Update ReadBuffer for AzureBlobStorage

[SmitaRKulkarni]

Changelog category (leave one):

• Performance Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Updated to use azure api DownloadTo instead of Download and ReadTo apis. DownloadTo reads to a buffer and uses parallel requests.

[robot-clickhouse-ci-1]

This is an automated comment for commit fa4d205 with description of existing statuses. It's updated for the latest CI running

❌ Click here to open a full report in a separate page

Check name	Description	Status
CI running	A meta-check that indicates the running CI. Normally, it's in success or pending state. The failed status indicates some problems with the PR	⏳ pending
ClickHouse build check	Builds ClickHouse in various configurations for use in further steps. You have to fix the builds that fail. Build logs often has enough information to fix the error, but you might have to reproduce the failure locally. The cmake options can be found in the build log, grepping for cmake. Use these options and follow the general build process	⏳ pending
Integration tests	The integration tests report. In parenthesis the package type is given, and in square brackets are the optional part/total tests	❌ failure
Mergeable Check	Checks if all other necessary checks are successful	❌ failure
Performance Comparison	Measure changes in query performance. The performance test report is described in detail here. In square brackets are the optional part/total tests	❌ failure

Successful checks

Check name	Description	Status
A Sync	There's no description for the check yet, please add it to tests/ci/ci_config.py:CHECK_DESCRIPTIONS	✅ success
AST fuzzer	Runs randomly generated queries to catch program errors. The build type is optionally given in parenthesis. If it fails, ask a maintainer for help	✅ success
ClickBench	Runs [ClickBench](https://github.com/ClickHouse/ClickBench/) with instant-attach table	✅ success
Compatibility check	Checks that clickhouse binary runs on distributions with old libc versions. If it fails, ask a maintainer for help	✅ success
Docker keeper image	The check to build and optionally push the mentioned image to docker hub	✅ success
Docker server image	The check to build and optionally push the mentioned image to docker hub	✅ success
Docs check	Builds and tests the documentation	✅ success
Fast test	Normally this is the first check that is ran for a PR. It builds ClickHouse and runs most of stateless functional tests, omitting some. If it fails, further checks are not started until it is fixed. Look at the report to see which tests fail, then reproduce the failure locally as described here	✅ success
Flaky tests	Checks if new added or modified tests are flaky by running them repeatedly, in parallel, with more randomization. Functional tests are run 100 times with address sanitizer, and additional randomization of thread scheduling. Integrational tests are run up to 10 times. If at least once a new test has failed, or was too long, this check will be red. We don't allow flaky tests, read the doc	✅ success
Install packages	Checks that the built packages are installable in a clear environment	✅ success
PR Check	There's no description for the check yet, please add it to tests/ci/ci_config.py:CHECK_DESCRIPTIONS	✅ success
Stateful tests	Runs stateful functional tests for ClickHouse binaries built in various configurations -- release, debug, with sanitizers, etc	✅ success
Stateless tests	Runs stateless functional tests for ClickHouse binaries built in various configurations -- release, debug, with sanitizers, etc	✅ success
Stress test	Runs stateless functional tests concurrently from several clients to detect concurrency-related errors	✅ success
Style check	Runs a set of checks to keep the code style clean. If some of tests failed, see the related log from the report	✅ success
Unit tests	Runs the unit tests for different release types	✅ success
Upgrade check	Runs stress tests on server version from last release and then tries to upgrade it to the version from the PR. It checks if the new server can successfully startup without any errors, crashes or sanitizer asserts	✅ success

[SmitaRKulkarni]

renamed this function, as it also reads data now. But we still need initialized bool for other checks, so kept that as it is.

[kssenii]

Does it really give a performance improvement? Seems like just a different api (and this new way with DownloadTo is better and more clear, so the change is good) but it seems to be not to be improving the perf anyhow.

[kssenii]

LGTM, but may be worth to update changelog entry to not-for-changelog, or if not updating then to make a bit more coherent description of the change in the changelog entry.

[kssenii]

If it is a azure disk read it will be better not to execute additional request to azure just to get file size (getFileSize is only used for external table engines) as it is already known before we create the read buffer. So may be just pass file_size as optional argument in constructor of ReadBufferFrpmAzureBlobStorage?

btw why do we need this check? There is no such check for other buffers AFAIK

[SmitaRKulkarni]

For other buffers, I see we send the request to read the full intended length of the file.
Here we are using readBytes function to read bytes to a buffer and only read as much as its capacity (data_capacity). So every time we read we extend the offset at one point it will reach intended size or file_size .

In getFileSize() function we only read the value once, it is just returned for future calls.

Please let me know if you have any suggestions

[kssenii]

In getFileSize() function we only read the value once, it is just returned for future calls.

Yes, I saw that it is fetched only once, but if it is avoidable - better to avoid, we already have a file size when we create this read buffer, better to use it.

For other buffers, I see we send the request to read the full intended length of the file.

But isn't it the same as here? Here we have a buffer and try to read data_capacity size to it, but if we reach the end of file while reading into buffer - we will just read nothing and the buffer will be empty, so we return false as well. Or am I missing something?

[SmitaRKulkarni]

But isn't it the same as here? Here we have a buffer and try to read data_capacity size to it, but if we reach the end of file while reading into buffer - we will just read nothing and the buffer will be empty, so we return false as well. Or am I missing something?

DownloadTo function takes Range as a parameter (offset, length) if length is zero it reads full file, so when we reach end of file, the range will be (offset=file_size, length), even if length is zero we see error message that the "The range specified is invalid for the current size of the resource".
Previously when we used to Download & then ReadToBuffer, there we had a check to_read_bytes = min(total_size-offset, length), and we would just read 0 bytes & return false.
Hope that clarifies the need. Please suggest if you have better options.

[kssenii]

Ah, ok understood. Also what if there are remaining N bytes to read from file and N is less that buffer_capacity, shouldn't it fail as well with incorrect range error? Seems currently code is not protected against this

[SmitaRKulkarni]

It is checked here : https://github.com/ClickHouse/ClickHouse/pull/61884/files/fa4d205ad943c7db334dc8788538d08b5cbdaa32#diff-3def5d979078691f30c3473edb5cadb9bdce981ec015f43c46588000f3d66281R175-R188

[kssenii]

But read_until_position is not always set. It is not set for remote_filesystem_read_method=read

[CurtizJ]

I think it will not improve performance but will decrease it instead because it will make a GET request for each call of nextImpl instead of one request in initialize method, as I understand the azure sdk. DownloadTo looks more suitable for reading small blobs that can be read with one call.