how streaming improves the delivery of a block" />
</center>
As our default block size is only 1MB, the difference should be very small on
As our default block size is only 1MB, the difference should be marginal on
fast networks: it takes only 8ms to transfer 1MB on a 1Gbps network,
thus adding at most 8ms of latency to a GetObject request (assuming no other
adding at most 8ms of latency to a `GetObject` request (assuming no other
data transfer is happening in parallel). However,
on a very slow network, or a very congested link with many parallel requests
handled, the impact can be much more important: on a 5Mbps network, it takes 1.6 seconds
to transfer our 1MB block, and streaming has the potential of heavily improving user experience.
handled, the impact can be much more important: on a 5Mbps network, it takes at least 1.6 seconds
to transfer our 1MB block, and streaming will heavily improve user experience.
We wanted to see if this theory holds in practice: we simulated a low latency
but slow network using `mknet` and did some requests with block streaming (Garage v0.8 beta) and
@ -138,11 +138,11 @@ whose results are shown on the following figure:
![Plot showing the TTFB observed on Garage v0.8, v0.7 and Minio](ttfb.png)
Garage v0.7, which does not support block streaming, gives us a TTFB between 1.6s
and 2s, which corresponds to the time to transfer the full block which we calculated above.
and 2s, which matches the time required to transfer the full block which we calculated above.
On the other side of the plot, we can see Garage v0.8 with a very low TTFB thanks to the
streaming feature (the lowest value is 43ms). Minio sits between the two
Garage versions: we suppose that it does some form of batching, but smaller
than 1MB.
than our initial 1MB default.
**Throughput** - As soon as we publicly released Garage, people started
benchmarking it, comparing its performances to writing directly on the
@ -152,7 +152,7 @@ situation, we did some optimizations, such as putting costly processing like has
and many others
([#342](https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/342),
[#343](https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/343)), which led us to
version 0.8 "Beta 1". We also noticed that some of the logic logic we wrote
version 0.8 "Beta 1". We also noticed that some of the logic we wrote
to better control resource usage
and detect errors, including semaphores and timeouts, was artificially limiting
performances. In another iteration, we made this logic less restrictive at the
@ -162,7 +162,7 @@ version 0.8 "Beta 2". Finally, we currently do multiple `fsync` calls each time
write a block. We know that this is expensive and did a test build without any
`fsync` call ([see the
commit](https://git.deuxfleurs.fr/Deuxfleurs/garage/commit/432131f5b8c2aad113df3b295072a00756da47e7))
that will not be merged, just to assess the impact of `fsync`. We refer to it
that will not be merged, only to assess the impact of `fsync`. We refer to it
as `no-fsync` in the following plot.
*A note about `fsync`: for performance reasons, operating systems often do not
@ -172,12 +172,12 @@ with other writes. If a power loss occurs before the OS has time to flush
data to disk, some writes will be lost. To ensure that a write is effectively
written to disk, the
[`fsync(2)`](https://man7.org/linux/man-pages/man2/fsync.2.html) system call must be used,
which blocks until the file or directory on which it is called has been flushed from volatile
which effectively blocks until the file or directory on which it is called has been flushed from volatile
memory to the persistent storage device. Additionally, the exact semantic of
`fsync` [differs from one OS to another](https://mjtsai.com/blog/2022/02/17/apple-ssd-benchmarks-and-f_fullsync/)
and, even on battle-tested software like Postgres, it was
["done wrong for 20 years"](https://archive.fosdem.org/2019/schedule/event/postgresql_fsync/).
Note that on Garage, we are currently working on our `fsync` policy and thus, for
Note that on Garage, we are still working on our `fsync` policy and thus, for
now, you should expect limited data durability in case of power loss, as we are
aware of some inconsistencies on this point (which we describe in the following
and plan to solve).*
@ -194,14 +194,14 @@ Minio, our reference point, gives us the best performances in this test.
Looking at Garage, we observe that each improvement we made had a visible
impact on performances. We also note that we have a progress margin in
terms of performances compared to Minio: additional benchmarks, tests, and
monitoring could help us better understand the remaining difference.
monitoring could help us better understand the remaining gap.
## A myriad of objects
Object storage systems do not handle a single object but huge numbers of them:
Amazon claims to handle trillions of objects on their platform, and Red Hat
communicates about Ceph being able to handle 10 billion objects. All these
tout Ceph as being able to handle 10 billion objects. All these
objects must be tracked efficiently in the system to be fetched, listed,
removed, etc. In Garage, we use a "metadata engine" component to track them.
