[zfs-discuss] Lower latency ZIL Option?: SSD behind Controller BB Write Cache

Eric D. Mudama
Fri, 28 Jan 2011 20:04:56 -0000

On Fri, Jan 28 at  8:25, Edward Ned Harvey wrote:
>> From: zfs-discuss-bounces at opensolaris.org [mailto:zfs-discuss-
>> bounces at opensolaris.org] On Behalf Of Eff Norwood
>>
>> We tried all combinations of OCZ SSDs including their PCI based SSDs
and
>> they do NOT work as a ZIL. After a very short time performance degrades
>> horribly and for the OCZ drives they eventually fail completely.
>
>This was something interesting I found recently.  Apparently for flash
>manufacturers, flash hard drives are like the pimple on the butt of the
>elephant. A vast majority of the flash production in the world goes into
>devices like smartphones, cameras, tablets, etc.  Only a slim minority goes
>into hard drives.  As a result, they optimize for these other devices, and
>one of the important side effects is that standard flash chips use an 8K
>page size.  But hard drives use either 4K or 512B.
>
>The SSD controller secretly remaps blocks internally, and aggregates small
>writes into a single 8K write, so there''s really no way for the OS
to know
>if it''s writing to a 4K block which happens to be shared with
another 4K
>block in the 8K page.  So it''s unavoidable, and whenever it
happens, the
>drive can''t simply write.  It must read modify write, which is
obviously
>much slower.

The reality is way more complicated, and statements like the above may
or may not be true on a vendor-by-vendor basis.

As time passes, the underlying NAND geometries are designed for
certain sets of advantages, continually subject to re-evaluation and
modification, and good SSD controllers on the top of NAND or other
solid-state storage will map those advantages effectively into our
problem domains as users.

Testing methodologies are improving over time as well, and eventually
it will be more clear which devices are suited to which tasks.

The suitability of a specific solution into a problem space will
always be a balance between cost, performance, reliability and time to
market.  No single solution (RAM SAN, RAM SSD, NAND SSD, BBU
controllers, rotating HDD, etc.) wins in every single area, or else we
wouldn''t be having this discussion.

--eric


--
Eric D. Mudama
edmudama at mail.bounceswoosh.org