Monthly Archives: April 2019

Dropping caches in Linux, to check if memory is actually being used

I encountered that Server, Xeon, 128 GB of RAM, with those 58 Spinning drives 10 TB and 2 SSD of 2 TB each, where I was testing the latest version of my Software.

Monitoring long term tests, data validation, checking for memory leaks…
I notice the Server is using 70 GB of RAM. Only 5.5 GB are used for buffers according to the usual tools (top, htop, free, cat /proc/meminfo, ps aux…) and no programs are eating that amount, so where is the RAM?.
The rest of the Servers are working well, including models: same mode, 4U60 with 64 GB of RAM, 4U90 with 128 GB and All-Flash-Array with 256 GB of RAM, only using around 8 GB of RAM even under load.
iSCSI sharings being used, with I/O, iSCSI initiators trying to connect and getting rejected, several requests for second, disk pulling, and that usual stuff. And this is the only unit using so many memory, so what?.
I checked some modules to see memory consumption, but nothing clear.
Ok, after a bit of investigation one member of the Team said “Oh, while you was on holidays we created a Ramdisk and filled it for some validations, we deleted that already but never rebooted the Server”.
Ok. The easy solution would be to reboot, but that would had hidden a memory leak it that was the cause.
No, I had to find the real cause.

I requested assistance of one my colleagues, specialist, Kernel Engineer.
He confirmed that processes were not taking that memory, and ask me to try to drop the cache.

So I did:

sync
echo 3 > /proc/sys/vm/drop_caches

Then the memory usage drop to 11.4 GB and kept like that while I maintain sustained the load.

That’s more normal taking in count that we have 16 Volumes shared and one host is attempting to connect to Volumes that do not exist any more like crazy, Services and Cronjobs run in background and we conduct tests degrading the pool, removing drives, etc..

After tests concluded memory dropped to 2 GB, which is what we use when we’re not under load.

Note: In order to know about the memory being used by Kernel slab cache in real time you can use command:

 slabtop

You can also check:

sudo vmstat -m

Solving an infinite loop in CentOS after inducing a Kernel Panic in a Server

This trick may be useful for you.
Almost surely if you power cycle, completely powering down your Server you’ll fix booting too.
Unfortunately we do not always have access to the Data Center or Remote Hands service available, so this trick may be useful for you.

Just reset your BMC card with this:
ipmitool -H 172.30.30.7 -U admin -P thepassword bmc reset cold

After this use the remote control tool to request a reboot and it will do and power on normally.

This may not work in all the Servers, it depends on a lot of aspects (firmware, bmc manufacturer, etc…) but can do the trick for you maybe.

Create a small partition on the drives for tests

Ok, as you know I work with ZFS, DRAID, Erasure Coding… and Cold Storage.
I work with big disks, SAS, SSD, and NVMe.
Sometimes I need to conduct some tests that involve filling completely to 100% the pool.
That’s very slow having to fill 14TB drives, with Servers with 60, 90 and 104 drives, for obvious reasons. So here is a handy script for partitioning those drives with a small partition, then you use the small partition for creating a pool that will fill faster.

1. Get the list of drives in the system
For example this script can help

DRIVES=`ls -al /dev/disk/by-id/ | grep "sd" | grep -v "part" | grep "wwn" | tr "./" "  " | awk '{ print $11; }'`

If your drives had a previous partition this script will detect them, and will use only the drives with wwn identifier.
Warning: some M.2 booting drives have wwn where others don’t. Use with caution.

2. Identify the boot device and remove from the list
3. Do the loop with for DRIVE in $DRIVES or manually:

for DRIVE in sdar sdcd sdi sdj sdbp sdbd sdy sdab sdbo sdk sdz sdbb sdl sdcq sdbl sdbe sdan sdv sdp sdbf sdao sdm sdg sdbw sdaf sdac sdag sdco sds sdah sdbh sdby sdbn sdcl sdcf sdbz sdbi sdcr sdbj sdd sdcn sdr sdbk sdaq sde sdak sdbx sdbm sday sdbv sdbg sdcg sdce sdca sdax sdam sdaz sdci sdt sdcp sdav sdc sdae sdf sdw sdu sdal sdo sdx sdh sdcj sdch sdaw sdba sdap sdck sdn sdas sdai sdaa sdcs sdcm sdcb sdaj sdcc sdad sdbc sdb sdq
do
(echo g; echo n; echo; echo; echo 41984000; echo w;) | fdisk /dev/$DRIVE
done