For this analysis, we compare different metadata engines in Garage and see how
@ -214,25 +214,25 @@ the only supported option was [sled](https://sled.rs/), but we started having
serious issues with it - and we were not alone
([#284](https://git.deuxfleurs.fr/Deuxfleurs/garage/issues/284)). With Garage
v0.8, we introduce an abstraction semantic over the features we expect from our
database, allowing us to switch from one back-end to another without touching
database, allowing us to switch from one metadata back-end to another without touching
the rest of our codebase. We added two additional back-ends: LMDB
(through [heed](https://github.com/meilisearch/heed)) and SQLite
(using [Rusqlite](https://github.com/rusqlite/rusqlite)). **Keep in mind that they
are both experimental: contrarily to sled, we have never run them in production
for a long time.**
are both experimental: contrarily to sled, we have yet to run them in production
for a significant time.**
Similarly to the impact of `fsync` on block writing, each database engine we use
has its own policy with `fsync`. Sled flushes its writes every 2 seconds by
has its own `fsync` policy. Sled flushes its writes every 2 seconds by
default (this is
[configurable](https://garagehq.deuxfleurs.fr/documentation/reference-manual/configuration/#sled-flush-every-ms)).
LMDB by default does an `fsync` on each write, which on early tests led to
LMDB default to an `fsync` on each write, which on early tests led to
abysmal performance. We thus added 2 flags,
[MDB\_NOSYNC](http://www.lmdb.tech/doc/group__mdb__env.html#ga5791dd1adb09123f82dd1f331209e12e)
and
[MDB\_NOMETASYNC](http://www.lmdb.tech/doc/group__mdb__env.html#ga5021c4e96ffe9f383f5b8ab2af8e4b16),
to deactivate `fsync`. On SQLite, it is also possible to deactivate `fsync` with
to deactivate `fsync` entirely. On SQLite, it is also possible to deactivate `fsync` with
`pragma synchronous = off`, but we have not started any optimization work on it yet:
our SQLite implementation currently calls `fsync` for all write operations. Additionally, we are
our SQLite implementation currently still calls `fsync` for all write operations. Additionally, we are
using these engines through Rust bindings that do not support async Rust,
with which Garage is built, which has an impact on performance as well.
**Our comparison will therefore not reflect the raw performances of
@ -242,20 +242,20 @@ Still, we think it makes sense to evaluate our implementations in their current
state in Garage. We designed a benchmark that is intensive on the metadata part
of the software, i.e. handling large numbers of tiny files. We chose again
`minio/warp` as a benchmark tool, but we
configured it here with the smallest possible object size it supported, 256
bytes, to put some pressure on the metadata engine. We evaluated sled twice:
configured it with the smallest possible object size it supported, 256
bytes, to put pressure on the metadata engine. We evaluated sled twice:
with its default configuration, and with a configuration where we set a flush
interval of 10 minutes to disable `fsync`.
interval of 10 minutes (longer than the test) to disable `fsync`.
*Note that S3 has not been designed for such workloads that store huge numbers of small objects;
*Note that S3 has not been designed for workloads that store huge numbers of small objects;
a regular database, like Cassandra, would be more appropriate. This test has
only been designed to stress our metadata engine, and is not indicative of
only been designed to stress our metadata engine and is not indicative of
real-world performances.*
![Plot of our metadata engines comparison with Warp](db_engine.png)
Unsurprisingly, we observe abysmal performances with SQLite, the engine which we have
the less tested and that still does an `fsync` for each write. Garage with LMDB performs twice better than
Unsurprisingly, we observe abysmal performances with SQLite, as it is the engine we did not put work on yet,
and that still does an `fsync` for each write. Garage with the `fsync`-disabled LMDB backend performs twice better than
with sled in its default version and 60% better than the "no `fsync`" sled version in our
benchmark. Furthermore, and not depicted on these plots, LMDB uses way less
disk storage and RAM; we would like to quantify that in the future. As we are
@ -263,7 +263,7 @@ only at the very beginning of our work on metadata engines, it is hard to draw
strong conclusions. Still, we can say that SQLite is not ready for production
workloads, and that LMDB looks very promising both in terms of performances and resource
usage, and is a very good candidate for being Garage's default metadata engine in
future releases. In the future, we will need to define a data policy for Garage to help us
future releases, once we figure out the proper `fsync` tuning. In the future, we will need to define a data policy for Garage to help us
arbitrate between performance and durability.
