Re: [pbs-devel] [PATCH v2 backup 1/5] add and implement chunk_from_offset for IndexFile

[Thomas Lamprecht <t.lamprecht@proxmox.com>]

On 22.07.20 15:56, Stefan Reiter wrote:
> Necessary for byte-wise seeking through chunks in an index.
> 
> Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
> ---
>  src/backup/dynamic_index.rs | 18 ++++++++++++++++++
>  src/backup/fixed_index.rs   | 11 +++++++++++
>  src/backup/index.rs         |  3 +++
>  3 files changed, 32 insertions(+)
> 
> diff --git a/src/backup/dynamic_index.rs b/src/backup/dynamic_index.rs
> index 4907fe1f..887b7cf3 100644
> --- a/src/backup/dynamic_index.rs
> +++ b/src/backup/dynamic_index.rs
> @@ -216,6 +216,24 @@ impl IndexFile for DynamicIndexReader {
>              digest: self.index[pos].digest.clone(),
>          })
>      }
> +
> +    fn chunk_from_offset(&self, offset: u64) -> Option<(usize, u64)> {
> +        let end_idx = self.index.len() - 1;
> +        let end = self.chunk_end(end_idx);
> +        let found_idx = self.binary_search(0, 0, end_idx, end, offset);
> +        let found_idx = match found_idx {
> +            Ok(i) => i,
> +            Err(_) => return None
> +        };
> +
> +        let found_start = if found_idx == 0 {
> +            0
> +        } else {
> +            self.chunk_end(found_idx - 1)
> +        };
> +
> +        Some((found_idx, offset - found_start))
> +    }
>  }
>  
>  struct CachedChunk {
> diff --git a/src/backup/fixed_index.rs b/src/backup/fixed_index.rs
> index 73d0dad0..b7e785d6 100644
> --- a/src/backup/fixed_index.rs
> +++ b/src/backup/fixed_index.rs
> @@ -219,6 +219,17 @@ impl IndexFile for FixedIndexReader {
>  
>          (csum, chunk_end)
>      }
> +
> +    fn chunk_from_offset(&self, offset: u64) -> Option<(usize, u64)> {
> +        if offset >= self.size {
> +            return None;
> +        }
> +
> +        Some((
> +            (offset / self.chunk_size as u64) as usize,
> +            offset % self.chunk_size as u64

modulo is really slow, but isn't chunk_size always a 2^x and thus we can
do the same here as we do in other places:

offset & (self.chunk_size - 1)

> +        ))
> +    }
>  }
>  
>  pub struct FixedIndexWriter {
> diff --git a/src/backup/index.rs b/src/backup/index.rs
> index efdf3b54..2eab8524 100644
> --- a/src/backup/index.rs
> +++ b/src/backup/index.rs
> @@ -22,6 +22,9 @@ pub trait IndexFile {
>      fn index_bytes(&self) -> u64;
>      fn chunk_info(&self, pos: usize) -> Option<ChunkReadInfo>;
>  
> +    /// Get the chunk index and the relative offset within it for a byte offset
> +    fn chunk_from_offset(&self, offset: u64) -> Option<(usize, u64)>;
> +
>      /// Compute index checksum and size
>      fn compute_csum(&self) -> ([u8; 32], u64);
>  
>