After the ValiDrive's testing has completed and its user has closed its map display, the introductory contents of ValiDrive's main dialog is replaced with a report providing additional information about the drive tested and ValiDrive's interaction with the drive:

The various items of the drive's report are detailed below:

• declared drive size – This is the exact number of bytes reported by the drive and its size followed by its size in commonly used units. This will be the approximate formatted file system size as seen and reported by by any operating system.
• validated drive size – After filling-in the entire drive map, ValiDrive scans from the front of the drive to find the first non-green (non-validated) region. It uses the end of this region as the drive's validated size, which it reports in exact bytes and common units.
• highest valid region – A read or write error might cause ValiDrive to prematurely terminate its upward validated drive size scan. So this “highest valid region” separately describes the last green (validated) region detected by ValiDrive, even if read or write errors may have been found earlier, which would have reduced the drive's apparent validated storage.
• hub or drive vendor – USB hubs may obscure their connected drive's vendor, product and serial number. But this information is generally correct when a USB drive is directly connected to the computer. ValiDrive shows whatever it has been able to determined about the connected drive, which may reflect an intervening hub, if any.
• hub or drive product – The same applies for the USB drive product as for its vendor. Please see the “hub or drive vendor” description above for more information.
• serial number – The same applies for the USB drive serial number as for its vendor. Please see the “hub or drive vendor” description above for more information.
• samples – This shows the total number of read and write operations which were performed. For any completed test this number will be 1,152 reads and 1,152 writes since this is 2 x 576 each, which is the total number of regions tested during the test with two reads and two writes performed for every region.
• minimum – The time required to perform every read and write operation is carefully measured to the microsecond. Once all (usually 1,152) reads and writes have been completed, ValiDrive shows the number of microseconds required for the fastest of the reads. In the example above, this was 340 microseconds for the single fastest read. That's 340 millionths of a second or 0.34 milliseconds (thousandths of a second.)
• maximum – As with the “minimum” above, this reflects the single longest (slowest) read or write encountered during ValiDrive's testing. In the example above, this was 4,557 microseconds for the slowest read, which is 4.557 milliseconds.
• average – As you doubtless know, this “average” measurement will be the average time required for all reads and for all writes during ValiDrive's testing. So it's the best overall indication of the drive's performing during this random access small-region reading and writing.
• median – After all 1,152 reads and writes have been completed, the set of sample times for reads and write are separately sorted from lowest to highest. The “median” is the value of the sample in the middle of the sorted set of samples where there are as many faster samples as there are slower samples.
• std dev – “Standard deviation” is a well known statistical measure of the dispersion the sampled data in relation to the average. A low or small standard deviation indicates that the sampled data are clustered tightly around the average, whereas a high or large standard deviation indicates that the data samples are more spread out. Although the report shows the standard deviation for the drive's read and write times in the interest of completeness, the more interesting measure is the “variance”, which is described next:
• variance – This “co-efficient of variation” (CV) is a statistical measure of the dispersion of data samples around the average. It is the ratio of the standard deviation to the average, which makes it useful for comparing the degree of variation from one set of samples to another, even if their averages are significantly different from one another. Again, this is the standard deviation divided by the average. Please see the discussion below for additional help understanding this value.
• total time – As you would expect, “total time” is the sum of all of the individual read and write times. So it shows the total amount of time required to read versus write to the drive during this testing.
• percent – This is the percentage of time spent reading versus time spent writing during ValiDrive's total testing time.

The ValiDrive performance details of two different thumb drives are shown above. The SanDisk 512GB drive is a classic tiny tumb drive, but it performs nicely. The total time required to run ValiDrive's test was 12.24 seconds (the sum of the total read and write times). By comparison, the 32GB "Micro Center" drives (no-name thumb drives purchased from Amazon in a 10-pack for ~$30 or $3 each) required a whopping 736.37 seconds or 12.27 minutes! – in other words about 60 times longer! The charts above reveal the reason for the massive discrepancy in performance:

Looking at the total time spent reading we see that the inexpensive 32GB drives significantly outperformed the much larger and more expensive SanDisk drive by more than 2 to 1. The inexpensive drive's average read time was 1.665 milliseconds whereas the average for the 512GB SanDisk drive was 4.467 milliseconds. However... writing is an entirely different story!

The $3 drive was nearly 100 times slower (93.95 times) to write than the larger 512GB SanDisk drive.

This massive discrepancy in apparent write performance is due to the fact that ValiDrive switches back and forth between reading and writing – transferring only 4k bytes of data each time – and less expensive drives require far more time to begin writing than more expensive and more sophisticated drives.

Writing to NAND flash memory requires the generation of high voltage, but reading requires low voltage. Before any NAND chip can write data into NAND flash memory it must internally generate a high voltage. This uses a “voltage pump” which takes time to charge up, and lower-end solid state storage does this much more slowly. Then, before reading, that high voltage must be dumped.

Since operating systems typically write entire files and even multiple files at once, a single “voltage charging” cycle can be amortized across the entire large write. So large writes are able to proceed at a reasonable pace. But since ValiDrive only writes 4k bytes before reading it back, causing the “writing charge” to be dumped before each read, ValiDrive accentuates this delay for drives which require a long write-cycle.

For any form of random access mass storage, the “Variance” in read and write timing can be an important indication of the general health of the storage system. We would expect healthy drives to generally spend the same amount of time reading or writing fixed amounts of data, independent of the data's location, whereas drives where something is wrong, very wrong, or beginning to go wrong, would be expected to exhibit more varying read and write performance. This is why ValiDrive introduces this potentially important concept of Variance.

As noted above, “standard deviation” is a measure of the dispersion of values within a sample set. Looking at the standard deviations of the write times of the two drives above, we see 5,605 for the fast-write cycle drive and a whopping 676,662 for the ultra-slow write-cycle drive. This would appear to indicate that the slower drive's write times have a massive dispersion. But that's not actually true because the standard deviation fails to take the average value into consideration.

The average write time for the faster write-cycle drive is 6,858 milliseconds and the average write time for the slower write-cycle drive is 637,543. So, for the standard deviation to have any diagnostic value it must be taken within the context of the average time. This is why ValiDrive calculates the Variance as the standard deviation divided by the average.

Now, when we compare the two drives' Variance we have 0.817 for the faster write-cycling drive and 1.061 for the slower write-cycling drive. So, even though their non-contextual raw standard deviations differ by a factor of 93!, their contextualized variance only differs by a factor of 0.77.

In other words, even though the inexpensive $3, 32GB thumb drive is exhibiting an extremely slow write-cycle under ValiDrive, that write-cycle timing is not varying very much, which suggests that all is well inside that drive... even though it takes a while to get going at the start of any writing activity. And it certainly does read quickly... more than twice as fast as the larger more expensive 512GB drive.