seife’s assorted rants

I knew for quite some time that XFS has an abysmal performance on huge deletes - e.g. deleting a Linux kernel source tree can take several minutes. But when I had to do multiple images with kiwi yesterday on my relatively new server machine at home, it started to really annoy me, so I invested some time to find out if there is a way to improve the situation.
This is what I started from (numbers are from bonnie++):

/dev/sda1 /space3 xfs rw,noatime,nodiratime,noquota 0 0

Version 1.01d       ------Sequential Output------ --Sequential Input- --Random-
                    -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine        Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
server           4G 69024  31 71616  10 32732   6 75462  39 77511   6 120.9   0
                    ------Sequential Create------ --------Random Create--------
                    -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
              files  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
                 16   341   1 +++++ +++   311   1   340   2 +++++ +++   249   1

The first hint I got from a colleague was to use “logbufs=8″ mount option. This increases the number of buffers that XFS uses for its log. Whatever that means. At the first try, it did not change anything until I realized that a “mount -o remount,…” was not enough, I had to unmount and newly mount the filesystem with the option to make it work.
This is the result, still not too impressive with regard to delete performance, the difference might be purely statistical noise:

/dev/sda1 /space3 xfs rw,noatime,nodiratime,logbufs=8,noquota 0 0

bonnie++:
Version 1.01d       ------Sequential Output------ --Sequential Input- --Random-
                    -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine        Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
server           4G 66413  31 68420  10 32467   5 76001  40 77143   6 107.2   0
                    ------Sequential Create------ --------Random Create--------
                    -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
              files  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
                 16   338   2 +++++ +++   313   1   332   2 +++++ +++   269   1

But the really good results came only after also increasing the logbuffer size with the “logbsize=262144″ option:

/dev/sda1 /space3 xfs rw,noatime,nodiratime,logbufs=8,logbsize=256k,noquota 0 0

Version 1.01d       ------Sequential Output------ --Sequential Input- --Random-
                    -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine        Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
server           4G 77932  33 85196  10 32134   6 59205  32 76792   6 184.5   0
                    ------Sequential Create------ --------Random Create--------
                    -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
              files  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
                 16  1495   5 +++++ +++   848   2  1468   7 +++++ +++   916   3

Now that’s a real improvement and I’ll keep it like that.

But beware, there are some caveats:

• The file system has to support the larger buffer sizes, which means you need a sufficiently new kernel. As I am running openSUSE 11.1 on this machine, this is not really a problem, but I had to enable the “version 2 log format” with “xfs_admin -j /dev/sda1″ because this partition was created using SLES10 and I am not sure if I still would be able to mount it on an old machine.
• It will use more memory. I guess that it needs at least 2 MB (256kB * 8 buffers) per filesystem, probably more. But on a box with 2GB of RAM, I’ll gladly spare that for better performance.

One note: those “measurements” were taken on a Core2 Duo machine using an abit IP35-E mainboard. The used disk drive was my old Maxtor STM3500630A 500GB IDE drive. I did not do any statistical corrections etc., so your mileage may vary.

Yesterday at FOSDEM 2009, I was asked in an interview done by linux-community.de’s Nils Magnus, “how do you get the pipe symbol on keyboards without that extra key” (which “international” keyboards have, but US keyboards don’t).
Apart from that question being off-topic, as the interview was supposed to be about my netbook talk I had held just before, it’s funny that I knew the answer offhand, since I am very often in the situation to use a german keymap on a US keyboard. So here it is:

Copy this into your .Xmodmap:

! use altgr-# as pipe - if i have an US keyboard
! without key 86, i can use this as pipe.
keycode 51 = numbersign apostrophe bar bar bar bar

Then make sure that “Xmodmap ~/.Xmodmap” is run during login. I think this is done by /etc/X11/xinit/xinitrc.common, but don’t quote me on that.

So now you have the pipe on “AltGr - #” (On a german keyboard. With the US keycaps it is “AltGr - |”, which is not too inconvenient).
The “<" and ">” characters, which are also on this key (again, with a german layout), can be easily be typed by “AltGr - Shift - Y” for “<" and "AltGr - Shift - X" for ">“.

Maybe this is helpful to somebody besides me.

Have fun