[kwlug-disc] SSDs and TRIM

Chris Irwin chris at chrisirwin.ca
Thu Apr 29 13:04:57 EDT 2010


On Thu, Apr 29, 2010 at 12:49, John Van Ostrand <john at netdirect.ca> wrote:
> Correct me if I'm ignorant, but won't putting an LVM on a dm device do what you want? You can use dm for TRIM and have the benefits of LVM.

I can neither confirm nor deny what you've said. I can only relay what
I've read so far:

LVM uses DM itself internally, no special steps required. DM has no
support for TRIM.

Somewhat annoying. The benefits of LVM are great. But the potential
pitfall of having degraded performance over time is not quite so
great.

Apparently some folks said the third-party utilities (at least one
called "wiper") should work with "single-device linear LVM". I read
that as meaning contiguous volumes. One must then calculate an offset
within the PV's partition where the filesystem actually starts. If
that offset is incorrect, non-volume sectors, and possibly in-use
sectors will be garbage collected. If your volume is not contiguous,
non-volume related sectors may be garbage collected. Both are
troublesome, but the latter means you couldn't trim any volume you've
ever grown. I grow, move, and shrink filesystems as needed. If I can't
do that, there is little point to using LVM at all.

-- 
Chris Irwin
<chris at chrisirwin.ca>



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