squashfs wo compression = speed up?

[livecdOrNothing]

Hello,

Today's computers could have enough memory (>8Gb) to avoid compression of filesystem in livecd. For example I generate the .iso file with size between 1.2 and 2.5 Gb which contains the squashfs file of the same size and where the filesystem inside (ext3img) is between 4 and 8 Gb. When booting in ram with live_ram parameter the system is already working faster than other os, but I assume that compression reduces the speed a bit.

So, my question is if you use "--skipcompression" option of livecd-iso-to-disk and obtain the 4Gb squashfs file in iso instead of 1.2 Gb will compression be totally absent (even if it is still called squashfs) or not?

Any opinion or suggestion is very welcome.

[stevea]

mksquashfs has several parameters to compress/not-compress various parts of the filesystem.

-noI do not compress inode table
-noD do not compress data blocks
-noF do not compress fragment blocks
-noX do not compress extended attributes
There are also several compression algorithms to choose from.

There is of course no advantage in using squashfs w/o compression, but yes it works as expected.

Btrfs supports compression too, fyi.

When you compress files, this reduces the amount of block I/O and increases the amount of data that appears in memory buffers, so it can make the disk I/O much less and therefore faster. However the negative side is that you must use CPU resource to uncompress on reads and compress on writes and that takes some time. Whether this is a net plus of minus depends on the application, the disk speed, the CPU performance and such.

[livecdOrNothing]

Thanks a lot for the reply - didn't know all those parameters and experimenting with them!!

Since I use rd.live.ram parameter everything is in ram and so the "disk speed" is maximum possible anyway. What I'm trying to achieve is to reduce even the CPU efforts spent on uncompressing the files from filesystem. For the moment, the more or less successful attempt is to have two squashfs.img files on the usb key (another post on same forum). One is compressed 1.2Gb, another is not compressed 5Gb and they correspond to two separate options on boot menu. Both of them would load to ram with rd.live.ram parameter, but depending on the computer memory I can boot with compressed filesystem (when memory is below 4Gb) or with uncompressed one when memory is above 8Gb.

May be an additional question. Is there a way to avoid compression when livecd copies squashfs to memory? If this can be decided at boot (kernel parameter) it would be nice. I suspect it is not possible, so my solution with two squashfs.img files takes more time to prepare but is sort of ok.

Also, if someone have an idea about performance profiling that can be compared on same computer, it would help too.