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Dropping cached memory in Linux

somedude — Fri, 30 Mar 2012 13:10:24 +0000

This is kind of a followup to the previous post about Linux seemingly being out of memory.

There is kernel parameter that can be altered on the run to free up memory used up by pagecache, dentries and inodes. In the following outputs we are interested in buffers/cache values. Note that sync command should be run first to flush out all cached objects.

Following is output of free command before anything is done:

root@ultra:~# sync;free total used free shared buffers cached Mem: 16446564 9924728 6521836 0 337568 8135860 -/+ buffers/cache: 1451300 14995264 Swap: 1998840 0 1998840

Now by echoing 1 into /proc/sys/vm/drop_caches memory used by pagecache is freed:

root@ultra:~# echo 1 > /proc/sys/vm/drop_caches root@ultra:~# free total used free shared buffers cached Mem: 16446564 1557852 14888712 0 504 150524 -/+ buffers/cache: 1406824 15039740 Swap: 1998840 0 1998840

By echoing 2 into the same file you can free up memory used by dentries and inodes:

root@ultra:~# echo 2 > /proc/sys/vm/drop_caches root@ultra:~# free total used free shared buffers cached Mem: 16446564 1427692 15018872 0 1404 151332 -/+ buffers/cache: 1274956 15171608 Swap: 1998840 0 1998840

The above two steps can be combined into one:

root@ultra:~# echo 3 > /proc/sys/vm/drop_caches

Alternatively sysctl -w vm.drop_caches=3 will achieve the same. Most of the information is described here, but I just do not feel like looking for it every time I need it.

Is Linux really out of memory?

somedude — Tue, 28 Feb 2012 11:56:23 +0000

If you have ever looked at RAM usage on a Linux server you might have noticed that the server might be running out of memory. This is misleading. Let’s take the following example:

[somedude@ultra ~]$ free total used free shared buffers cached Mem: 16432988 16378844 54144 0 647204 14692400 -/+ buffers/cache: 1039240 15393748 Swap: 16383992 4501164 11882828

At glance it looks like you have 54144 bytes of memory free. This is not true. This number specifies that 54144 bytes are not used for anything. To get amount of memory that is free for use by programs, you need to look at the number below 54144. In this case 15393748 bytes is available memory for use by programs.

In Linux, unused memory is used for disk caching to speed things up. In the above example, when system attempts to load program into memory, chunk of disk cache is freed up so program can load.

So, in the above, the system has in reality 93% of memory available for programs.

Sometimes this might be a little confusing for newcomers.

Flashing firmware on SunFire T2000

somedude — Fri, 20 Jan 2012 15:14:25 +0000

Not so long ago I had to flash firmware on SunFire T2000. It’s not a complicated thing.

I need to do something with Service Controller and in the process I found out that there is no scadm utility on T2000. Why? I am not completely sure. Maybe someone can chime in. It’s probably because T2000 runs ALOM CMT which is not the same as ALOM.

Anyway, there are two methods of doing all this. You can use SC’s flashupdate command to grab firmware off an FTP server. Unfortunately I did not have Net management port hooked up, only serial. Long story…

I had to transfer the firmware archive to the server and then, using included utility upload it to System Controller and perform firmware flashing.

Before starting note your current firmware version by logging into System Controller:

sc> showhost Sun-Fire-T2000 System Firmware 6.3.9 2007/08/16 23:53 Host flash versions: Hypervisor 1.3.4 2007/03/28 06:03 OBP 4.25.8 2007/08/16 10:59 POST 4.25.8 2007/08/16 11:26

Start by transferring the firmware archive to the server and unzipping it:

bash-3.00# ls 139434-09 bash-3.00# cd 139434-09 bash-3.00# ls 139434-09.html Install.info LEGAL_LICENSE.TXT Legal README.139434-09 Sun_Fire_T2000_metadata.xml Sun_System_Firmware-6_7_12-SPARC_Enterprise_T2000.bin Sun_System_Firmware-6_7_12-Sun_Fire_T2000.bin copyright sysfwdownload sysfwdownload.README

Using included sysfwdownload utility upload the firmware to System Controller. This takes roughly 15-20 minutes.

bash-3.00# ./sysfwdownload Sun_System_Firmware-6_7_12-Sun_Fire_T2000.bin .......... (9%).......... (18%).......... (27%).......... (37%).......... (46%).......... (55%).......... (64%).......... (74%).......... (83%).......... (92%)......... (100%) Download completed successfully. bash-3.00#

At this point firmware is on System Controller. Now you need to shut down the system.

bash-3.00# shutdown -g0 -y -i0

Power down the system from SC console and proceed with firmware flashing.

{8} ok sc> poweroff Are you sure you want to power off the system [y/n]? y sc> SC Alert: SC Request to Power Off Host. SC Alert: Host system has shut down.

