# Indices in a File Image¶

Since our math indexer creates a large amount of directories and files on disk proportionally to the number of expressions you are going to index. If you are using a typical file system on Linux, e.g. EXT4, you are limited to create so many directories/files by the number of inodes on your file system.

To overcome this problem, as well as improve efficiency of math index, it is better to create a disk image as a loopback device to be partitioned by some file systems which do not put restriction on inodes. The file system should be efficient in crucial aspects of benchmarks that have great impact on search performance. Later, we can simply mount this disk image to be used as our index “disk”.

After some investigation, we choose to use ReiserFS as our default index file system due to its overall fast sequential read and random seek (see ref1 and ref2 ).

To create, mount and unmount a ReiserFS disk image, we provide a few simple scripts located under $PROJECT/indexer/scripts. Creating and mounting a disk image are simple as follows: $ cd $PROJECT/indexer$ ./scripts/vdisk-creat.sh reiserfs
$sudo ./scripts/vdisk-mount.sh reiserfs  A vdisk.img is created as our ReiserFS disk image, and is mounted to ./tmp so we can just use indexer and searcher on ./tmp like a normal directory. After you are done, unmount ./tmp: $ sudo ./scripts/vdisk-umount.sh


## 1. Lacking kernel support for ReiserFS support¶

If you are running on kernel without ReiserFS support, modify scripts argument above and change file system to btrfs for similar performance.

For server distributions support ReiserFS, Debian 8.5 is recommended one. On Debian, install reiserfsprogs for userland ReiserFS supports (e.g. mkfs).

$apt-get install reiserfsprogs  ## 2. dd command reports exhausted memory¶ When you experience dd command not being able to create certain size of image file: dd: memory exhausted by input buffer of size 1073741824 bytes (1.0 GiB)  Try either reduce the bs argument number of dd, or use a disk swap file: dd if=/dev/zero of=/swapspace bs=1M count=4000 mkswap /swapspace swapon /swapspace  ## 3. TRIM in SSD¶ If you are doing indexing on an SSD drive (which is recommended because it is often more than 4 times faster than hard disk in terms of random write performance), it is highly suggested to enable SSD TRIM whenever it is supported, due to SSD Write Amplification (WA) effect. Without TRIM, the intensive writing onto SSD drive can cause very slow indexing performance and reduce SSD life span. TRIM can be invoked either continuously by mounting your SSD drive with discard option sudo mount -o discard,noatime /dev/nvme0n1 ./mount‑point  where noatime option stops to record the timestamp of accessing files (and directories) to further reduce the number of writing operations performed on SSD. Or, by periodically run fstrim command: $ sudo fstrim -v ./nvme0n1/


alternatively, The util‑linux package provides fstrim.service and fstrim.timer systemd unit files. Enabling the timer will activate the service weekly:

$sudo systemctl enable fstrim.timer$ sudo systemctl start fstrim.timer
\$ journalctl ‑‑unit fstrim.timer # show logs