[Disksim-users] Other problems that I have w./ Disksim

[YipKei Kwok]

Hi,

Here are some other questions that I have w./ Disksim.

I discovered these questions under the following environment.
- trace workload
- 1 I/O driver, 1 sys logorg, 1 ctrl, RAID 0 of 1 disk (4 disks if specified)

1. Some requests in the trace not being delivered to the disk
As I am looking for a reason why the number of requests getting into
the disk is less than the number of requests specified in the trace, I
found that some requests specified in the trace are not actually being
delivered to the disk. To investigate the problem, I started from a
tiny problem set.

When there is/are only 1 or 2 request(s) in the trace, there is no
request being delivered to the disk. Finally, as I keep increasing the
number of requests, the "Total Request handled" count of the overall
I/O System and the Disk become 1 when there are 3 requests in the
trace. However, the buffer hit count is 0. So, it means that the
disappearance of the 2 requests is not because of cache hit. So, where
did they go?

2. Different values of "Overall I/O System Total Requests handled"
with the same trace but different RAID setup.

There are 92359 requests in our trace. If a RAID 0 of 1 disk is used,
the "Overall I/O System Total Requests handled" is 86339. However, if
a RAID 0 of 4 disks is used, the value of "Overall I/O System Total
Requests handled" increases to 92358. I would assume that the number
of requests handled should be independent of the number of disks in
the RAID. Did I make a wrong assumption, or did I miss something?
Also, I cannot find the definition of "Overall I/O System" from the
user manual. Could anyone help me with the definition please?

3. Swelling number of requests along the I/O hierarchy
I used a RAID 0 of 1 disk and the other one with 4 disks. I found
that, when only 1 disk is used, the values of "Total Requests handled"
of the "Overall I/O System", "Sys logorg", IOdriver" and Disk are the
same. But, when 4 disks are used, the values of "Total Requests
handled" of the last 3 are the same but that of the "Overall I/O
System" is less than the rest 3. How can we explain it?

Please see below for experimental results.
-- from statistical output of an unmodified Disksim installation
--- e.g. when 92359 requests are requested (RAID 0, 1 disk)
---- Overall I/O System Total Requests handled = 86339
---- Sys logorg #0 disk Total Requests handled = 86339
---- IOdriver Total Requests handled = 86339
---- Disk Total Requests handled = 86339

--- when 92359 requests are requested (RAID 0, 4 disk)
---- Overall I/O Subsystem Total Requests handled = 92358
---- Sys logorg #0 disk Total Requests handled = 105515
---- IOdriver Total Requests handled = 105515
----- IOdriver #0 device #0 Total Requests handled = 26042
----- IOdriver #0 device #1 Total Requests handled = 26557
----- IOdriver #0 device #2 Total Requests handled = 26504
----- IOdriver #0 device #3 Total Requests handled = 26412
---- Disk Total Requests handled = 105515
----- Disk #0 Total Requests handled = 26042
----- Disk #1 Total Requests handled = 26557
----- Disk #2 Total Requests handled = 26504
----- Disk #3 Total Requests handled = 26412

I am looking forward to hear from you. More experiment information
will be provided upon requested.

Thank you!

Regards,
Yipkei

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