Next, make sure keyswitch is set to NORMAL. If it is set to LOCKED you will not be able to flash the firmware or send STOP-A to the system. If keyswitch is set to NORMAL start actual flashing process using flashupdate command:

sc> showkeyswitch Keyswitch is in the NORMAL position. sc> flashupdate -s 127.0.0.1 SC Alert: System poweron is disabled. ............................................................. ............................................................. ............................................................ Update complete. Reset device to use new software. SC Alert: SC firmware was reloaded

At this point firmware is flashed. You still have to reset SC so it loads the new firmware:

sc> resetsc Are you sure you want to reset the SC [y/n]? y User Requested SC Shutdown ALOM BOOTMON v1.7.11 ALOM Build Release: 001 Reset register: 00000000 ALOM POST 1.0 Dual Port Memory Test, PASSED. TTY External - Internal Loopback Test TTY External - Internal Loopback Test, PASSED. TTYC - Internal Loopback Test TTYC - Internal Loopback Test, PASSED. TTYD - Internal Loopback Test TTYD - Internal Loopback Test, PASSED. Memory Data Lines Test Memory Data Lines Test, PASSED. Memory Address Lines Test Slide address bits to test open address lines Test for shorted address lines Memory Address Lines Test, PASSED. Boot Sector FLASH CRC Test Boot Sector FLASH CRC Test, PASSED. Return to Boot Monitor for Handshake ALOM POST 1.0 Status = 00007fff Returned from Boot Monitor and Handshake Loading the runtime image... VxWorks running. Starting Advanced Lights Out Manager CMT v1.7.11 Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved. Current mode: NORMAL Attaching network interface lo0... done. Attaching network interface motfec0.... done. Booting from Segment 0 Oracle Advanced Lights Out Manager CMT v1.7.11 SC Alert: SC System booted. Full VxDiag Tests BASIC TOD TEST Read the TOD Clock: TUE NOV 22 22:04:15 2011 Wait, 1 - 3 seconds Read the TOD Clock: TUE NOV 22 22:04:17 2011 BASIC TOD TEST, PASSED ETHERNET CPU LOOPBACK TEST 50 BYTE PACKET - a 0 in field of 1's. 50 BYTE PACKET - a 1 in field of 0's. 900 BYTE PACKET - pseudo-random data. ETHERNET CPU LOOPBACK TEST, PASSED Full VxDiag Tests - PASSED Status summary - Status = 7FFF VxDiag - - PASSED POST - - PASSED LOOPBACK - - PASSED I2C - - PASSED EPROM - - PASSED FRU PROM - - PASSED ETHERNET - - PASSED MAIN CRC - - PASSED BOOT CRC - - PASSED TTYD - - PASSED TTYC - - PASSED MEMORY - - PASSED MPC885 - - PASSED Please login:

After logging in make sure SC is running new version of firmware:

sc> showhost Sun-Fire-T2000 System Firmware 6.7.12 2011/07/06 20:03 Host flash versions: OBP 4.30.4.d 2011/07/06 14:29 Hypervisor 1.7.3.c 2010/07/09 15:14 POST 4.30.4.b 2010/07/09 14:24

That’s it, now you can poweron the system.

Unable to access console login prompt from ALOM

somedude — Tue, 13 Dec 2011 09:19:50 +0000

Not so long ago, I logged into System Controller of SunFire T2000 running Solaris 10 and tried to access server’s console. For some reason, usual console command did not work. It simply did not return anything.

I figured maybe something was stuck, so I reset System Controller and tried again. Again, I got nothing, which was strange, because usually couple of ‘Enters’ bring up console login prompt.

This had worked before so I knew there was not some configuration issue elsewhere. I vaguely remembered that I had a similar issue in Solaris 9. If you look at /etc/inittab on Solaris 9 and prior you will see something like this:

co:234:respawn:/usr/lib/saf/ttymon -g -h -p "`uname -n` console login: " -T sun -d /dev/console -l console -m ldterm,ttcompat

Killing ttymon process would cause init respawn it again in specified run-levels. This fixed my problem then. On Solaris 10 it’s different. Console login prompt is handled by SMF:

bash-3.00# svcs -a | grep console-login online 14:41:04 svc:/system/console-login:default

So, restarting svc:/system/console-login:default should fix the issue:

bash-3.00# svcadm restart svc:/system/console-login:default

Now you should be able to get console login prompt from ALOM.

ZFS and swapping

somedude — Tue, 15 Nov 2011 10:19:27 +0000

ZFS is great. Obviously. Generally when I do a server I install system on UFS and then mirror disks with SVM. I keep data and apps on ZFS. That’s nice, especially if ZFS is on a SAN. If something happens with the server I can move ZFS pools to a different server, if necessary.

The server build process is just a preference. You can easily install Solaris 10 on ZFS and be done with it.

Not so long ago I ran into situation, where I needed to add some swap. For various reasons I did not want to shuffle slicing on system disks while system is running. Nor could I add a swap file on one of UFS filesystems. So The last option I had was to use one of the zpools:

bash-3.00# zpool status pool: dudespool state: ONLINE scrub: none requested config: NAME STATE READ WRITE CKSUM dudespool ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 c2t1d0 ONLINE 0 0 0 c2t2d0 ONLINE 0 0 0 errors: No known data errors

I was going to create a file to use it as swap area:

bash-3.00# cd /dudespool bash-3.00# mkfile 50M swap bash-3.00# swap -a /dudespool/swap "/dudespool/swap" may contain holes - can't swap on it.

So that was that. As it turns out, ZFS can not be used for swap files. You have to create zvol and swap on that.

bash-3.00# zfs create -V 50M dudespool/swapvol bash-3.00# swap -a /dev/zvol/dsk/dudespool/swapvol bash-3.00# swap -l swapfile dev swaplo blocks free /dev/md/dsk/d10 85,10 8 1220928 1220928 /dev/zvol/dsk/dudespool/swapvol 181,1 8 102392 102392

There is more info on the net, just google it…