Auditing IPEAK SPT

Application
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Operating System
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OS Disk Cache
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OS File System
     |<-------- WinTrace32 records all that reaches this point ---> no overlap between
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     |<-------- RankDisk plays back all from this point onwards -------^
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OS Disk Host Adapter Driver
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Disk Host Adapter
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Hard Drive Buffer
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Hard Drive Platter

The perfect, non-overlap system delivered by WinTrace32 and RankDisk in theory delivers the best conceivable combination between controlled trials and relevant access patterns. The million-dollar question: Are we sure that WinTrace32 and RankDisk do exactly what they claim?

Does WinTrace32 accurately record disk requests?

Another program from Intel, IOMeter, allows users to define access patterns through settings such as block sizes, reads vs. writes, sequential accesses vs. random accesses, etc. Accordingly, if we take a given access pattern with known settings and run through it while recording the accesses using WinTrace32, AnalyzeTrace should then be able to examine the file and note the similarities and/or differences between what IOMeter claims to be sending to the drive and what WinTrace32 claims to see going to the drive.

Let’s take a closer look at the File Server access pattern, included in the default wrkloads.icf file that comes bundled with IOMeter:

Size, % Access, % Read, and % Random are the most important settings. % Access determines how much of the total pattern will consist of the other three settings. In other words, for the first entry, 10% (% Access) of the access pattern will consist of 512 byte requests (Size) of which 80% will be reads and 20% will be writes (% Read). 100% of these requests will be random. The same explanation applies to all the entries that follow.

Through these settings, Intel attempts to synthetically reproduce the disk accesses of a typical file server. One significant setting, the number of outstanding I/Os (queue depth), is set on another screen:

We went ahead and ran this pattern on an ATA drive (WD’s Caviar WD1200BB) for 10 minutes with WinTrace32 running in the background to capture the accesses. WinTrace32’s facilities must be manually started and stopped. We attempted to start up the IOMeter accesses immediately after beginning the recording and to stop recording immediately after IOMeter completed the 10 minute trial. Let’s see how IOMeter’s input variables and WinTrace32’s captures stack up:

The first thing that jumps out in this AnalyzeTrace screen capture is the preponderance of 8 sector (512 bytes per sector, thus 4 KB) accesses- 60%. Surely enough, the File Server Access Pattern Spec calls for 60% of accesses to be 4 KB in length. 10% of accesses should be one sector (512 bytes) in length. Again, this is exactly what AnalyzeTrace reports. The exercise checks out perfectly: all of the reported transfer sizes in this trace file match up exactly with what IOMeter requested.

Throughout this IOMeter run, queue depth was kept at a constant 4. And, unsurprisingly, this is exactly what AnalyzeTrace reports.

According to the IOMeter File Server settings, 80% of all requests should be reads while 20% should be writes. AnalyzeTrace reports that of 48605 total requests, 39043 were reads… or 80.3%. The remaining 9562 requests (19.7%) are writes.

Since the File Server access pattern calls for 100% random accesses, there should be no requests that were 0 sectors away (and if there are any, it’s pure coincidence!). As expected, AnalyzeTrace reports no back-to-back requests. Interestingly, on this chart’s scale, seek distances aren’t even noticeable until they’re at least 256 MB away from the previous access… and only a tiny amount meet that minimum! A vast contrast from our captured desktop access patterns in which half the seeks occur within 8 KB of each other. This problem, lack of localization, plagued our now-defunct Workstation access pattern and caused it to miss the mark when assessing desktop performance.

IOMeter’s csv output file lists the WD1200BB’s average response time in this trial at 49.48 milliseconds. The average value derived from the trace’s service time graph displayed above is 49.66 milliseconds, for all intents and purposes identical.

Finally, let’s take a quick look at the accesses generated during this trial plotted along the drive’s capacity with respect to time. As one would expect, AnalyzeTrace reports an even, random distribution. Incidentally, contrasting this graph with that of the StorageReview.com Office DriveMark 2002 (a trace drawn from actual computer use by yours truly) reveals the localization of single-user accesses.

From the above its clear that WinTrace32 delivers on its end of the bargain- it succinctly and precisely captures exactly what is sent to it. Combined with AnalyzeTrace, WinTrace32 offers us an excellent view of exactly what kind of disk accesses occur with any given arbitrary system use. But what about RankDisk? Does it replay exactly what it should? Let’s take a look at the other half!

