Detecting file I/O read-ahead
Detecting file I/O read-ahead
- Subject: Detecting file I/O read-ahead
- From: Chris Page <email@hidden>
- Date: Mon, 27 Jun 2005 00:05:51 -0700
[Originally posted to email@hidden. Someone there suggested I try darwin-dev.]
I'm specifically interested in Mac OS X, but I assume the answer will be the same for Darwin.
I've observed that reading a large file from front to back is significantly faster than reading back to front. I assume this is due to the OS performing read-ahead in file I/O. Is there some way, say, using
fs_usage, to see when read-ahead is happening? The traces I'm seeing for an application I'm developing only include the expected
pread calls, but no apparent additional read I/O.
Traces for some other programs, such as
wc, include
RdData[async] entries mixed in among
read calls, which look suspiciously like they may be read-ahead, but I haven't been able to find any documentation describing what those log entries mean, and I don't see them in the log for my app.
--
Chris Page
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