> ## Documentation Index
> Fetch the complete documentation index at: https://private-7c7dfe99-parallel-read-in-order-multi-part.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# s3_upload_* session settings

> ClickHouse session settings in the s3_upload_* generated group.

export const VersionHistory = ({rows = []}) => {
  if (rows.length === 0) {
    return null;
  }
  const headers = ["Version", "Default value", "Comment"];
  const border = "1px solid rgba(128, 128, 128, 0.3)";
  const cell = {
    border,
    padding: "0.25rem 0.5rem",
    textAlign: "start",
    verticalAlign: "top"
  };
  return <details className="not-prose" style={{
    border,
    borderRadius: "0.5rem",
    margin: "0.5rem 0",
    padding: "0.5rem 0.75rem",
    fontSize: "0.8125rem",
    lineHeight: "1.125rem"
  }}>
      <summary style={{
    cursor: "pointer",
    fontWeight: 600,
    opacity: 0.72
  }}>
        Version history
      </summary>
      <table style={{
    borderCollapse: "collapse",
    width: "100%",
    margin: "0.5rem 0 0"
  }}>
        <thead>
          <tr>
            {headers.map(header => <th key={header} style={{
    ...cell,
    fontWeight: 600,
    opacity: 0.72
  }}>
                {header}
              </th>)}
          </tr>
        </thead>
        <tbody>
          {rows.map((row, row_index) => <tr key={row.id ?? row_index}>
              {(row.items ?? []).map((item, item_index) => <td key={item_index} style={{
    ...cell,
    overflowWrap: "anywhere"
  }}>
                  {item?.label}
                </td>)}
            </tr>)}
        </tbody>
      </table>
    </details>;
};

export const SettingsInfoBlock = ({type, default_value, changeable_without_restart}) => {
  return <div className="not-prose" style={{
    display: "flex",
    flexWrap: "wrap",
    alignItems: "baseline",
    columnGap: "0.5rem",
    rowGap: "0.125rem",
    margin: "0.375rem 0",
    fontSize: "0.8125rem",
    lineHeight: "1.125rem"
  }}>
      <div style={{
    fontWeight: 600,
    opacity: 0.72
  }}>Type</div>
      <div style={{
    overflowWrap: "anywhere"
  }}>{type}</div>
      <div style={{
    fontWeight: 600,
    opacity: 0.72,
    marginInlineStart: "0.5rem"
  }}>Default</div>
      <div style={{
    overflowWrap: "anywhere"
  }}>{default_value}</div>
      {changeable_without_restart && <div style={{
    fontWeight: 600,
    opacity: 0.72,
    marginInlineStart: "0.5rem"
  }}>
          Changeable without restart
        </div>}
      {changeable_without_restart && <div style={{
    overflowWrap: "anywhere"
  }}>
          {changeable_without_restart}
        </div>}
    </div>;
};

These settings are available in [system.settings](/reference/system-tables/settings) and are autogenerated from [source](https://github.com/ClickHouse/ClickHouse/blob/master/src/Core/Settings.cpp).

<h2 id="s3_upload_checksum_algorithm">
  s3\_upload\_checksum\_algorithm
</h2>

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.9"},{"label": ""},{"label": "New setting to choose the checksum algorithm for S3 uploads."}]}]} />

The checksum algorithm used when ClickHouse uploads data to `S3`. `CRC32` and `SHA256` are sent as flexible `x-amz-checksum-*` headers, while `MD5` is sent as a `Content-MD5` header.

By default the value is empty and ClickHouse lets the AWS SDK compute `Content-MD5`. In FIPS mode `MD5` is unavailable, so the SDK silently omits it and the upload carries no checksum at all: set `CRC32` or `SHA256` to attach a flexible checksum instead. Where this setting applies, an explicit `MD5` is then rejected.

It takes effect when the `S3` client is created, so it applies to query-scoped uses such as the `s3` table function and `BACKUP ... TO S3`; a later per-query `SET` does not reconfigure an already-created long-lived client such as an `S3` disk.

`S3Express` buckets require a flexible checksum and do not accept `Content-MD5`: an explicit `CRC32` or `SHA256` is honored, an empty value uses `CRC32`, and an explicit `MD5` is rejected.

`GCS` endpoints (`storage.googleapis.com`) ignore this setting entirely and keep the SDK's `Content-MD5` behavior, because `GCS` rejects `SigV4`-signed requests carrying the AWS `x-amz-checksum-*` headers.

Server-side copies ignore this setting.

<h2 id="s3_upload_part_size_multiply_factor">
  s3\_upload\_part\_size\_multiply\_factor
</h2>

<SettingsInfoBlock type="UInt64" default_value="2" />

Multiply s3\_min\_upload\_part\_size by this factor each time s3\_multiply\_parts\_count\_threshold parts were uploaded from a single write to S3.

<h2 id="s3_upload_part_size_multiply_parts_count_threshold">
  s3\_upload\_part\_size\_multiply\_parts\_count\_threshold
</h2>

<SettingsInfoBlock type="UInt64" default_value="500" />

Each time this number of parts was uploaded to S3, s3\_min\_upload\_part\_size is multiplied by s3\_upload\_part\_size\_multiply\_factor.