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Re: [linux-lvm] Testing TRIM with LVM



On Apr 13, 2011 3:41am, Zdenek Kabelac <zdenek kabelac gmail com> wrote:
> Dne 13.4.2011 03:47, DarkNovaNick gmail com napsal(a):
>
> > On Apr 12, 2011 6:47pm, Mike Snitzer snitzer redhat com> wrote:
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> >> On Tue, Apr 12 2011 at 10:59am -0400,
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> >
>
> >> DarkNovaNick gmail com DarkNovaNick gmail com> wrote:
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> >
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> >
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> >
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> >> > I recently purchased a Crucial C300 SSD and set it up as my primary
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> >
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> >> > drive. I wanted to use LVM, and my reading indicates that LVM added
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> >
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> >> > support to pass-through TRIM commands with kernel 2.6.37. I'm
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> >
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> >> > running Ubuntu 10.10, but I installed the latest 11.04 kernel, so
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> >
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> >> > I'm running kernel 2.6.38-8. I added "discard" to fstab so my
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> >
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> >> > mountpoint looks like:
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> >
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> >> >
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> >
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> >> > /dev/mapper/vg0-vol0 on / type ext4
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> >
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> >> > (rw,noatime,nodiratime,errors=remount-ro,discard,commit=0)
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> >> If the discards aren't working then ext4 will print a warning and stop
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> >
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> >> issuing them, see: http://git.kernel.org/linus/a30eec2a8650a77f7
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> >
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> >
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> >
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> >> So if you don't see "discard not supported, disabling" in your dmesg (or
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> >> /var/log/messages) then TRIM is likely working as expected.
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> >> > I found directions on various sites like:
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> >> >
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> >> http://duopetalflower.blogspot.com/2010/11/enterprise-kernel-6-has-ssd-trim.html
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> >>
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> >
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> >> > on how to confirm if TRIM is working. I ran (as root):
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> >
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> >> >
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> >> > dd if=/dev/urandom of=tempfile count=100 bs=512k oflag=direct
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> >
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> >> > hdparm --fibmap tempfile
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> >
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> >> > (then took the first begin_LBA)
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> >
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> >> > hdparm --read-sector 191710208 /dev/sda
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> >
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> >> >
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> >
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> >> > and it printed:
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> >> >
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> >> > /dev/sda:
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> >> > reading sector 191710208: succeeded
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> >
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> >> > 3254 3a32 3834 313a 2b36 3030 303a 2030
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> >
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> >> > 4544 5542 2047 3728 3a29 5320 7661 6e69
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> >
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> >> > 2067 6e69 6564 2078 6f66 2072 7270 646f
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> >
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> >> > ............
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> >
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> >> >
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> >> > Then I ran:
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> >> > rm tempfile
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> >> > sync
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> >> > hdparm --read-sector 191710208 /dev/sda
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> >
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> >> >
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> >
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> >> > and it still returned:
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> >> > /dev/sda:
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> >
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> >> > reading sector 191710208: succeeded
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> >
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> >> > 3254 3a32 3834 313a 2b36 3030 303a 2030
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> >
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> >> > 4544 5542 2047 3728 3a29 5320 7661 6e69
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> >
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> >> > 2067 6e69 6564 2078 6f66 2072 7270 646f
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> >
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> >> > .............
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> >> >
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> >> > If TRIM is working, the sector is supposed to contain all zeros.
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> >
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> >> That is only if the SSD implements TRIM so that it zeroes the discarded
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> >
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> >> blocks. You can check if the device at least knows enough to report
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> >
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> >> discard_zeroes_data:
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> >> cat /sys/block/sda/queue/discard_zeroes_data
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> >> Also, even if the SSD does zero, it may take some time -- well after the
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> >
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> >> discard has completed -- for the SSD to actually zero the blocks (this
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> >
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> >> is true of some lesser quality SSDs).
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> >> > Am I doing something wrong or do I need to do something more to get
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> >> > LVM to pass down the TRIM command?
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> >> LVM (device mapper specifically) passes discards perfectly well. But
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> >
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> >> the snapshot and dm-crypt targets don't have discard support. All other
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> >
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> >> targets do have discard support. What is your output for: dmsetup table
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> >> Mike
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> > I get 0 back when I do "cat /sys/block/sda/queue/discard_zeroes_data" so does
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> > this mean that my drive doesn't zero out the TRIMed blocks? I don't really
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> > care if it does or does not, I just want to make sure the drive is getting the
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> > TRIM commands as the benchmarks I've seen show performance degradation if they
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> > aren't used.
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>
>
> As I've the same drive - what kind of performance degradation do you see?
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>
>
> Note: this drive (unlike many others) is processing TRIM command on
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> background. So 'traditional' benchmark do not provide valuable results.
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> (As we've been evaluating this drive somewhat to compare with Intel).
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>
>
> Outcome is - when you issue TRIM command - it's immediately finished - but the
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> performance of drive is temporarily decreased while the drive is performing
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> block trimming for some time.  Thus you might get 'impressive' results when
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> some benchmarks are evaluating TRIM performance ;)
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>
>
> Zdenek
>
>

I may not have been clear, but the benchmarks I am referring to show the performance degradation if you are *not* using TRIM (i.e. just relying on the drive's internal garbage collector). Which is why I want to make sure the TRIM is working so it doesn't get into this poor state.

Nick
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