Improving Image Conversion Performance with /dev/shm tmpfs in Qunar's Group Buying Backend

Qunar's group‑buying backend originally used ImageMagick's convert and Google gm commands to compress and reformat uploaded images, generating up to 13 different sizes per image. When the number of concurrent editors grew, the IO load on the single physical server became a bottleneck, causing slowdowns and unresponsiveness.

The proposed solution focused on reducing IO by using a memory‑based filesystem (tmpfs) mounted at /dev/shm . Standard IO‑optimization methods such as faster disks (SCSI/SSD) or increasing filesystem cache were either impractical or already maximized, leaving the in‑memory filesystem as the viable option.

By creating a directory on /dev/shm and linking it to the backend's temporary image conversion path, all intermediate files are written to RAM instead of disk. This change lowered the server load from around 40 to about 1.x, without any code modifications or system reboots.

Typical commands used:

mkdir /dev/shm/tuanpic
ln -s /dev/shm/tuanpic /tuan/image/convert/temp/path

To control the size of the tmpfs mount, the /etc/fstab entry can be edited:

tmpfs    /your/mount/point    tmpfs    defaults,size=5%    0  0

or the filesystem can be mounted directly:

sudo mkdir /your/mount/point
sudo mount -t tmpfs none -o size=2G /your/mount/point

Because the tmpfs resides in RAM, it must be managed carefully: intermediate files are removed after being copied to their final disk location to avoid exhausting memory and causing OOM conditions.

The same technique was previously applied in 2011 to store PHP session files in /dev/shm , eliminating session blocking and outperforming alternatives like memcached.