I wonder how snzip handles IO.

[yuslee80]

We are compressing the data using snzip in a specific time zone. To explain more, we are compressing /dev/sdb's data into /dev/sdc.

If you look at the picture below, you can see the Read IO(r/s) and Write Sector Size (wKB/s) indicators of the device in /dev/sdb.
By the way, while /dev/sdb's data is being compressed to /dev/sdc, there is almost no disk I/O for /dev/sdc. If you are compressing data to /dev/sdb -> /dev/sdc, shouldn't w/s of /dev/sdc increase?

Periodically some Write IO occurs in /dev/sdc.

Device            r/s     w/s     rkB/s     wkB/s   rrqm/s   wrqm/s  %rrqm  %wrqm r_await w_await aqu-sz rareq-sz wareq-sz  svctm  %util
sdc              0.00   14.00      0.00   6812.00     0.00     0.00   0.00   0.00    0.00    9.07   0.13     0.00   486.57   1.93   2.70
sdc1             0.00   14.00      0.00   6812.00     0.00     0.00   0.00   0.00    0.00    9.07   0.13     0.00   486.57   1.93   2.70

My question is: if you are compressing to /dev/sdb -> /dev/sdc, /dev/sdb should be continuously outputting Read IO, /dev/sdc should be continuously outputting Write IO, but /dev/sdc should not be doing periodic Write IO. Why is that?