[linux-lvm] Why the dramatic increase in filesystem performance when usingxfs????

["Gary Mansell" <Gary.Mansell@ricardo.com>]

Hi,

I have always run ext3 filesystem with journalling on Redhat AS as it is
the only supported filesystem.

One of my colleagues runs xfs, though, and on comparable hardware
configs he gets twice the performance compared to my ext3 tests.

The test that I perform is to create a file at least twice the size of
the RAM installed in the system to avoid the possibility of cacheing,
measuring the time to write and read the file back gives me the
performance figure that I am after. I realise that this is a very simple
test of large sequential IO but it is good enough for my needs.

ie

Write test:

# time dd if=/dev/zero of=./testfile bs=16384 count=250000 ; time sync

Read test:

# time dd if=./testfile of=/dev/null bs=16384


As the xfs performance comes back about twice the performance of ext3
for this test I am of the opinion that xfs must be cheating somehow. It
has always been my opinion that the IO bottleneck is the hardware and
not the filesystem hence changing the filesystem but using the same
hardware should not make a huge difference to performance (you still
have to get the same amount of data out to disk at the end of the day)

I am struggling to comprehend how xfs can cheat, though, as it can't
cache such a huge file as there is not enough memory. Is it perhaps
cheating because the file is comprised entirely of zero's?

Can someone please enlighten me

Thanks in advance

Gary Mansell
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