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Kyrylo Kovalenko
Kyrylo Kovalenko

Reputation: 2171

API for monitoring individual files IO performace on Windows

What Windows API can I use to monitor I/O performance metrics for a specific file or set of files? Performance counters seem to offer only higher level objects such as LogicalDisk and PhysicalDisk. I'm looking for something that Windows Resource Monitor uses under Disk->Disk Activity, i.e read/write bps and response time.

Upvotes: 2

Views: 284

Answers (1)

Χpẘ
Χpẘ

Reputation: 3451

I did a quick search for "Perfmon individual files" and didn't see anything promising.

But I'm not sure measuring the performance of individual files will be all that meaningful. I/O activity is coalesced in the I/O stack in several places, the result being that at different levels the OS can't distinguish file I/O for one file versus another.

Assuming the app isn't doing any buffering/caching on it's own, the first place can be in buffering that happens in "C" (or similar) runtime libraries. Another place where coalescing occurs is in the file system (I'm assuming NTFS). I/O for file directories can be coalesced across multiple files in the same directory. I/O can be coalesced based on the file system's block size. So if multiple MFT entries share a block they can all be read/written at once. NTFS also implements caching and other I/O optimizations (read ahead). The performance of the cache can be affected by other processes running at the same time by either accessing the same file(s) you want to measure (helping to keep the file in cache) or by accessing other files (helping to evict your file from cache).

Coalescing also happens below the file system at the logical disk level. Single I/Os may service multiple files.

At the disk driver level single I/O requests may again involve multiple files. Additionally the driver (or more likely the drive firmware) can reorder disk I/Os based on knowledge it has about the drive "geometry" to gain additional throughput at the (possible) expense of response time. In this case I/O to your files may suffer compared to what it would see if the other processes weren't doing I/O at the same time.

Many disks implement caching in DRAM. This cache will also be affected by other processes in the same way that Window's cache is. Again affecting measured performance due to other process's activity.

If you still want to measure though, one way to circumvent the limitations in Perfmon is to put files or sets of files on different drives. The drives don't necessarily have to be different physical drives, they could be VHDs, or some other kind of virtual disk-on-physical disk. I know the Volume Snapshot Service (VSS) SDK has a little utility to create virtual drives out of files.

But putting your files on their own physical disks will probably give much more consistent results.

Upvotes: 1

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