Two file I/O questions

[Niek Bouman]

In the seastar `/apps` folder there is an application `iotune` which performs some disk I/O measurements and then outputs the results to a file, which can then be provided to any seastar application with the `--io-properties-file` command-line argument.

- In case you do _not_ perform this tuning, can something be said about the performance penalty?

Another question:
The Seastar docs say that for good performance, it is advised to respect the `disk_read_max_length()` and `disk_write_max_length` parameters:
https://docs.seastar.io/master/classseastar_1_1file.html#a5ca4c63b1c643bd070c3c99840048511

My question is how to properly respect these limits when using the `dma_read` and `dma_write` overloads that take a `std::vector< iovec >`. Does the limit apply to each `iovec`-length separately, or should the _sum_ of the lengths of all `iovec`s in the vector not exceed this limit?

[avi@scylladb.com]

On Tue, 2025-02-25 at 05:50 -0800, Niek Bouman wrote:

In the seastar `/apps` folder there is an application `iotune` which performs some disk I/O measurements and then outputs the results to a file, which can then be provided to any seastar application with the `--io-properties-file` command-line argument.

- In case you do _not_ perform this tuning, can something be said about the performance penalty?

In fact bandwidth can be better, but there will be no latency isolation between scheduling groups.

Another question:
The Seastar docs say that for good performance, it is advised to respect the `disk_read_max_length()` and `disk_write_max_length` parameters:
https://docs.seastar.io/master/classseastar_1_1file.html#a5ca4c63b1c643bd070c3c99840048511

My question is how to properly respect these limits when using the `dma_read` and `dma_write` overloads that take a `std::vector< iovec >`. Does the limit apply to each `iovec`-length separately, or should the _sum_ of the lengths of all `iovec`s in the vector not exceed this limit?

Sum of the lengths.

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