*To `fsync` or not to `fsync`? Performance is nothing without reliability, so we
@ -300,35 +300,35 @@ We wrote our own benchmarking tool for this test,
[s3billion](https://git.deuxfleurs.fr/Deuxfleurs/mknet/src/branch/main/benchmarks/s3billion)[^ref2].
The benchmark procedure consists in
concurrently sending a defined number of tiny objects (8192 objects of 16
bytes by default) and measuring the time it takes. This step is then repeated a given
bytes by default) and measuring the wall clock time to the last object upload. This step is then repeated a given
number of times (128 by default) to effectively create a target number of
objects on the cluster (1M by default). On our local setup with 3
nodes, both Minio and Garage with LMDB were able to achieve this target. In the
following plot, we show how much time it took Garage and Minio to handle
each batch.
Before looking at the plot, **you must keep in mind some important points about
Before looking at the plot, **you must keep in mind some important points regarding
the internals of both Minio and Garage**.
Minio has no metadata engine, it stores its objects directly on the filesystem.
Sending 1 million objects on Minio results in creating one million inodes on
the storage server in our current setup. So the performances of the filesystem
probably have a substantial impact on the results we observe.
probably have a substantial impact on the observed results.
In our precise setup, we know that the
filesystem we used is not adapted at all for Minio (encryption layer, fixed
number of inodes, etc.). Additionally, we mentioned earlier that we deactivated
`fsync` for our metadata engine in Garage, whereas Minio has some `fsync` logic here slowing down the
creation of objects. Finally, object storage is designed for big objects, for which the
costs measured here are negligible. In the end, again, we use Minio as a
costs measured here are negligible. In the end, again, we use Minio only as a
reference point to understand what performance budget we have for each part of our
software.
Conversely, Garage has an optimization for small objects. Below 3KB, a separate file is
not created on the filesystem but the object is directly stored inline in the
metadata engine. In the future, we plan to evaluate how Garage behaves at scale with
>3KB objects, which we expect to be way closer to Minio, as it will have to create
objects above 3KB, which we expect to be way closer to Minio, as it will have to create
at least one inode per object. For now, we limit ourselves to evaluating our
metadata engine and thus focus only on 16-byte objects.
metadata engine and focus only on 16-byte objects.
![Showing the time to send 128 batches of 8192 objects for Minio and Garage](1million-both.png)
@ -339,27 +339,27 @@ time to complete a batch of inserts is constant, while on Garage it still increa
It could be interesting to know if Garage's batch completion time would cross Minio's one
for a very large number of objects. If we reason per object, both Minio's and
Garage's performances remain very good: it takes respectively around 20ms and
5ms to create an object. At 100 Mbps, the upload of a 10MB file takes
5ms to create an object. In a real-world scenario, at 100 Mbps, the upload of a 10MB file takes
800ms, and goes up to 8sec for a 100MB file: in both cases
handling the object metadata is only a fraction of the upload time. The
handling the object metadata would be only a fraction of the upload time. The
only cases where a difference would be noticeable would be when uploading a lot of very
small files at once, which again is an unusual usage of the S3 API.
small files at once, which again would be an unusual usage of the S3 API.
Let us now focus on Garage's metrics only to better see its specific behavior:
![Showing the time to send 128 batches of 8192 objects for Garage only](1million.png)
Two effects are now more visible: 1., batch completion time increases with the
number of objects in the bucket and 2., measurements are dispersed, at least
more than for Minio. We expect this batch completion time increase to be logarithmic,
but we don't have enough data points to conclude safety: additional
number of objects in the bucket and 2., measurements are scattered, at least
more than for Minio. We expected this batch completion time increase to be logarithmic,
but we don't have enough data points to conclude confidently it is the case: additional
measurements are needed. Concerning the observed instability, it could
be a symptom of what we saw with some other experiments in this machine,
be a symptom of what we saw with some other experiments on this setup,
which sometimes freezes under heavy I/O load. Such freezes could lead to
request timeouts and failures. If this occurs on our testing computer, it might
occur on other servers as well: it would be interesting to better understand this
issue, document how to avoid it, and potentially change how we handle our I/O
internally in Garage. But still, this was a very stressful test that will probably not be encountered in
issue, document how to avoid it, and potentially change how we handle I/O
internally in Garage. But still, this was a very heavy test that will probably not be encountered in
many setups: we were adding 273 objects per second for 30 minutes straight!
