SSD

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This article describes how to set up SSDs (Solid State Drives) on Gentoo Linux. It presumes the user has obtained knowledge of setting up, partitioning, and formatting mechanical hard drives.

Considerations

Rootfs

The -odiscard option on a rootfs mount should not be used. discard is the "TRIM" command that tells the SSD to do its magic. Having discard running constantly could potentially cause performance degradation on older SSDs. Modern SSDs use discard by default. Rather the following command can be used manually or be setup as a cron job (see below) to run twice a day, which should suffice for the rootfs:

root #fstrim -v /

Mount point

If you're going to have a low write directory mounted on SSD, using the "discard" option will be fine in /etc/fstab. See below. If you're going to mount a database to an SSD, you probably want timed TRIM commands rather than the discard option.

After hardware installation

Partitioning

The fdisk utility (sys-apps/util-linux, version 2.24.1-r2 and up) can be used for GPT disk partitioning.

Use type fdisk /dev/sda to run fdisk, use M option for help. The following is an example for GPT partition table with BIOS motherboard. Type G+Enter to create a new empty GPT partition table on the SSD. Type N+Enter, Enter, Enter, +2M+Enter, T+Enter, 4+Enter to create BIOS boot partition.

root #fdisk /dev/sda
   Command (m for help): n
   Partition number (1-128, default 1): 
   First sector (2048-117231374, default 2048): 
   Last sector, +sectors or +size{K,M,G,T,P} (2048-117231374, default 117231374): +2M
   
   Created a new partition 1 of type 'Linux filesystem' and of size 2 MiB.
   
   Command (m for help): t
   Selected partition 1
   Partition type (type L to list all types): 4
   Changed type of partition 'Linux filesystem' to 'BIOS boot partition'.

Next create a 2GB swap partition:

   Command (m for help): n
   Partition number (2-128, default 2): 
   First sector (6144-117231374, default 6144): 
   Last sector, +sectors or +size{K,M,G,T,P} (6144-117231374, default 117231374): +2G
   
   Created a new partition 2 of type 'Linux filesystem' and of size 2 GiB.
   
   Command (m for help): t
   Partition number (1,2, default 2):
   Partition type (type L to list all types): 15
   
   Changed type of partition 'Linux filesystem' to 'Linux swap'.

Finally create system root partition and write the changes:

   Command (m for help): n
   Partition number (3-128, default 3): 
   First sector (4200448-117231374, default 4200448): 
   Last sector, +sectors or +size{K,M,G,T,P} (4200448-117231374, default 117231374): 
   
   Created a new partition 3 of type 'Linux filesystem' and of size 53.9 GiB.
   
   Command (m for help): p
   Disk /dev/sda: 55.9 GiB, 60022480896 bytes, 117231408 sectors
   Units: sectors of 1 * 512 = 512 bytes
   Sector size (logical/physical): 512 bytes / 512 bytes
   I/O size (minimum/optimal): 512 bytes / 512 bytes
   Disklabel type: gpt
   Disk identifier: E19B261A-21EA-4529-B736-1A4E48FC77A1
   
   Device           Start          End   Size Type
   /dev/sda1         2048         6143     2M BIOS boot partition
   /dev/sda2         6144      4200447     2G Linux swap
   /dev/sda3      4200448    117231374  53.9G Linux filesystem
   
   Command (m for help): w
   
   The partition table has been altered.
   Calling ioctl() to re-read partition table.
   Syncing disks.

Although fdisk is a tool familiar to many Linux users, other common partitioning tools exist: gdisk, gparted, and parted. All of them (newer versions) handle partition alignment automatically. When in doubt whether partition is aligned, check that each partition starts on a MiB boundary (multiple of 2048 sectors). This should be 'safe' for any device (SSD or not).

LVM

LVM aligns to MiB boundaries and allows discards by default. No special configuration is required.

In order to TRIM all unused space in the VG use the blkdiscard utility:

root #lvcreate -l100%FREE -n trim yourvg
root #blkdiscard /dev/yourvg/trim
root #lvremove yourvg/trim

Alternatively, there is a discard option in lvm.conf which makes LVM discard entire LV on lvremove, lvreduce, pvmove and other actions that free physical extents (PE) in a VG. However, enabling it will immediately render the system unable to undo any changes to the LV layout.

FILE /etc/lvm/lvm.conf
devices {
  [...]
  issue_discards = 1
}

LUKS

For TRIM to work on encrypted LUKS devices, they have to be opened with the --allow-discards option.

root #cryptsetup luksOpen --allow-discards /dev/thing luks

When root-device exists on LUKS, enabling TRIM depends on the Initramfs implementation. When using genkernel for creating your initramfs, pass the following kernel option:

FILE /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="[...] root_trim=yes"

When using dracut for creating your initramfs, pass the following kernel option:

FILE /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="[...] rd.luks.allow-discards"

Formatting

If you can find your erase block size, you can add some extended attributes that may help performance. For software raid, you really should know the erase block size. Consider this information when making your purchase. Regardless of whether you're using RAID or not, setting extended values is known to be beneficial. https://raid.wiki.kernel.org/index.php/RAID_setup#ext2.2C_ext3.2C_and_ext4

http://blog.nuclex-games.com/2009/12/aligning-an-ssd-on-linux/

The modern FAQ found on the Arch wiki too:

https://wiki.archlinux.org/index.php/SSD

Without knowing the erase block size

  1. Formatting the rootfs partition /dev/sda3:
  2. Using 4096 byte blocks by default aligns the SSD for writes (see [defaults] section of /etc/mke2fs.conf and also
  3. man mkfs.ext4 about "-b block-size" and "-T usage-type[,...]" inside it):
root #mkfs.ext4 /dev/sda3

