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Huge cache usage while making hdd image with dd


On Windows 8.1 Update 1 Professional with cygwin 2.4.1 (not the
latest, I know, I'll recheck under 2.5.1 later),
I am running this command:

dd if=/dev/sdb bs=1M | tee sdb.1TB.test | sha256sum > sdb.1TB.test.sha256

the destination folder in on an external drive (NTFS, more detail below).

The CPU usage is about 40% (4 core Intel i5-2320) and the current
copying speed is 60 MB/s (measured by signaling USR1 to dd and
doubleckecking in the task manager).

The weird thing is OS disk cache usage.
Task manager show this for RAM usage (16GB physical RAM in system, 16
GB fixed size pagefile - on sdb):
In use: 1.7 GB
Available: 13.7 GB
Cached: 14,3 GB (!)
Committed: 2,8/32 GB
Paged pool: 277 MB
Non-paged pool: 126 MB

$ cat /proc/meminfo
MemTotal:       16758408 kB
MemFree:        14409844 kB
HighTotal:             0 kB
HighFree:              0 kB
LowTotal:       16758408 kB
LowFree:        14409844 kB
SwapTotal:      16777216 kB
SwapFree:       16233228 kB

perfmon.exe shows about 4% pagefile usage.

The symptoms are:
 - starting programs is slow. Starting explorer.exe takes 2-3 seconds
versus instant as normal (C: is a SSD)
 - after a while if left alone programs get stuck (swapped). For
example Firefox. If left alone (unused) for a minute and the I try to
click a link or scroll the page, it takes about 10 seconds during
which the window title shows "Not responding"

A simple copy operation should not occupy so much cache IMO.
I post here, as cygwin is involved.

Any idea what is going on?
Where to look?

Some system info:

Intel i5-2320
Intel H61 chipset

C: Samsung SSD 830  128GB

source drive (sdb) is an internal Hitachi HDS721010CLA332 (1 TB)

 * Seagate Desktop Expansion 5TB external drive, connected over USB3.0

David BalaÅic

Problem reports:
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