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Add comment explaining why RandomAccessFileReader* is not passed in at construction time

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Andrew Chang 2024-11-25 14:05:49 -08:00
parent f20d12adc8
commit 5924a20443
1 changed files with 12 additions and 0 deletions
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12
file/file_prefetch_buffer.h
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@ -300,6 +300,14 @@ class FilePrefetchBuffer {
// offset : the file offset to start reading from.
// n : the number of bytes to read.
//
// Note: Why do we pass in the RandomAccessFileReader* for every single call
// to Prefetch/PrefetchAsync/TryReadFromCache? Why can't we just pass it in at
// construction time?
// Although the RandomAccessFileReader* is often available when creating
// the FilePrefetchBuffer, this is not true for BlobDB (see
// BlobSource::GetBlob). The file reader gets retrieved or created inside
// BlobFileCache::GetBlobFileReader, after we have already allocated a new
// FilePrefetchBuffer.
Status Prefetch(const IOOptions& opts, RandomAccessFileReader* reader,
uint64_t offset, size_t n);
@ -488,6 +496,10 @@ class FilePrefetchBuffer {
// Whether we reuse the file system provided buffer
// Until we also handle the async read case, only enable this optimization
// for the synchronous case when num_buffers_ = 1.
// Note: Although it would be more convenient if we could determine
// whether we want to reuse the file system buffer at construction time,
// this would not work in all cases, because not all clients (BlobDB in
// particular) have a RandomAccessFileReader* available at construction time.
bool UseFSBuffer(RandomAccessFileReader* reader) {
return reader->file() != nullptr && !reader->use_direct_io() &&
fs_ != nullptr &&