On 7/25/2011 4:49 AM, joerg.schill...@fokus.fraunhofer.de wrote:
Erik Trimble<erik.trim...@oracle.com> wrote:
On 7/25/2011 3:32 AM, Orvar Korvar wrote:
How long have you been using a SSD? Do you see any performance decrease? I
mean, ZFS does not support TRIM, so I wonder about long term effects...
Frankly, for the kind of use that ZFS puts on a SSD, TRIM makes no
impact whatsoever.
TRIM is primarily useful for low-volume changes - that is, for a
filesystem that generally has few deletes over time (i.e. rate of change
is low).
Using a SSD as a ZIL or L2ARC device puts a very high write load on the
device (even as an L2ARC, there is a considerably higher write load than
a "typical" filesystem use). SSDs in such a configuration can't really
make use of TRIM, and depend on the internal SSD controller block
re-allocation algorithms to improve block layout.
Now, if you're using the SSD as primary media (i.e. in place of a Hard
Drive), there is a possibility that TRIM could help. I honestly can't
be sure that it would help, however, as ZFS's Copy-on-Write nature means
that it tends to write entire pages of blocks, rather than just small
blocks. Which is fine from the SSD's standpoint.
Writing to an SSD is: clear + write + verify
As the SSD cannot know that the rewritten blocks have been unused for a while,
the SSD cannot handle the clear operation at a time when there is no interest
in the block, the TRIM command is needed to give this knowledge to the SSD.
Jörg
Except in many cases with ZFS, that data is irrelevant by the time it
can be used, or is much less useful than with other filesystems.
Copy-on-Write tends to end up with whole SSD pages of blocks being
rendered "unused", rather than individual blocks inside pages. So, the
SSD often can avoid the read-erase-modify-write cycle, and just do
erase-write instead. TRIM *might* help somewhat when you have a
relatively quiet ZFS filesystem, but I'm not really convinced of how
much of a benefit it would be.
As I've mentioned in other posts, ZIL and L2ARC are too "hot" for TRIM
to have any noticeable impact - the SSD is constantly being used, and
has no time for GC. It's stuck in the read-erase-modify-write cycle
even with TRIM.
--
Erik Trimble
Java Platform Group Infrastructure
Mailstop: usca22-317
Phone: x67195
Santa Clara, CA
Timezone: US/Pacific (UTC-0800)
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