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From: Li Zefan <lizf <at> cn.fujitsu.com>
Subject: Re: Verifying TRIM support on SSDs when using BtrFS
Newsgroups: gmane.comp.file-systems.btrfs
Date: Friday 9th September 2011 05:04:36 UTC (over 7 years ago)
(resend with linux-btrfs Cced)

> Hi All,
> My company is considering using BtrFS for some of our production
> systems, but before we can use it, I must positively verify that the
> TRIM command is being issued to the SSDs. 營 attempted to verify TRIM
> first using examples that work for Ext4
> <http://andyduffell.com/techblog/?p=852>,
but I had to make changes to
> the procedure to work with BtrFS.
> The server is Ubuntu 11.04 server 64-bit release (mkfs.btrfs version
> 0.19). I have installed the Linux 3.0.0 kernel as the BtrFS changelog
> states that bulk TRIM is not available in the kernel shipped with
> Ubuntu 11.04 (2.6.38).
> My testing methodology:
> * Manually TRIM the disks before starting: for i in {0..10} ; do let
> A="$i * 65536" ; hdparm --trim-sector-ranges $A:65535
> --please-destroy-my-drive /dev/sda ; done
> * Verify the drive was TRIM'd: ./sectors.pl |grep + | tee sectors-$(date
> * Partition the drive: fdisk /dev/sda
> * Make the file system: mkfs.btrfs /dev/sda1
> * Mount: sudo mount -t btrfs -o ssd /dev/sda1 /mnt
> * Create a file: dd if=/dev/urandom of=/mnt/testfile bs=1k count=50000
> oflag=direct
> * Verify the file is on the disk: ./sectors.pl | tee sectors-$(date +%s)
> * Delete the test file: rm /mnt/testfile
> * See that the test file is TRIM'd from the disk: ./sectors.pl | tee
> sectors-$(date +%s)
> * Verify the TRIM'd blocks: diff the two most recent sectors-* files
> At this point, the pre-delete and post delete verifications still show
> the same disk blocks in use. I should instead see a reduction in the
> number of in use blocks. Waiting an hour (in case it takes a while for
> the TRIM command to be issued) after the test file is deleted still
> shows the same blocks in use.
> I have also tried mounting with the "-o ssd,discard" options, but that
> doesn't seem to help at all.
> I also posted this question at
> <http://serverfault.com/questions/307397/verify-trim-support-with-btrfs-on-ssd>
> but a good answer has yet to surface.  If you are interested, you can
> see my sectors.pl and a little more detail at that URL.
> Is my testing methodology flawed?  Am I missing something here?
> Thanks for your help.

Have you tried to use this methodology to test ext4? So you can verify if
it's flawed.

I used a much simpler way to test this, and I found btrfs works fine:

  # mount -t btrfs -o discard /dev/sdc5 /mnt
  # dd if=/dev/urandom of=/mnt/tmp bs=1M count=5
  # sync
  # hexdump /mnt/tmp
  0000000 064c ded5 c386 4721 060c 13e8 b090 b4a0
  # hexdump /dev/sdc5 | grep '064c ded5'
  0c00000 064c ded5 c386 4721 060c 13e8 b090 b4a0

Now verify the discard feature:

  # rm /mnt/tmp
  # sync
  # echo 3 > /proc/sys/vm/drop_caches
  # hexdump /dev/sdc5 | grep '064c ded5'
  (grep returns nothing, as expected)

So there's nothing wrong in btrfs.

Try the above test but without discard option, and you'll see the sectors
will not be zeroed.

Li Zefan

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