To conclude this part, Garage can ingest 1 million tiny objects while remaining
@ -382,45 +382,45 @@ core value proposition of Garage. For example, our production cluster is
hosted [on refurbished Lenovo Thinkcentre Tiny desktop computers](https://guide.deuxfleurs.fr/img/serv_neptune.jpg)
behind consumer-grade fiber links across France and Belgium (if you are reading this,
congratulation, you fetched this webpage from it!). That's why we are very
careful that our internal protocol (named RPC protocol in our documentation)
careful that our internal protocol (referred to as "RPC protocol" in our documentation)
remains as lightweight as possible. For this analysis, we quantify how network
latency and the number of nodes in the cluster impact the duration of the most
latency and number of nodes in the cluster impact the duration of the most
important kinds of S3 requests.
**Latency amplification** - With the kind of networks we use (consumer-grade
fiber links across the EU), the observed latency between nodes is in the 50ms range.
When latency is not negligible, you will observe that request completion
time is a factor of the observed latency. That's to be expected: in many cases, the
node of the cluster you are contacting can not directly answer your request, and
has to reach other nodes of the cluster to get the requested information. Each
node of the cluster you are contacting cannot directly answer your request, and
has to reach other nodes of the cluster to get the data. Each
of these sequential remote procedure calls - or RPCs - adds to the final S3 request duration, which can quickly become
expensive. This ratio between request duration and network latency is what we
refer to as *latency amplification*.
For example, on Garage, a GetObject request does two sequential calls: first,
it fetches the descriptor of the requested object, which contains a reference
For example, on Garage, a `GetObject` request does two sequential calls: first,
it fetches the descriptor of the requested object from the metadata engine, which contains a reference
to the first block of data, and then only in a second step it can start retrieving data blocks
from storage nodes. We can therefore expect that the
request duration of a small GetObject request will be close to twice the
request duration of a small `GetObject` request will be close to twice the
network latency.
We tested the latency amplification theory with another benchmark of our own named
[s3lat](https://git.deuxfleurs.fr/Deuxfleurs/mknet/src/branch/main/benchmarks/s3lat)
which does a single request at a time on an endpoint and measures its response
which does a single request at a time on an endpoint and measures the response
time. As we are not interested in bandwidth but latency, all our requests
involving an object are made on a tiny file of around 16 bytes. Our benchmark
involving objects are made on tiny files of around 16 bytes. Our benchmark
tests 5 standard endpoints of the S3 API: ListBuckets, ListObjects, PutObject, GetObject and
RemoveObject. Here are the results:
![Latency amplification](amplification.png)
As Garage has been optimized for this use case from the beginning, we don't see
As Garage has been optimized for this use case from the very beginning, we don't see
any significant evolution from one version to another (Garage v0.7.3 and Garage
v0.8.0 Beta 1 here). Compared to Minio, these values are either similar (for
ListObjects and ListBuckets) or way better (for GetObject, PutObject, and
RemoveObject). This can be easily understood by the fact that Minio has not been designed for
environments with high latencies. Instead, it expects to run on clusters that are built
ListObjects and ListBuckets) or significantly better (for GetObject, PutObject, and
RemoveObject). This can be easily explained by the fact that Minio has not been designed with
environments with high latencies in mind. Instead, it is expected to run on clusters that are built
in a singe data center. In a multi-DC setup, different clusters could then possibly be interconnected with their asynchronous
[bucket replication](https://min.io/docs/minio/linux/administration/bucket-replication.html?ref=docs-redirect)
feature.
@ -488,7 +488,7 @@ terabytes of data and billions of objects on long-lasting experiments.
In the meantime, stay tuned: we have released
[a first release candidate for Garage v0.8](https://git.deuxfleurs.fr/Deuxfleurs/garage/releases/tag/v0.8.0-rc1),
and are already working on a number of features for the next version.
and are already working on several features for the next version.
For instance, we are working on a new layout that will have enhanced optimality properties,
as well as a theoretical proof of correctness
([#296](https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/296)). We are also