On 7/25/2011 6:43 AM, Edward Ned Harvey wrote:
From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
boun...@opensolaris.org] On Behalf Of Erik Trimble
Honestly, I think TRIM isn't really useful for anyone.
I'm going to have to disagree.
There are only two times when TRIM isn't useful:
1) Your demand of the system is consistently so low that it never adds up
to anything meaningful... Basically you always have free unused blocks so
adding more unused blocks to the pile doesn't matter at all, or you never
bother to delete anything... Or it's just a lightweight server processing
requests where network latency greatly outweighs any disk latency, etc. AKA
your demand is very low.
or
2) Your demand of the system is consistently so high that even with TRIM,
the device would never be able to find any idle time to perform an erase
cycle on blocks marked for TRIM.
In case #2, it is at least theoretically possible for devices to become
smart enough to process the TRIM block erasures in parallel even while there
are other operations taking place simultaneously. I don't know if device
mfgrs implement things that way today. There is at least a common
perception (misperception?) that devices cannot process TRIM requests while
they are 100% busy processing other tasks.
Or your disk is always 100% full. I guess that makes 3 cases, but the 3rd
one is esoteric.
What I'm saying is that #2 occurs all the time with ZFS, at least as a
ZIL or L2ARC. TRIM is really only useful when the SSD has some
"downtime" to work. As a ZIL or L2ARC, the SSD *has* no pauses, and
can't do GC in the background usefully (which is what TRIM helps).
Instead, what I've seen is that the increased "smarts" of the new
generation SSD controllers do a better job of on-the-fly reallocation.
--
Erik Trimble
Java Platform Group Infrastructure
Mailstop: usca22-317
Phone: x67195
Santa Clara, CA
Timezone: US/Pacific (UTC-0800)
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