On 13 January 2018, the RPC node at steemd.privex.io went down because the Steem daemon (steemd) exhausted the 256GB of RAM on that node. The Graphene in-memory database just kept getting bigger.
To get their RPC node back online, 
My team (
The fix worked, and despite running out of 256GB of RAM, the 
Unfortunately, the only way to prevent this ever-inflating memory usage from becoming unmaintainable would be to overhaul how Graphene accesses what it needs from the blockchain. There are already mitigations in place, like the LOW_MEMORY_NODE compile-time option or disabling unneeded plugins, but as the blockchain grows, so will memory usage.
I've got some good news, though:
We are now recommending the use of zram as a new best practice for all new and existing Steem nodes.
@gridcoin.science Uses zramOur witness
Above, you can see that the steemd memory-mapped file is 21GiB large, but zram has compressed some of it:
Thanks to zram, we're able to run a witness below the commonly accepted minimum RAM requirement.
When either the CPU struggles to keep up with zram swapping or when zram swap space runs low, we plan to fail over to the backup witness briefly, increase the RAM of the primary witness, catch up the blockchain, and resume operations from the primary witness.
Currently, there's barely any CPU load, so we expect that zram will last us a while.
Ubuntu makes it dead simple to set up zram.
Debian 7 only: You need to enable the backports repository in
/etc/apt/sources.list:deb http://ftp.debian.org/debian wheezy-backports main
All Debian releases: Manually download and install the
zram-configpackage version 0.5 from Ubuntu:sudo apt update wget 'http://archive.ubuntu.com/ubuntu/pool/universe/z/zram-config/zram-config_0.5_all.deb' sudo dpkg -i zram-config_0.5_all.deb sudo apt install -f rm -v zram-config_0.5_all.debThen go directly to step #3.
If you haven't already, enable the "universe" repository:
sudo add-apt-repository "deb http://archive.ubuntu.com/ubuntu $(lsb_release -sc) universe"
Install the zram-config package:
sudo apt update
sudo apt install zram-config
By default, zram-config sets up zram swap half the size of your RAM, but our testing revealed that the steemd in-memory database has a zram lzo compression ratio of greater than 2×, which means you can comfortably double the default zram swap size.
Set the calculated zram swap capacity to be equal to that of RAM:
Ubuntu 14.04 only:
sudo sed -i 's|/ 2 /|/ 1 /|g' /etc/init/zram-config.conf
zram-config 0.5 (all other releases as of September 2018):
sudo sed -i 's|/ 2 /|/ 1 /|g' /usr/bin/init-zram-swapping
Start up zram-config:
Ubuntu 16.04 and newer or Debian 8 and newer:
sudo systemctl restart zram-config
All releases:
sudo service zram-config restart
You should now see zram swap:
$ swapon --show
NAME TYPE SIZE USED PRIO
/dev/zram0 partition 2G 0B 5
/dev/zram1 partition 2G 0B 5
/dev/zram2 partition 2G 0B 5
/dev/zram3 partition 2G 0B 5
/dev/zram4 partition 2G 0B 5
/dev/zram5 partition 2G 0B 5
/dev/zram6 partition 2G 0B 5
/dev/zram7 partition 2G 0B 5
$ zramctl
NAME ALGORITHM DISKSIZE DATA COMPR TOTAL STREAMS MOUNTPOINT
/dev/zram0 lzo 2G 4K 81B 12K 1 [SWAP]
/dev/zram1 lzo 2G 4K 81B 12K 1 [SWAP]
/dev/zram2 lzo 2G 4K 81B 12K 1 [SWAP]
/dev/zram3 lzo 2G 4K 81B 12K 1 [SWAP]
/dev/zram4 lzo 2G 4K 81B 12K 1 [SWAP]
/dev/zram5 lzo 2G 4K 81B 12K 1 [SWAP]
/dev/zram6 lzo 2G 4K 81B 12K 1 [SWAP]
/dev/zram7 lzo 2G 4K 81B 12K 1 [SWAP]
Optional, but highly recommended:
If you do not already have regular disk swap (either a swap file or a swap partition), create one and set it to enable on boot:
This sets up a 4GiB swap file (bs=1M count=4096):
sudo dd if=/dev/zero of=/swapfile bs=1M count=4096
sudo mkswap /swapfile
echo "/swapfile swap swap defaults 0 0" | sudo tee -a /etc/fstab
Extra swap will keep steemd running longer, even if you run out of zram swap. You are at elevated risk of missing blocks when using disk swap because disk swap is much slower than zram swap.
