Message-ID: <4B814920.1040204@cheapcomplexdevices.com>
Date: Sun, 21 Feb 2010 06:54:24 -0800
From: Ron Mayer <rm_pg@cheapcomplexdevices.com>
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To: Bruce Momjian <bruce@momjian.us>
CC: Ron Mayer <rm_pg@cheapcomplexdevices.com>,
	pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: SSD + RAID
References: <201002211410.o1LEAht05941@momjian.us>
In-Reply-To: <201002211410.o1LEAht05941@momjian.us>
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Bruce Momjian wrote:
> Agreed, thought I thought the problem was that SSDs lie about their
> cache flush like SATA drives do, or is there something I am missing?

There's exactly one case I can find[1] where this century's IDE
drives lied more than any other drive with a cache:

  Under 120GB Maxtor drives from late 2003 to early 2004.

and it's apparently been worked around for years.

Those drives claimed to support the "FLUSH_CACHE_EXT" feature (IDE
command 0xEA), but did not support sending 48-bit commands which
was needed to send the cache flushing command.

And for that case a workaround for Linux was quickly identified by
checking for *both* the support for 48-bit commands and support for the
flush cache extension[2].


Beyond those 2004 drive + 2003 kernel systems, I think most the rest
of such reports have been various misfeatures in some of Linux's
filesystems (like EXT3 that only wants to send drives cache-flushing
commands when inode change[3]) and linux software raid misfeatures....

...and ISTM those would affect SSDs the same way they'd affect SATA drives.


[1] http://lkml.org/lkml/2004/5/12/132
[2] http://lkml.org/lkml/2004/5/12/200
[3] http://www.mail-archive.com/linux-kernel@vger.kernel.org/msg272253.html