diff options
Diffstat (limited to 'src/backend/access/transam/xlogprefetch.c')
| -rw-r--r-- | src/backend/access/transam/xlogprefetch.c | 12 |
1 files changed, 7 insertions, 5 deletions
diff --git a/src/backend/access/transam/xlogprefetch.c b/src/backend/access/transam/xlogprefetch.c index 28764326bcc..2178c9086e6 100644 --- a/src/backend/access/transam/xlogprefetch.c +++ b/src/backend/access/transam/xlogprefetch.c @@ -31,12 +31,14 @@ * stall; this is counted with "skip_fpw". * * The only way we currently have to know that an I/O initiated with - * PrefetchSharedBuffer() has that recovery will eventually call ReadBuffer(), - * and perform a synchronous read. Therefore, we track the number of + * PrefetchSharedBuffer() has completed is to wait for the corresponding call + * to XLogReadBufferInRedo() to return. Therefore, we track the number of * potentially in-flight I/Os by using a circular buffer of LSNs. When it's - * full, we have to wait for recovery to replay records so that the queue - * depth can be reduced, before we can do any more prefetching. Ideally, this - * keeps us the right distance ahead to respect maintenance_io_concurrency. + * full, we have to wait for recovery to replay enough records to remove some + * LSNs, and only then can we initiate more prefetching. Ideally, this keeps + * us just the right distance ahead to respect maintenance_io_concurrency, + * though in practice it errs on the side of being too conservative because + * many I/Os complete sooner than we know. * *------------------------------------------------------------------------- */ |