Charlie Drage

Installing an SSD for HDD caching (SSD-Cache)

SSD-Cache. Whether you’re using a new laptop or accessing Facebook, in some way or another, SSD-cache is being used.

SSD-Cache is the power of writing and reading information from SSD before the slow-process of writing to HDD is done. Facebook introduced Flashcache 4 years ago by Mohan Srinivasan and Paul Saab. With the introduction of SSD’s to high-end data centers and the initial high cost, utilizing the storage capability of HDD with the speed of SSD made sense. As of 2015, even Facebook still uses SSD-Cached storage for most of their database servers.

Why do you need it? Simple. Be able to use the high read/write cycles of SSD’s with the storage capability of HDD’s.

Since then, Flashcache has been forked and developed into EnhanceIO, which is still in (semi) active development. Despite the issues with writeback mode (which we won’t use), it’s still as stable as can be, still being used at multiple companies.

Installing EnhanceIO is simple.


In our setup, we assume that we have three (1TB) HDD’s and one (256GB) SSD drive located in our (Debian) server rack. We have already setup RAID-5 over the three HDD’s and now we want to use SSD-Cache to speed up the read/write process.

# Get the initial files
apt-get install build-essential dkms
git clone

# Compile and install
cd EnhanceIO/Driver/enhanceio/
make && make install

# Copy over manuals
cd ~/EnhanceIO/CLI
cp eio_cli /sbin/
cp eio_cli.8 /usr/share/man/man8

# Initialize the modules 
modprobe enhanceio_lru
modprobe enhanceio_fifo
modprobe enhanceio

# To initial the modules on startup, simply add to /etc/modules before loop

# The best part. Create the drive. Assuming /dev/md0 is our raid partition and /dev/sdd is our SSD drive
eio_cli create -d /dev/md0 -s /dev/sdd -p fifo -m wt -c ssdcache


Let’s get into detail with our eio_cli command.

eio_cli create -d /dev/md0 -s /dev/sdd -p fifo -m wt -c ssdcache

-d /dev/md0 # Specifies the drive(s) we want to cache
-s /dev/sdd # Init's the SSD drive were using to cache.
-p fifo # Our cache replacement policy. Can be changed between random, fifo and lru.
-m wt # Our caching mode. Can be changed between wt (write-through), wb (write-back) and read

More details on the mode can be found here.

And there you have it! SSD-cache storage ahoy.