Perhaps most telling is AnalyzeTrace’s summaries for the two files:

File Server Capture, Original	2nd Generation Capture
1	1

Both trace files feature identical amounts of reads and writes when it comes to both the number of requests as well as the amount of data transferred. In the “capture of the capture” there is neither a stray access that wasn’t in the original nor an access in the original file that did not subsequently occur in the replay.

Let’s take a brief-blow-by-blow look at some other relevant AnalyzeTrace screens:

Distribution of Transfer Sizes
File Server Capture, Original	2nd Generation Capture
1	1
Average Value = 22.0845 sectors	Average Value = 22.0845 sectors

Both captures reflect an exacting reproduction of IOMeter’s set parameters in the File Server access pattern.

Distribution of Queue Lengths
File Server Capture, Original	2nd Generation Capture
1	1

In the original trial, IOMeter did its best to keep queue depths at 4. The chart to the left proves that WinTrace32 could record this constant queue depth. The chart to the right proves that RankDisk can play it back perfectly.

Distribution of Seek Distances
File Server Capture, Original	2nd Generation Capture
1	1

No surprises here. Since both the original trace as well as the trace of the original trace feature the exact same sector accesses, both seek distance graphs are identical.

Distribution of Service Times
File Server Capture, Original	2nd Generation Capture
1	1
Average Value = 49.6615 milliseconds	Average Value = 49.6423 milliseconds

It’s not unreasonable to expect that a given physical actuator will minutely vary in the time it takes to fulfill an access that is logically identical. One cause may be heat: the platters (or actuator) may be slightly expanded, contracted, etc. Even so, the “head-to-tail” (i.e., the amount of time that passes between the start of a given request and its completion) service times turned in by both the first and second-generation captures remain remarkably close to IOMeter’s original reported score of 49.48 milliseconds.

Disk Access Sequence
File Server Capture, Original	2nd Generation Capture

This plot of the location of all requests within the access pattern is, by nature, identical in both traces. Enough said.

RankDisk itself, after completing the playback of a trace, turns out the average service time recorded… though not in a “head-to-tail” fashion. Rather, RankDisk’s output is “head-to-head,” the average time measured between the start of one request and the start of the next.

Replayed on the same drive that the trace was captured from (the WD1200BB), this File Server pattern yielded a RankDisk score of 12.37 milliseconds. IOMeter, for its part, does not turn in a “head-to-head” response time in milliseconds. It does, however, report the number of I/O operations per second. A look at the IOMeter output file reveals that the WD1200BB completed an average of 80.83501 I/Os per second in the original trial. There are, of course, 1000 milliseconds within one second. One can then extrapolate I/Os per second from RankDisk’s output to yield a comparable score (this, in fact, is done in the SR DriveMarks… results are reported in IO/s sec rather than RankDisk’s native average milliseconds per request). The result? RankDisk reports that in the File Server access pattern, at a queue depth of 4 I/Os, the WD1200BB performs (1000/12.37) 80.84 I/Os per second… this being the replay of a trace of an original trial in which the WD performed 80.83 I/Os per second! Absolutely amazing!

RankDisk delivers. It plays back any given captured trace file with incredible precision and reliability.

It is plain to see that all attributes of the original disk accesses as generated by IOMeter (a program that allows us to literally define what kind of accesses are to take place) are preserved in both the recording and the playback of the trace file. It is reasonable to assume that if IPEAK can perfectly reproduce a given program’s (IOMeter) disk accesses, it can reproduce the disk accesses generated by any other program or set of programs. The StorageReview.com Desktop DriveMarks are just that- exacting RankDisk playback of precise WinTrace32 captures of typical desktop usage.

The ramifications may be startling, but they are indisputable: For desktop usage, an ATA drive such as Western Digital’s Caviar WD1000BB-SE delivers performance comparable to or even exceeding that of today’s 10,000 RPM SCSI drives. Thus, if you’re out to purchase a drive for your non-server machine and you settle on a 10k SCSI drive just because “it’s 10,000 RPM” or “it’s SCSI,” you are cheating yourself out of a good deal of capacity, a good deal of performance, and/or a good deal of saved money!

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