[Bug 1213631] Re: Trim doesn't work with lvm and dm-crypt

Sun Sep 29 08:27:49 UTC 2013

It turns out to be a caching issue.

When a region of the SSD is trimmed, that data is lost. However for some
unfathomable reason, Linux keeps the data in its caches. As a result,
the 2nd dd call returns wrong data, making it seem as if the TRIM didn't
work regardless whether it really did or not.

I am not sure if this is a kernel bug or not. It should not have any ill
effects in practice (after all who usually tries to read from a freshly
TRIMmed region?) but even so it seems like a waste of cache memory if
it's occupied by invalidated data.

Anyway, drop caches before issuing the dd command.

    echo 1 | sudo tee /proc/sys/vm/drop_caches

    sudo dd bs=4096 skip=2224384 count=256 if=/dev/mapper/lubuntu--vg-
root | hexdump -C

Then see if it still returns y.y.y. or not. If it does not - trim has
been working as it is supposed to be.

Also the issue_discards in LVM is not strictly necessary, it only
affects lvremove, lvreduce, etc. Filesystem based TRIM seem to be passed
on by LVM unconditionally.

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Title:
  Trim doesn't work with lvm and dm-crypt

Status in cryptsetup:
  New
Status in lvm2 - Logical Volume Manager:
  New
Status in The Util-Linux-ng Set of Utilities:
  New
Status in “lvm2” package in Ubuntu:
  Confirmed

Bug description:
  I want to use fstrim together with lvm and dm-crypt but it doesn't
  work (see test procedure below). The problem seems to be somewhere in
  the lvm layer. Is there any fix or workaround for this?

  My configuration:

  `cat /etc/crypttab`

      sda3_crypt UUID=[...] none luks,discard

  and

  `cat /etc/lvm/lvm.conf`

      # [...]
      devices  {
            # [ ... ]
            issue_discards = 1
            # [ ... ]
         }
      # [...]

  The SSD is a Samsung 840 Pro.

  Here is the output of `sudo dmsetup table`

      lubuntu--vg-root: 0 465903616 linear 252:0 2048
      lubuntu--vg-swap_1: 0 33308672 linear 252:0 465905664
      sda3_crypt: 0 499222528 crypt aes-xts-plain64 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 0 8:3 4096 1 allow_discards

  Here is my `/etc/fstab`:

      # <file system> <mount point>   <type>  <options>       <dump>  <pass>
      /dev/mapper/lubuntu--vg-root /               ext4    errors=remount-ro 0       1
      # /boot was on /dev/sda2 during installation
      UUID=f700d855-96d0-495e-a480-81f52b965bda /boot           ext2    defaults        0       2
      # /boot/efi was on /dev/sda1 during installation
      UUID=2296-2E49  /boot/efi       vfat    defaults        0       1
      /dev/mapper/lubuntu--vg-swap_1 none            swap    sw              0       0
      # tmp
      tmpfs /tmp tmpfs nodev,nosuid,noexec,mode=1777          0       0 

  Testing procedure:

  yes | dd iflag=fullblock bs=1M count=1 of=trim.test

  filefrag -s -v trim.test

  Filesystem type is: ef53
  File size of trim.test is 1048576 (256 blocks, blocksize 4096)
   ext logical physical expected length flags
     0       0  2224384             256 eof
  trim.test: 1 extent found

  df trim.test

  Filesystem                   1K-blocks     Used Available Use% Mounted on
  /dev/mapper/lubuntu--vg-root 229164268 38651596 178865084  18% /

  sudo dd bs=4096 skip=2224384 count=256 if=/dev/mapper/lubuntu--vg-root | hexdump -C 

  00000000  79 0a 79 0a 79 0a 79 0a  79 0a 79 0a 79 0a 79 0a  |y.y.y.y.y.y.y.y.|
  *
  256+0 records in
  256+0 records out
  1048576 bytes (1,0 MB) copied, 0,00328209 s, 319 MB/s
  00100000

  rm trim.test

  sync

   sudo fstrim -v /                       
  /: 695181312 bytes were trimmed

  sync

  sudo dd bs=4096 skip=2224384 count=256 if=/dev/mapper/lubuntu--vg-root | hexdump -C
  00000000  79 0a 79 0a 79 0a 79 0a  79 0a 79 0a 79 0a 79 0a  |y.y.y.y.y.y.y.y.|
  *
  256+0 records in
  256+0 records out
  1048576 bytes (1,0 MB) copied, 0,00166157 s, 631 MB/s
  00100000

  I.e. the y pattern persists which indicates that TRIM doesn't work. See also my post on unix-stackexchange: http://unix.stackexchange.com/questions/85865/trim-with-lvm-and-dm-crypt

  ProblemType: Bug
  DistroRelease: Ubuntu 13.04
  Package: lvm2 2.02.95-6ubuntu4
  ProcVersionSignature: Ubuntu 3.8.0-27.40-generic 3.8.13.4
  Uname: Linux 3.8.0-27-generic x86_64
  ApportVersion: 2.9.2-0ubuntu8.3
  Architecture: amd64
  Date: Sun Aug 18 14:31:46 2013
  InstallationDate: Installed on 2013-08-03 (14 days ago)
  InstallationMedia: Lubuntu 13.04 "Raring Ringtail" - Release amd64 (20130423.1)
  MarkForUpload: True
  SourcePackage: lvm2
  UpgradeStatus: No upgrade log present (probably fresh install)

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