If you are not already storing the steemd memory-mapped file in a tmpfs (ramdisk) mount:
In witness_node_data_dir/config.ini, set shared-file-dir to a tmpfs mount (/dev/shm by default):
shared-file-dir = /dev/shm
In the same config.ini, set shared-file-size to something sane. In January 2018, the default for witnesses is 54G (54GiB). Anything below 22G (22GiB) in January 2018 will fail for witnesses because the steemd in-memory database is about to reach that size.
We suggest that you use double your RAM size plus however much disk swap you have minus 1GiB for other things that may be running in RAM. If you have 16GiB of RAM and 4GiB of disk swap, set shared-file-size = 35G (16GiB × 2 + 4GiB - 1GiB = 35GiB).
Regardless of how big you set the file size, steemd will only use as much space as it needs.
Remount the /dev/shm tmpfs so that it can hold the entire shared_memory.bin.
If your shared-file-size = 35G, consider setting the tmpfs file size to 36352M ((35GiB + 0.5GiB buffer) * 1024 = 36352MiB):
mount -o remount,size=36352M /dev/shm
If you have the files shared_memory.bin and shared_memory.meta already, copy them over to /dev/shm so that you don't have to replay the blockchain.
Start the Steem daemon:
steemd if you copied the files in the previous stepsteemd --replay-blockchain if you need to replay the blockchainThese instructions should be pretty portable across Linux distros as long as you install the util-linux package because it contains /sbin/zramctl.
sudo apt install util-linuxsudo dnf install util-linuxsudo yum install util-linuxsudo pacman -S util-linuxsudo emerge util-linuxIf zram doesn't show up in lsmod | grep zram, run sudo modprobe zram.
If you get a message that starts with modprobe: FATAL: Module zram not found, then you'll need to boot up with a kernel that has zram (standard with Linux 3.14 and newer).
Run zramctl -f to confirm that /dev/zram0 is the first zram device available.
If it's not /dev/zram0, that means you already started up zram somewhere else. This guide recommends that zram be used exclusively for steemd's memory-mapped file and assumes that /dev/zram0 is the device you choose to use.
Determine how much space to allocate to the zram device. Just use however much RAM you have:
$ totalmem=$(LC_ALL=C free -b | grep -e "^Mem:" | sed -e 's/^Mem: *//' -e 's/ *.*//')
$ echo "$totalmem"
16742518784
Create a zram device with the size you determined in the previous step:
$ sudo zramctl -f -s "$totalmem"
/dev/zram0
Format the new zram device as swap:
$ sudo mkswap /dev/zram0
Setting up swapspace version 1, size = 15.6 GiB (16742514688 bytes)
no label, UUID=632f5bc3-d5cf-4983-a5ba-bcbcfe9dd238
Mount the new swap device:
$ sudo swapon /dev/zram0
Go to step #5 of the Ubuntu/Debian instructions above.
I want to contribute to alleviating the operational growing pains of the Steemit platform. Growing RAM usage, which increases costs of running Steem nodes, continues to be a nagging problem. Collectively, that's a lot of RAM. I hope this zram tutorial helps to squeeze out more value from the hardware available while being transparent to the software.
Perhaps if using zram becomes standard operating practice, we can have more reliable witnesses to support the long-term endurance of Steem (and by extension, Graphene).
Reducing RAM usage isn't all, though. The witness
To support this witness, visit https://steemit.com/~witnesses and add gridcoin.science to the box at the bottom of the page, click vote, and authorize using your Active Key.
We want to continue innovating and sharing our findings. Please let me or