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SSD

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How to set up SSDs on Gentoo Linux. This article assumes you know how to set up, partition, and format a mechanical hard drive.

Considerations

rootfs

You don't want to use -odiscard on a rootfs mount. discard is the "TRIM" command that tells the SSD to do its magic. Having this running potentially constantly may cause performance degredation, and there are articles to suggest this all over the web. Rather, you want to use:

root # fstrim -v /

on a command line, as root. You can set this up as a cron (see below) to run twice a day, which should suffice for rootfs.

mountpoint

If you're going to have a low write directory mounted on SSD, using "discard" option will be fine in 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

STUB: No parted/gparted (see comments)

LVM

If you are using LVM, you need to allow discards on the logical volumes too.

File/etc/lvm/lvm.conf

devices {
  [...]
  issue_discards = 1
} 

LUKS

If you have your root-device on LUKS, you need to set a kernel option:

File/etc/default/grub

GRUB_CMDLINE_LINUX_DEFAULT="[...] root_trim=yes"

fdisk

  #assuming /dev/sda is the SSD
  #Per OCZ.
  fdisk -S 32 -H 32 /dev/sda

Set up the first partition to start on the second 512-cylinder unit:

  Device Boot         Start         End      Blocks   Id  System
  /dev/sda1               2         129       65536   83  Linux
  /dev/sda2             130    31457279    15662080   83  Linux

so long as the first start number is 2 the rest will fall in line if you use size parameters rather than sector paramters (e.g start <enter> end <+4GB>)

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/

Without knowing the erase block size

  #Formatting the rootfs partition /dev/sda2:
  #Using 4096 byte blocks aligns the SSD for writes:
  mkfs -t ext4 -b 4096 /dev/sda2

With knowing the erase block size

  #Formatting the rootfs partition /dev/sda2:
  #Using 4096 byte blocks aligns the SSD for writes;
  #Using ERASE_BLOCK_SIZE / 4 as the stride and stripe width size;
  #In this example, OCZ Vertex drives have 512 kibbibyte -
  #Erase Block size, therefore stride/stripe-width = 512/4 = 128:
  mkfs -t ext4 -b 4096 -E stride=128,stripe-width=128 /dev/sda2

Mounting

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

  /dev/sda2          /          ext4          defaults,relatime,discard          0 1

Or this:

  /dev/sda2          /          ext4          defaults,relatime                  0 1

If you choose the latter, see the section on the cronjob needed if 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:

  mount -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 your ssd supports trimming.
  • simply install it by running it once without any option and deinstall 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)

fstrimDemon

If your computer is not turned on when cron would trigger its job, fstrim would not be called. You can install fstrimDemon instead.

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's generally advised to not use more than 1/2 of your available system memory for tmpfs unless you know what you're doing. So let's assume you have 12GB of memory and want to allow up to 6 gigs of tmpfs. Edit /etc/fstab and add:

  tmpfs			/tmp		tmpfs		noatime,nodiratime,size=6G 	0 0

You can then tell portage to use this as it's compile scratch space in /etc/portage/make.conf:

  PORTAGE_TMPDIR="/tmp"
  1. this may fail on large compiles unless you have >20GB of system memory. scratch space for libreoffice can exceed 8GB, and the entire compile will fail if it runs out of space.
  2. Ram used by TMPFS is freed back to the pool when it is no longer in use, so when using portage or other software that uses /tmp you will notice a huge performance gain.

XDG cache

If you are running gentoo on desktop, many programs, using X windows systems (like: chromium, firefox, skype) are making frequent disk I/O every few seconds to cache. Default cache dir is ~/.cache, which is on hard drive.

Edit /etc/env.d/30xdg-data-local and add line:

  XDG_CACHE_HOME="/tmp/.cache"

Don't forget:

root # env-update