With knowing the erase block size | this info can be outdated

  1. Formatting the rootfs partition /dev/sda3:
  2. Using 4096 byte blocks aligns the SSD for writes;
  3. Using ERASE_BLOCK_SIZE / 4 as the stride and stripe width size;
  4. In this example, OCZ Vertex drives have 512 kibbibyte -
  5. Erase Block size, therefore stride/stripe-width = 512/4 = 128:
root #mkfs.ext4 -E stride=128,stripe-width=128 /dev/sda3

blkdiscard

If you are formatting a previously used device and your preferred mkfs does not support bulk discards when creating the filesystem, you can use blkdiscard (from sys-apps/util-linux-2.23 or later) before creating the filesystem.

Mounting

Given the considerations above, either add something similar to this:

FILE /etc/fstab
/dev/sda3          /          ext4          defaults,relatime,discard          0 1

Or:

FILE /etc/fstab
/dev/sda3          /          ext4          defaults,relatime                  0 1

If you choose the latter, see the cron section below you don't want to manually TRIM the drive.

Once you've set up /etc/fstab you can run the following command to have the drive mounted:

root #mount -a

In the same way you can add discard option to the swap line in fstab too. The following example shows you how SSD disk swap can be parallelized with SATA disk swap:

FILE /etc/fstab
/dev/sda2          none           swap         sw,pri=3,discard          0 0
/dev/sdb2          none           swap         sw,pri=3                  0 0

Once you've set up swap in fstab you can run the following command to make swap available:

root #swapon -a

Notes

cron

Run fstrim -v / from cron twice a day to automatically do "discard":

  #Mins  Hours  Days   Months  Day of the week   command
  15     1,13   *      *       *                 /sbin/fstrim -v /

There is also a semi-automatic cronjob available on GitHub called SSDcronTRIM which has the following features:

  • Distribution Independent script (developed on my Gentoo system).
  • The script decides every time depending on the disk usage how often (monthly, weekly, daily, hourly) each partition has to be trimmed.
  • Recognizes if it should install itself into /etc/cron.{monthly,weekly,daily,hourly}, /etc/cron.d or any other defined directory and if it should make an entry into crontab.
  • Checks if the kernel meets the requirements, the filesystem is able to and if the SSD supports trimming.
  • Simply install it by running it once without any option and uninstall it with the -d option

Future version should implement:

  • Use of nice and ionice to let trimm only run when the disc is not busy.
  • Overgive options to fstrim (e.g. '-m 1M')
  • support for encrypted filesystem (LUKS)

fstrimDaemon

If your computer turned off when cron scheduled its job, fstrim would not be called at all. You can install fstrimDaemon to solve this problem.

tmpfs

When using an SSD, administrators generally want to reduce the amount of writes performed. On Gentoo, Portage is a huge source of writes, and putting these writes into system memory rather than a disk is beneficial. If you have more than 8GB of ram you may want to consider using tmpfs, which is a way to dedicate some system memory to storage on a temporary basis. It is generally advised to not use more than half of the available system memory for tmpfs.

For the next example, presume the system has 12GB of memory and the user would like to allow up to 6GBs for tmpfs. Edit /etc/fstab and add:

FILE /etc/fstab
tmpfs			/tmp		tmpfs		noatime,nodiratime,size=6G 	0 0

After the fstab entry is set, tell Portage to use the above location to compile temporary space. This is done in /etc/portage/make.conf:

# This may fail on large compiles unless you have greater than 20GB of system memory is available. Scratch space for Libreoffice can exceed 8GB; compile will fail if it runs out of space.
# RAM used by [[Portage_TMPDIR_on_tmpfs|tmpfs]] is freed back to the pool when it is no longer in use, so when using Portage or other software that uses /tmp there should be a huge performance gain.
PORTAGE_TMPDIR="/tmp"

XDG cache

When running Gentoo on desktop, many programs, using X windows systems (Chromium, Firefox, Skype, etc.) are making frequent disk I/O every few seconds to cache. Default cache directory is ~/.cache, which is on hard drive and should be moved to tmpfs.

Create the file /etc/profile.d/xdg_cache_home.sh and add the following lines to it:

FILE /etc/profile.d/xdg_cache_home.sh
#!/bin/bash
 
export XDG_CACHE_HOME="/tmp/${USER}/.cache"

Do not forget to run env-update && source /etc/profile:

root #env-update && source /etc/profile
Note
The above command only updates the variables in the current terminal, new consoles, and their children. Thus, if the user is working in X11, he needs to either type
user $source /etc/profile
in every new terminal opened or restart X so that all new terminals source the new variables. If a login manager is used, it is necessary to become root and restart the /etc/init.d/xdm service.
Warning

With above change, auto unlocking the gnome keyring didn't work anymore. /var/log/messages said the following:

  May  5 10:02:52 localhost gnome-keyring-daemon[5561]: couldn't bind to control socket: /home/david/.cache/keyring-hRt5QC/control: No such file or directory