Re: Very slow file write/create operations

[Frank Wille <frank%phoenix.owl.de@localhost>]

Robert Elz wrote:

> | 2. Shouldn't the user- and sys-times more or less add into "real"
> | Where did the system waste all that time?
>
> Waiting for something to finish.   With your tests, I'd guess that
> is synchronous writes to the ciss raid array - which are (by default)
>  used for all metadata operations
> [...]
> If you can make a new filesystem on the raid (it should not need to 
> be very big, use twice the size of du /etc (get the units correct)
> as a rough estimate), and mount that one -o async and repeat the test
> copying /etc onto that (new) filesystem.

As I now understand synchronous writes are the default, unless I specify 
-o async. Then the negative effect of synchronous writes is especially 
visible with RAID configurations? Otherwise I should have noticed that 
before.

Anyway, I didn't make the -o async test yet, because it requires adding 
more disks, or making the swap partition smaller. But I noticed that I 
didn't change the file system to WAPBL after installation!

So I added the "log" option in /etc/fstab, rebooted and repeated the test:

# time cp -Rp /etc /root
        0.05 real         0.00 user         0.05 sys
# time rm -rf /root/etc
        0.01 real         0.00 user         0.01 sys

That's what I would expect! The extreme delay is gone. I can even add a 
sync and it's still factor 20:

# time cp -Rp /etc /root; time sync
        0.05 real         0.00 user         0.04 sys
        1.14 real         0.00 user         0.00 sys
# time rm -rf /root/etc; time sync
        0.01 real         0.00 user         0.01 sys
        0.26 real         0.00 user         0.00 sys

This matches, or is even a little faster, than the old server.
Thanks.

--
Frank Wille