Jason wrote:
I occasionally run Windows' Disk Cleanup to purge cruft
left over from updates. I started it today and it reported
that it could liberate 21GB! on C:. The list of files to
delete included an entry I'd never seen befo "Compress
your OS drive" (20GB), but that entry was NOT checked so
would be skipped if I pressed OK. Is Cleanup proposing to
actually compress the drive or is there a hulking file
somewhere that it concludes I don't need?


This is my result today.

https://i.postimg.cc/6p4S2Kxm/cleanmgr.gif

The OS has the ability to apply NTFS "new style" compression
to selected files. The "compact" command is used. There
is an attribute that indicates whether the OS is a candidate
for compression, but I can't really be sure they know how
to operate that properly. I specifically select
/CompactOS:never on OS volumes here, forcing Windows to
re-expand the crap it compressed. I even do that on the
SSD drives. CompactOS is not applied to any of your data
partitions, and only applies to C: if/when applicable.

In any case, WinSXS (maybe 6GB of stuff), some of the
files in there may be candidates for compression. I don't
think the intention is to compress System32 "kernel.dll"
or equivalent operational files. I thought the design intent
was to compress "little used" files, and WinSXS has some
of those.

In my example picture, the largest numeric value is related
to my Downloads folder (which happens to be my work
folder and has a Chromium build in it). I have to be
careful not to tick the box for "Downloads" in the picture.

The "Windows Update Cleanup" could be my SoftwareDistribution,
which should really be self-cleaning. I don't plan on
ticking the box on that one either. So let's make a list
of candidates and see how they match up. Hmmm... Not
what i was expecting.

Windows Update Cleanup 3.92GB === proposed cleanup

SoftwareDistribution 321MB (candidate list)
C:\$WINDOWS.~BT approx zero
C:\$WINDOWS.~WS approx zero

C:\Windows\Installer 3.25GB for 1,755 .msi files (MSI is compressed internally)

WinSXS 8.88GB for 63,632 files

It's nice that no combination of candidates equals 3.92GB.
Maybe that's an estimate of the damage they could do to
WinSXS ?

*******

I don't generally recommend selecting any compression option as:

1) Compression is slow.
2) It has the potential to fragment things (at least the
"old" NTFS compression had that capability. I've noticed that
The "write unit" size on Win10 has increased to what could be
64KB, so it's possible they've paved over that problem.
3) Such Compression selections only make sense for 32GB
tablets, where the user is fighting with the OS for space.
And who doesn't want to do more Flash write cycles on
an eMMC chip that requires unsoldering to replace ?
Tablets are the perfect place for beating the **** out
of Flash.

Paul