[linux-lvm] Testing TRIM with LVM

[DarkNovaNick at gmail.com]

On Apr 13, 2011 3:41am, Zdenek Kabelac <zdenek.kabelac at gmail.com> wrote:
> Dne 13.4.2011 03:47, DarkNovaNick at gmail.com napsal(a):

> > On Apr 12, 2011 6:47pm, Mike Snitzer snitzer at redhat.com> wrote:

> >> On Tue, Apr 12 2011 at 10:59am -0400,

> >

> >> DarkNovaNick at gmail.com DarkNovaNick at gmail.com> wrote:

> >

> >

> >

> >> > I recently purchased a Crucial C300 SSD and set it up as my primary

> >

> >> > drive. I wanted to use LVM, and my reading indicates that LVM added

> >

> >> > support to pass-through TRIM commands with kernel 2.6.37. I'm

> >

> >> > running Ubuntu 10.10, but I installed the latest 11.04 kernel, so

> >

> >> > I'm running kernel 2.6.38-8. I added "discard" to fstab so my

> >

> >> > mountpoint looks like:

> >

> >> >

> >

> >> > /dev/mapper/vg0-vol0 on / type ext4

> >

> >> > (rw,noatime,nodiratime,errors=remount-ro,discard,commit=0)

> >

> >

> >

> >> If the discards aren't working then ext4 will print a warning and stop

> >

> >> issuing them, see: http://git.kernel.org/linus/a30eec2a8650a77f7

> >

> >

> >

> >> So if you don't see "discard not supported, disabling" in your dmesg  
> (or

> >

> >> /var/log/messages) then TRIM is likely working as expected.

> >

> >

> >

> >> > I found directions on various sites like:

> >

> >> >

> >>  
> http://duopetalflower.blogspot.com/2010/11/enterprise-kernel-6-has-ssd-trim.html

> >>

> >

> >> > on how to confirm if TRIM is working. I ran (as root):

> >

> >> >

> >

> >> > dd if=/dev/urandom of=tempfile count=100 bs=512k oflag=direct

> >

> >> > hdparm --fibmap tempfile

> >

> >> > (then took the first begin_LBA)

> >

> >> > hdparm --read-sector 191710208 /dev/sda

> >

> >> >

> >

> >> > and it printed:

> >

> >> >

> >

> >> > /dev/sda:

> >

> >> > reading sector 191710208: succeeded

> >

> >> > 3254 3a32 3834 313a 2b36 3030 303a 2030

> >

> >> > 4544 5542 2047 3728 3a29 5320 7661 6e69

> >

> >> > 2067 6e69 6564 2078 6f66 2072 7270 646f

> >

> >> > ............

> >

> >> >

> >

> >> > Then I ran:

> >

> >> > rm tempfile

> >

> >> > sync

> >

> >> > hdparm --read-sector 191710208 /dev/sda

> >

> >> >

> >

> >> > and it still returned:

> >

> >> > /dev/sda:

> >

> >> > reading sector 191710208: succeeded

> >

> >> > 3254 3a32 3834 313a 2b36 3030 303a 2030

> >

> >> > 4544 5542 2047 3728 3a29 5320 7661 6e69

> >

> >> > 2067 6e69 6564 2078 6f66 2072 7270 646f

> >

> >> > .............

> >

> >> >

> >

> >> > If TRIM is working, the sector is supposed to contain all zeros.

> >

> >

> >

> >> That is only if the SSD implements TRIM so that it zeroes the discarded

> >

> >> blocks. You can check if the device at least knows enough to report

> >

> >> discard_zeroes_data:

> >

> >

> >

> >> cat /sys/block/sda/queue/discard_zeroes_data

> >

> >

> >

> >> Also, even if the SSD does zero, it may take some time -- well after  
> the

> >

> >> discard has completed -- for the SSD to actually zero the blocks (this

> >

> >> is true of some lesser quality SSDs).

> >

> >

> >

> >> > Am I doing something wrong or do I need to do something more to get

> >

> >> > LVM to pass down the TRIM command?

> >

> >

> >

> >> LVM (device mapper specifically) passes discards perfectly well. But

> >

> >> the snapshot and dm-crypt targets don't have discard support. All other

> >

> >> targets do have discard support. What is your output for: dmsetup table

> >

> >

> >

> >> Mike

> >

> >

> >

> >

> > I get 0 back when I do "cat /sys/block/sda/queue/discard_zeroes_data"  
> so does

> > this mean that my drive doesn't zero out the TRIMed blocks? I don't  
> really

> > care if it does or does not, I just want to make sure the drive is  
> getting the

> > TRIM commands as the benchmarks I've seen show performance degradation  
> if they

> > aren't used.



> As I've the same drive - what kind of performance degradation do you see?



> Note: this drive (unlike many others) is processing TRIM command on

> background. So 'traditional' benchmark do not provide valuable results.

> (As we've been evaluating this drive somewhat to compare with Intel).



> Outcome is - when you issue TRIM command - it's immediately finished -  
> but the

> performance of drive is temporarily decreased while the drive is  
> performing

> block trimming for some time. Thus you might get 'impressive' results when

> some benchmarks are evaluating TRIM performance ;)



> Zdenek



I may not have been clear, but the benchmarks I am referring to show the  
performance degradation if you are *not* using TRIM (ie 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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