[zfs-discuss] Pure SSD Pool

Erik Trimble
Tue, 12 Jul 2011 17:34:06 -0000

FYI - virtually all non-super-low-end SSDs are already significantly
over-provisioned, for GC and scratch use inside the controller.

In fact, the only difference between the OCZ "extended" models and the
non-extended models (e.g. Vertex 2 50G (OCZSSD2-2VTX50G) and Vertex 2
Extended 60G (OCZSSD2-2VTXE60G)) is the amount of extra flash dedicated
to scratch. Both the aforementioned drives have 64G of flash chips -
it''s just that the 50G one uses significantly more for scratch, and
thus, will perform better under heavy use.

Over-provisioning at the filesystem level is unlikely to significantly
improve things, as the SSD controller generally only uses what it
considered "scratch" as such - that is, while not using 10G at the
filesystem level might seem useful, overall, my understanding of SSD''s
controller usage patterns is that this generally isn''t that much of a
performance gain.

E.g. you''d be better off buying the 50G Vertex 2 and fully using it
than
the 60G model and only using 50G on it.

-Erik



On Tue, 2011-07-12 at 10:10 -0700, Henry Lau wrote:
> It is hard to say, 90% or 80%. SSD has already reserved overprovisioning
places
> for garbage collection and wear leveling. The OS level only knows file LBA,
not
> the physical LBA mapping to flash pages/block. Uberblock updates and COW
from
> ZFS will use a new page/block each time. A TRIM command from ZFS level
should be
> a better solution but RAID is still a problem for TRIM at the OS level.
>
> Henry
>
>
>
> ----- Original Message ----
> From: Jim Klimov <jimklimov at cos.ru>
> Cc: ZFS Discussions <zfs-discuss at opensolaris.org>
> Sent: Tue, July 12, 2011 4:18:28 AM
> Subject: Re: [zfs-discuss] Pure SSD Pool
>
> 2011-07-12 9:06, Brandon High ?????:
> > On Mon, Jul 11, 2011 at 7:03 AM, Eric Sproul<esproul at
omniti.com>  wrote:
> >> Interesting-- what is the suspected impact of not having TRIM
support?
> > There shouldn''t be much, since zfs isn''t changing
data in place. Any
> > drive with reasonable garbage collection (which is pretty much
> > everything these days) should be fine until the volume gets very full.
>
> I wonder if in this case it would be beneficial to slice i.e. 90%
> of an SSD for use in ZFS pool(s) and leave the rest of the
> disk unassigned to any partition or slice? This would reserve
> some sectors as never-written-to-by-OS. Would this ease the
> life for SSD devices without TRIM between them ans the OS?
>
> Curious,
> //Jim
>
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--
Erik Trimble
Java Platform Group - Infrastructure
Mailstop:  usca22-317
Phone:  x67195
Santa Clara, CA
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