[zfs-discuss] Pure SSD Pool

Garrett D''Amore
Tue, 12 Jul 2011 15:48:57 -0000

I think high end SSDs, like those from Pliant, use a significant amount of
"over allocation", and internal remapping and internal COW, so that
they can automatically garbage collect when they need to, without TRIM.  This
only works if the drive has enough extra free space that it knows about (because
it uses overallocation for example).

TRIM support is still something we want in ZFS, for a variety of reasons,
including SSD performance.  I think you can expect to hear more on this front
before too much longer, so stay tuned.

  -- Garrett D''Amore

On Jul 12, 2011, at 7:42 AM, "Eric Sproul" <esproul at
omniti.com> wrote:

> On Tue, Jul 12, 2011 at 1:06 AM, Brandon High <bhigh at freaks.com>
wrote:
>> 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.
>
> But that''s exactly the problem-- ZFS being copy-on-write will
> eventually have written to all of the available LBA addresses on the
> drive, regardless of how much live data exists.  It''s the rate of
> change, in other words, rather than the absolute amount that gets us
> into trouble with SSDs.  The SSD has no way of knowing what blocks
> contain live data and which have been freed, because the OS never
> tells it (that''s what TRIM is supposed to do).  So after ZFS has
> written to almost every LBA, it starts writing to addresses previously
> used (and freed by ZFS, but unknown to the SSD), so the SSD has to
> erase the cell before it can be written anew.  This incurs a heavy
> performance penalty and seems like a worst-case-scenario use case.
>
> Now, others have hinted that certain controllers are better than
> others in the absence of TRIM, but I don''t see how GC could know
what
> blocks are available to be erased without information from the OS.
>
> Those with deep knowledge of SSD models/controllers: how does the
> Intel 320 perform under ZFS as primary storage (not ZIL or L2ARC)?
>
> Eric
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